# GNAT User's Guide

## GNAT, The GNU Ada 95 Compiler

## Document revision level 1.408.2.6

## GNAT Version 3.14p

## Date: 2001/05/10 16:08:26

# Table of Contents

- About This Guide
  - What This Guide Contains
  - What You Should Know before Reading This Guide
  - Related Information
  - Conventions
- Getting Started with GNAT
  - Running GNAT
  - Running a Simple Ada Program
  - Running a Program with Multiple Units
  - Using the `gnatmake` Utility
- The GNAT Compilation Model
  - Source Representation
  - Foreign Language Representation
    - Latin-1
    - Other 8-Bit Codes
    - Wide Character Encodings
  - File Naming Rules
  - Using Other File Names
  - Alternative File Naming Schemes
  - Generating Object Files
  - Source Dependencies
  - The Ada Library Information Files
  - Binding an Ada Program
  - Mixed Language Programming
    - Interfacing to C
    - Calling Conventions
  - Building Mixed Ada & C++ Programs
    - Interfacing to C++
    - Linking a Mixed C++ & Ada Program
    - A Simple Example
    - Adapting the Run Time to a New C++ Compiler
  - Comparison between GNAT and C/C++ Compilation Models
  - Comparison between GNAT and Conventional Ada Library Models
- Compiling Using `gcc`
  - Compiling Programs
  - Switches for `gcc`
    - Output and Error Message Control
    - Debugging and Assertion Control
    - Style Checking
    - Run-Time Checks
    - Stack Overflow Checking
    - Run-Time Control
    - Using `gcc` for Syntax Checking
    - Using `gcc` for Semantic Checking
    - Compiling Ada 83 Programs
    - Character Set Control
    - File Naming Control
    - Subprogram Inlining Control
    - Auxiliary Output Control
    - Debugging Control
  - Search Paths and the Run-Time Library (RTL)
  - Order of Compilation Issues
  - Examples
- Binding Using `gnatbind`
  - Running `gnatbind`
  - Generating the Binder Program in C
  - Consistency-Checking Modes
  - Binder Error Message Control
  - Elaboration Control
  - Output Control
  - Binding with Non-Ada Main Programs
  - Binding Programs with No Main Subprogram
  - Summary of Binder Switches
  - Command-Line Access
  - Search Paths for `gnatbind`
  - Examples of `gnatbind` Usage
- Linking Using `gnatlink`
  - Running `gnatlink`
  - Switches for `gnatlink`
  - Setting Stack Size from `gnatlink`
  - Setting Heap Size from `gnatlink`
- The GNAT Make Program `gnatmake`
  - Running `gnatmake`
  - Switches for `gnatmake`
  - Mode Switches for `gnatmake`
  - Notes on the Command Line
  - How `gnatmake` Works
  - Examples of `gnatmake` Usage
- Renaming Files Using `gnatchop`
  - Handling Files with Multiple Units
  - Operating gnatchop in Compilation Mode
  - Command Line for `gnatchop`
  - Switches for `gnatchop`
  - Examples of `gnatchop` Usage
- Configuration Pragmas
  - Handling of Configuration Pragmas
  - The Configuration Pragmas File
- Elaboration Order Handling in GNAT
  - Elaboration Code in Ada 95
  - Checking the Elaboration Order in Ada 95
  - Controlling the Elaboration Order in Ada 95
  - Controlling Elaboration in GNAT - Internal Calls
  - Controlling Elaboration in GNAT - External Calls
  - Default Behavior in GNAT - Ensuring Safety
  - Elaboration Issues for Library Tasks
  - Mixing Elaboration Models
  - What to Do If the Default Elaboration Behavior Fails
  - Elaboration for Access-to-Subprogram Values
  - Summary of Procedures for Elaboration Control
  - Other Elaboration Order Considerations
- The Cross-Referencing Tools `gnatxref` and `gnatfind`
  - `gnatxref` Switches
  - `gnatfind` Switches
  - Project Files
  - Regular Expressions in `gnatfind` and `gnatxref`
  - Examples of `gnatxref` Usage
    - General Usage
    - Using gnatxref with vi
  - Examples of `gnatfind` Usage
- File Name Krunching Using `gnatkr`
  - About `gnatkr`
  - Using `gnatkr`
  - Krunching Method
  - Examples of `gnatkr` Usage
- Preprocessing Using `gnatprep`
  - Using `gnatprep`
  - Switches for `gnatprep`
  - Form of Definitions File
  - Form of Input Text for `gnatprep`
- The GNAT Library Browser `gnatls`
  - Running `gnatls`
  - Switches for `gnatls`
  - Example of `gnatls` Usage
- GNAT and Libraries
  - Creating an Ada Library
  - Installing an Ada Library
  - Using an Ada Library
  - Rebuilding the GNAT Run-Time Library
- Using the GNU `make` Utility
  - Using gnatmake in a Makefile
  - Automatically Creating a List of Directories
  - Generating the Command Line Switches
  - Overcoming Command Line Length Limits
- Finding Memory Problems with `gnatmem`
  - Running `gnatmem` (GDB Mode)
  - Running `gnatmem` (GMEM Mode)
  - Switches for `gnatmem`
  - Example of `gnatmem` Usage
  - GDB and GMEM Modes
  - Implementation Note
    - `gnatmem` Using `GDB` Mode
    - `gnatmem` Using `GMEM` Mode
- Finding Memory Problems with GNAT Debug Pool
- Creating Sample Bodies Using `gnatstub`
  - Running `gnatstub`
  - Switches for `gnatstub`
- Reducing the Size of Ada Executables with `gnatelim`
  - About `gnatelim`
  - `Eliminate` Pragma
  - Tree Files
  - Preparing Tree and Bind Files for `gnatelim`
  - Running `gnatelim`
  - Correcting the List of Eliminate Pragmas
  - Making Your Executables Smaller
  - Summary of the gnatelim Usage Cycle
- Other Utility Programs
  - Using Other Utility Programs with GNAT
  - The `gnatpsys` Utility Program
  - The `gnatpsta` Utility Program
  - The External Symbol Naming Scheme of GNAT
  - Ada Mode for `Glide`
    - General Features:
    - Ada Mode Features That Help Understanding Code:
    - Glide Support for Writing Ada Code:
  - Converting Ada Files to html with `gnathtml`
  - Installing `gnathtml`
- Running and Debugging Ada Programs
  - The GNAT Debugger GDB
  - Running GDB
  - Introduction to GDB Commands
  - Using Ada Expressions
  - Calling User-Defined Subprograms
  - Breaking on Ada Exceptions
  - Ada Tasks
  - Debugging Generic Units
  - GNAT Abnormal Termination or Failure to Terminate
  - Naming Conventions for GNAT Source Files
  - Getting Internal Debugging Information
- Performance Considerations
  - Controlling Run-Time Checks
  - Optimization Levels
  - Debugging Optimized Code
  - Inlining of Subprograms
- Index

@input texiplus

(C) Copyright 1995-2000, Ada Core Technologies, Inc.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.

Silicon Graphics and IRIS are registered trademarks and IRIX is a trademark of Silicon Graphics, Inc.

IBM PC is a trademark of International Business Machines Corporation.

UNIX is a registered trademark of AT&T Bell Laboratories.

The following are trademarks of Compaq Computers: DEC, DEC Ada, DECthreads, DIGITAL, DECset, OpenVMS, and VAX.

The following are trademarks of Microsoft Corporation: Windows NT, Windows 95, Windows 98.

The following are trademarks of Wind River Systems: VxWorks, Tornado.

# About This Guide

This guide describes the use of GNAT, a compiler and software development toolset for the full Ada 95 programming language. It describes the features of the compiler and tools, and details how to use them to build Ada 95 applications.

- What This Guide Contains
- What You Should Know before Reading This Guide
- Related Information
- Conventions

## What This Guide Contains

This guide contains the following chapters:

- section Getting Started with GNAT, describes how to get started compiling and running Ada programs with the GNAT Ada programming environment.
- section The GNAT Compilation Model, describes the compilation model used by GNAT.
- section Compiling Using `gcc`, describes how to compile Ada programs with `gcc`, the Ada compiler.
- section Binding Using `gnatbind`, describes how to perform binding of Ada programs with `gnatbind`, the GNAT binding utility.
- section Linking Using `gnatlink`, describes `gnatlink`, a program that provides for linking using the GNAT run-time library to construct a program. `gnatlink` can also incorporate foreign language object units into the executable.
- section The GNAT Make Program `gnatmake`, describes `gnatmake`, a utility that automatically determines the set of sources needed by an Ada compilation unit, and executes the necessary compilations binding and link.
- section Renaming Files Using `gnatchop`, describes `gnatchop`, a utility that allows you to preprocess a file that contains Ada source code, and split it into one or more new files, one for each compilation unit.
- section The Cross-Referencing Tools `gnatxref` and `gnatfind`, discusses `gnatxref` and `gnatfind`, two tools that provide an easy way to navigate through sources.
- section File Name Krunching Using `gnatkr`, describes the `gnatkr` file name krunching utility, used to handle shortened file names on operating systems with a limit on the length of names.
- section Preprocessing Using `gnatprep`, describes `gnatprep`, a preprocessor utility that allows a single source file to be used to generate multiple or parameterized source files, by means of macro substitution.
- section The GNAT Library Browser `gnatls`, describes `gnatls`, a utility that displays information about compiled units, including dependences on the corresponding sources files, and consistency of compilations.
- section GNAT and Libraries, describes the process of creating and using Libraries with GNAT. It also describes how to recompile the GNAT run-time library.
- section Using the GNU `make` Utility, describes some techniques for using the GNAT toolset in Makefiles.
- section Finding Memory Problems with `gnatmem`, describes `gnatmem`, a utility that monitors dynamic allocation and deallocation activity in a program, and displays information about incorrect deallocations and sources of possible memory leaks.
- section Finding Memory Problems with GNAT Debug Pool, describes how to use the GNAT-specific Debug Pool in order to detect as early as possible the use of incorrect memory references.
- section Creating Sample Bodies Using `gnatstub`, discusses `gnatstub`, a utility that generates empty but compilable bodies for library units.
- section Reducing the Size of Ada Executables with `gnatelim`, describes `gnatelim`, a tool which detects unused subprograms and helps the compiler to create a smaller executable for the program.
- section Other Utility Programs, discusses several other GNAT utilities, including `gnatpsta` and `gnatpsys`.
- section Running and Debugging Ada Programs, describes how to run and debug Ada programs.
- section Building Mixed Ada & C++ Programs, gives hints on how to interface with c++.
- section Performance Considerations, reviews the trade offs between using defaults or options in program development.

## What You Should Know before Reading This Guide

This user's guide assumes that you are familiar with Ada 95 language, as described in the International Standard ANSI/ISO/IEC-8652:1995, Jan 1995.

## Related Information

For further information about related tools, refer to the following documents:

- GNAT Reference Manual, which contains all reference material for the GNAT implementation of Ada 95.
- Ada 95 Language Reference Manual, which contains all reference material for the Ada 95 programming language.
- Debugging with GDB contains all details on the use of the GNU source-level debugger.
- GNU Emacs Manual contains full information on the extensible editor and programming environment Emacs.

## Conventions

Following are examples of the typographical and graphic conventions used in this guide:

- `Functions`, `utility program names`, `standard names`, and `classes`.
- ``Option flags'`
- ``File Names'`, ``button names'`, and ``field names'`.
- Variables.
- Emphasis.
- [optional information or parameters]
- Examples are described by text

      and then shown this way.

Commands that are entered by the user are preceded in this manual by the characters "`$` " (dollar sign followed by space). If your system uses this sequence as a prompt, then the commands will appear exactly as you see them in the manual. If your system uses some other prompt, then the command will appear with the `$` replaced by whatever prompt character you are using.

# Getting Started with GNAT

This chapter describes some simple ways of using GNAT to build executable Ada programs.

- Running GNAT
- Running a Simple Ada Program
- Running a Program with Multiple Units
- Using the gnatmake Utility

## Running GNAT

Three steps are needed to create an executable file from an Ada source file:

1. The source file(s) must be compiled.
2. The file(s) must be bound using the GNAT binder.
3. All appropriate object files must be linked to produce an executable.

All three steps are most commonly handled by using the `gnatmake` utility program that, given the name of the main program, automatically performs the necessary compilation, binding and linking steps.

## Running a Simple Ada Program

Any text editor may be used to prepare an Ada program. If `glide` is used, the optional Ada mode may be helpful in laying out the program. The program text is a normal text file. We will suppose in our initial example that you have used your editor to prepare the following standard format text file:

    with Ada.Text_IO; use Ada.Text_IO;
    procedure Hello is
    begin
       Put_Line ("Hello WORLD!");
    end Hello;

This file should be named ``hello.adb'`. With the normal default file naming conventions, GNAT requires that each file contain a single compilation unit whose file name is the unit name with periods replaced by hyphens, and whose extension is ``.ads'` for a spec and ``.adb'` for a body. You can override this default file naming convention by use of the special pragma `Source_File_Name` (see section Using Other File Names). Alternatively, if you want to rename your files according to this default convention, which is probably more convenient if you will be using GNAT for all your compilations, then the `gnatchop` utility can be used to generate correctly-named source files (see section Renaming Files Using `gnatchop`).

You can compile the program using the following command (`$` is used as the command prompt in the examples in this document):

    $ gcc -c hello.adb

`gcc` is the command used to run the compiler. This compiler is capable of compiling programs in several languages, including Ada 95 and C. It assumes that you have given it an Ada program if the file extension is either ``.ads'` or ``.adb'`, and it will then call the GNAT compiler to compile the specified file.

The `-c` switch is required. It tells `gcc` to only do a compilation. (For C programs, `gcc` can also do linking, but this capability is not used directly for Ada programs, so the `-c` switch must always be present.)

This compile command generates a file ``hello.o'` which is the object file corresponding to your Ada program. It also generates a file ``hello.ali'` which contains additional information used to check that an Ada program is consistent. To build an executable file, use `gnatbind` to bind the program and `gnatlink` to link it. The argument to both `gnatbind` and `gnatlink` is the name of the ``ali'` file, but the default extension of ``.ali'` can be omitted. This means that in the most common case, the argument is simply the name of the main program:

    $ gnatbind hello
    $ gnatlink hello

A simpler method of carrying out these steps is to use `gnatmake`, which is a master program that invokes all of the required compilation, binding and linking tools in the correct order. In particular, `gnatmake` automatically recompiles any sources that have been modified since they were last compiled, or sources that depend on such modified sources, so that a consistent compilation is ensured.

    $ gnatmake hello.adb

The result is an executable program called ``hello'`, which can be run by entering:

    $ hello

assuming that the current directory is on the search path for executable programs.

and, if all has gone well, you will see

    Hello WORLD!

appear in response to this command.

## Running a Program with Multiple Units

Consider a slightly more complicated example that has three files: a main program, and the spec and body of a package:

    package Greetings is
       procedure Hello;
       procedure Goodbye;
    end Greetings;

    with Ada.Text_IO; use Ada.Text_IO;
    package body Greetings is
       procedure Hello is
       begin
          Put_Line ("Hello WORLD!");
       end Hello;

       procedure Goodbye is
       begin
          Put_Line ("Goodbye WORLD!");
       end Goodbye;
    end Greetings;

    with Greetings;
    procedure Gmain is
    begin
       Greetings.Hello;
       Greetings.Goodbye;
    end Gmain;

Following the one-unit-per-file rule, place this program in the following three separate files:

``greetings.ads'`
  spec of package `Greetings`
``greetings.adb'`
  body of package `Greetings`
``gmain.adb'`
  body of main program

To build an executable version of this program, we could use four separate steps to compile, bind, and link the program, as follows:

    $ gcc -c gmain.adb
    $ gcc -c greetings.adb
    $ gnatbind gmain
    $ gnatlink gmain

Note that there is no required order of compilation when using GNAT. In particular it is perfectly fine to compile the main program first. Also, it is not necessary to compile package specs in the case where there is an accompanying body; you only need to compile the body. If you want to submit these files to the compiler for semantic checking and not code generation, then use the `-gnatc` switch:

       $ gcc -c greetings.ads -gnatc

Although the compilation can be done in separate steps as in the above example, in practice it is almost always more convenient to use the `gnatmake` tool. All you need to know in this case is the name of the main program's source file. The effect of the above four commands can be achieved with a single one:

    $ gnatmake gmain.adb

In the next section we discuss the advantages of using `gnatmake` in more detail.

## Using the `gnatmake` Utility

If you work on a program by compiling single components at a time using `gcc`, you typically keep track of the units you modify. In order to build a consistent system, you compile not only these units, but also any units that depend on the units you have modified. For example, in the preceding case, if you edit ``gmain.adb'`, you only need to recompile that file. But if you edit ``greetings.ads'`, you must recompile both ``greetings.adb'` and ``gmain.adb'`, because both files contain units that depend on ``greetings.ads'`.

`gnatbind` will warn you if you forget one of these compilation steps, so that it is impossible to generate an inconsistent program as a result of forgetting to do a compilation. Nevertheless it is tedious and error-prone to keep track of dependencies among units. One approach to handle the dependency-bookkeeping is to use a makefile. However, makefiles present maintenance problems of their own: if the dependencies change as you change the program, you must make sure that the makefile is kept up-to-date manually, which is also an error-prone process.

The `gnatmake` utility takes care of these details automatically. Invoke it using either one of the following forms:

    $ gnatmake gmain.adb
    $ gnatmake gmain

The argument is the name of the file containing the main program; you may omit the extension. `gnatmake` examines the environment, automatically recompiles any files that need recompiling, and binds and links the resulting set of object files, generating the executable file, ``gmain'`. In a large program, it can be extremely helpful to use `gnatmake`, because working out by hand what needs to be recompiled can be difficult.

Note that `gnatmake` takes into account all the Ada 95 rules that establish dependencies among units. These include dependencies that result from inlining subprogram bodies, and from generic instantiation. Unlike some other Ada make tools, `gnatmake` does not rely on the dependencies that were found by the compiler on a previous compilation, which may possibly be wrong when sources change. `gnatmake` determines the exact set of dependencies from scratch each time it is run.

# The GNAT Compilation Model

- Source Representation
- Foreign Language Representation
- File Naming Rules
- Using Other File Names
- Alternative File Naming Schemes
- Generating Object Files
- Source Dependencies
- The Ada Library Information Files
- Binding an Ada Program
- Mixed Language Programming
- Building Mixed Ada & C++ Programs
- Comparison between GNAT and C/C++ Compilation Models
- Comparison between GNAT and Conventional Ada Library Models

This chapter describes the compilation model used by GNAT. Although similar to that used by other languages, such as C and C++, this model is substantially different from the traditional Ada compilation models, which are based on a library. The model is initially described without reference to the library-based model. If you have not previously used an Ada compiler, you need only read the first part of this chapter. The last section describes and discusses the differences between the GNAT model and the traditional Ada compiler models. If you have used other Ada compilers, this section will help you to understand those differences, and the advantages of the GNAT model.

## Source Representation

Ada source programs are represented in standard text files, using Latin-1 coding. Latin-1 is an 8-bit code that includes the familiar 7-bit ASCII set, plus additional characters used for representing foreign languages (see section Foreign Language Representation for support of non-USA character sets). The format effector characters are represented using their standard ASCII encodings, as follows:

`VT`
  Vertical tab, `16#0B#`
`HT`
  Horizontal tab, `16#09#`
`CR`
  Carriage return, `16#0D#`
`LF`
  Line feed, `16#0A#`
`FF`
  Form feed, `16#0C#`

Source files are in standard text file format. In addition, GNAT will recognize a wide variety of stream formats, in which the end of physical physical lines is marked by any of the following sequences: `LF`, `CR`, `CR-LF`, or `LF-CR`. This is useful in accommodating files that are imported from other operating systems.

The end of a source file is normally represented by the physical end of file. However, the control character `16#1A#` (`SUB`) is also recognized as signalling the end of the source file. Again, this is provided for compatibility with other operating systems where this code is used to represent the end of file.

Each file contains a single Ada compilation unit, including any pragmas associated with the unit. For example, this means you must place a package declaration (a package spec) and the corresponding body in separate files. An Ada compilation (which is a sequence of compilation units) is represented using a sequence of files. Similarly, you will place each subunit or child unit in a separate file.

## Foreign Language Representation

GNAT supports the standard character sets defined in Ada 95 as well as several other non-standard character sets for use in localized versions of the compiler (see section Character Set Control).

- Latin-1
- Other 8-Bit Codes
- Wide Character Encodings

### Latin-1

The basic character set is Latin-1. This character set is defined by ISO standard 8859, part 1. The lower half (character codes `16#00#` ... `16#7F#)` is identical to standard ASCII coding, but the upper half is used to represent additional characters. These include extended letters used by European languages, such as French accents, the vowels with umlauts used in German, and the extra letter A-ring used in Swedish.

For a complete list of Latin-1 codes and their encodings, see the source file of library unit `Ada.Characters.Latin_1` in file ``a-chlat1.ads'`. You may use any of these extended characters freely in character or string literals. In addition, the extended characters that represent letters can be used in identifiers.

### Other 8-Bit Codes

GNAT also supports several other 8-bit coding schemes:

Latin-2
  Latin-2 letters allowed in identifiers, with uppercase and lowercase equivalence.
Latin-3
  Latin-3 letters allowed in identifiers, with uppercase and lowercase equivalence.
Latin-4
  Latin-4 letters allowed in identifiers, with uppercase and lowercase equivalence.
IBM PC (code page 437)
  This code page is the normal default for PCs in the U.S. It corresponds to the original IBM PC character set. This set has some, but not all, of the extended Latin-1 letters, but these letters do not have the same encoding as Latin-1. In this mode, these letters are allowed in identifiers with uppercase and lowercase equivalence.
IBM PC (code page 850)
  This code page is a modification of 437 extended to include all the Latin-1 letters, but still not with the usual Latin-1 encoding. In this mode, all these letters are allowed in identifiers with uppercase and lowercase equivalence.
Full Upper 8-bit
  Any character in the range 80-FF allowed in identifiers, and all are considered distinct. In other words, there are no uppercase and lowercase equivalences in this range. This is useful in conjunction with certain encoding schemes used for some foreign character sets (e.g. the typical method of representing Chinese characters on the PC).
No Upper-Half
  No upper-half characters in the range 80-FF are allowed in identifiers. This gives Ada 83 compatibility for identifier names.

For precise data on the encodings permitted, and the uppercase and lowercase equivalences that are recognized, see the file ``csets.adb'` in the GNAT compiler sources. You will need to obtain a full source release of GNAT to obtain this file.

### Wide Character Encodings

GNAT allows wide character codes to appear in character and string literals, and also optionally in identifiers, by means of the following possible encoding schemes:

Hex Coding
  In this encoding, a wide character is represented by the following five character sequence:

      ESC a b c d

  Where `a`, `b`, `c`, `d` are the four hexadecimal characters (using uppercase letters) of the wide character code. For example, ESC A345 is used to represent the wide character with code `16#A345#`. This scheme is compatible with use of the full Wide_Character set.
Upper-Half Coding
  The wide character with encoding `16#abcd#` where the upper bit is on (in other words, "a" is in the range 8-F) is represented as two bytes, `16#ab#` and `16#cd#`. The second byte cannot be a format control character, but is not required to be in the upper half. This method can be also used for shift-JIS or EUC, where the internal coding matches the external coding.
Shift JIS Coding
  A wide character is represented by a two-character sequence, `16#ab#` and `16#cd#`, with the restrictions described for upper-half encoding as described above. The internal character code is the corresponding JIS character according to the standard algorithm for Shift-JIS conversion. Only characters defined in the JIS code set table can be used with this encoding method.
EUC Coding
  A wide character is represented by a two-character sequence `16#ab#` and `16#cd#`, with both characters being in the upper half. The internal character code is the corresponding JIS character according to the EUC encoding algorithm. Only characters defined in the JIS code set table can be used with this encoding method.
UTF-8 Coding
  A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2. Depending on the character value, the representation is a one, two, or three byte sequence:

      @leftskip=.7cm
      16#0000#-16#007f#: 2#0xxxxxxx#
      16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
      16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#

  where the xxx bits correspond to the left-padded bits of the 16-bit character value. Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, but in this implementation, all UTF-8 sequences of four or more bytes length will be treated as illegal).
Brackets Coding
  In this encoding, a wide character is represented by the following eight character sequence:

      [ " a b c d " ]

  Where `a`, `b`, `c`, `d` are the four hexadecimal characters (using uppercase letters) of the wide character code. For example, ["A345"] is used to represent the wide character with code `16#A345#`. It is also possible (though not required) to use the Brackets coding for upper half characters. For example, the code `16#A3#` can be represented as `["A3"]`. This scheme is compatible with use of the full Wide_Character set, and is also the method used for wide character encoding in the standard ACVC (Ada Compiler Validation Capability) test suite distributions.

Note: Some of these coding schemes do not permit the full use of the Ada 95 character set. For example, neither Shift JIS, nor EUC allow the use of the upper half of the Latin-1 set.

## File Naming Rules

The default file name is determined by the name of the unit that the file contains. The name is formed by taking the full expanded name of the unit and replacing the separating dots with hyphens and using lowercase for all letters.

An exception arises if the file name generated by the above rules starts with one of the characters a,g,i, or s, and the second character is a minus. In this case, the character tilde is used in place of the minus. The reason for this special rule is to avoid clashes with the standard names for child units of the packages System, Ada, Interfaces, and GNAT, which use the prefixes s- a- i- and g- respectively.

The file extension is ``.ads'` for a spec and ``.adb'` for a body. The following list shows some examples of these rules.

``main.ads'`
  Main (spec)
``main.adb'`
  Main (body)
``arith_functions.ads'`
  Arith_Functions (package spec)
``arith_functions.adb'`
  Arith_Functions (package body)
``func-spec.ads'`
  Func.Spec (child package spec)
``func-spec.adb'`
  Func.Spec (child package body)
``main-sub.adb'`
  Sub (subunit of Main)
``a~bad.adb'`
  A.Bad (child package body)

Following these rules can result in excessively long file names if corresponding unit names are long (for example, if child units or subunits are heavily nested). An option is available to shorten such long file names (called file name "krunching"). This may be particularly useful when programs being developed with GNAT are to be used on operating systems with limited file name lengths. See section Using `gnatkr`.

Of course, no file shortening algorithm can guarantee uniqueness over all possible unit names; if file name krunching is used, it is your responsibility to ensure no name clashes occur. Alternatively you can specify the exact file names that you want used, as described in the next section. Finally, if your Ada programs are migrating from a compiler with a different naming convention, you can use the gnatchop utility to produce source files that follow the GNAT naming conventions. (For details see section Renaming Files Using `gnatchop`.)

## Using Other File Names

In the previous section, we have described the default rules used by GNAT to determine the file name in which a given unit resides. It is often convenient to follow these default rules, and if you follow them, the compiler knows without being explicitly told where to find all the files it needs.

However, in some cases, particularly when a program is imported from another Ada compiler environment, it may be more convenient for the programmer to specify which file names contain which units. GNAT allows arbitrary file names to be used by means of the Source_File_Name pragma. The form of this pragma is as shown in the following examples:

    pragma Source_File_Name (My_Utilities.Stacks,
      Spec_File_Name => "myutilst_a.ada");
    pragma Source_File_name (My_Utilities.Stacks,
      Body_File_Name => "myutilst.ada");

As shown in this example, the first argument for the pragma is the unit name (in this example a child unit). The second argument has the form of a named association. The identifier indicates whether the file name is for a spec or a body; the file name itself is given by a string literal.

The source file name pragma is a configuration pragma, which means that normally it will be placed in the ``gnat.adc'` file used to hold configuration pragmas that apply to a complete compilation environment. For more details on how the ``gnat.adc'` file is created and used see section Handling of Configuration Pragmas

GNAT allows completely arbitrary file names to be specified using the source file name pragma. However, if the file name specified has an extension other than ``.ads'` or ``.adb'` it is necessary to use a special syntax when compiling the file. The name in this case must be preceded by the special sequence `-x` followed by a space and the name of the language, here `ada`, as in:

    $ gcc -c -x ada peculiar_file_name.sim

`gnatmake` handles non-standard file names in the usual manner (the non-standard file name for the main program is simply used as the argument to gnatmake). Note that if the extension is also non-standard, then it must be included in the gnatmake command, it may not be omitted.

## Alternative File Naming Schemes

In the previous section, we described the use of the `Source_File_Name` pragma to allow arbitrary names to be assigned to individual source files. However, this approach requires one pragma for each file, and especially in large systems can result in very long ``gnat.adc'` files, and also create a maintenance problem.

GNAT also provides a facility for specifying systematic file naming schemes other than the standard default naming scheme previously described. An alternative scheme for naming is specified by the use of `Source_File_Name` pragmas having the following format:

    pragma Source_File_Name (
       Spec_File_Name  => FILE_NAME_PATTERN
     [,Casing          => CASING_SPEC]
     [,Dot_Replacement => STRING_LITERAL]);

    pragma Source_File_Name (
       Body_File_Name  => FILE_NAME_PATTERN
     [,Casing          => CASING_SPEC]
     [,Dot_Replacement => STRING_LITERAL]);

    pragma Source_File_Name (
       Subunit_File_Name  => FILE_NAME_PATTERN
     [,Casing             => CASING_SPEC]
     [,Dot_Replacement    => STRING_LITERAL]);

    FILE_NAME_PATTERN ::= STRING_LITERAL
    CASING_SPEC ::= Lowercase | Uppercase | Mixedcase

The `FILE_NAME_PATTERN` string shows how the file name is constructed. It contains a single asterisk character, and the unit name is substituted systematically for this asterisk. The optional parameter `Casing` indicates whether the unit name is to be all upper-case letters, all lower-case letters, or mixed-case. If no `Casing` parameter is used, then the default is all lower-case.

The optional `Dot_Replacement` string is used to replace any periods that occur in subunit or child unit names. If no `Dot_Replacement` argument is used then separating dots appear unchanged in the resulting file name. Although the above syntax indicates that the `Casing` argument must appear before the `Dot_Replacement` argument, but it is also permissible to write these arguments in the opposite order.

As indicated, it is possible to specify different naming schemes for bodies, specs, and subunits. Quite often the rule for subunits is the same as the rule for bodies, in which case, there is no need to give a separate `Subunit_File_Name` rule, and in this case the `Body_File_name` rule is used for subunits as well.

The separate rule for subunits can also be used to implement the rather unusual case of a compilation environment (e.g. a single directory) which contains a subunit and a child unit with the same unit name. Although both units cannot appear in the same partition, the Ada Reference Manual allows (but does not require) the possibility of the two units coexisting in the same environment.

The file name translation works in the following steps:

- If there is a specific `Source_File_Name` pragma for the given unit, then this is always used, and any general pattern rules are ignored.
- If there is a pattern type `Source_File_Name` pragma that applies to the unit, then the resulting file name will be used if the file exists. If more than one pattern matches, the latest one will be tried first, and the first attempt resulting in a reference to a file that exists will be used.
- If no pattern type `Source_File_Name` pragma that applies to the unit for which the corresponding file exists, then the standard GNAT default naming rules are used.

As an example of the use of this mechanism, consider a commonly used scheme in which file names are all lower case, with separating periods copied unchanged to the resulting file name, and specs end with ".1.ada", and bodies end with ".2.ada". GNAT will follow this scheme if the following two pragmas appear:

    pragma Source_File_Name
      (Spec_File_Name => "*.1.ada");
    pragma Source_File_Name
      (Body_File_Name => "*.2.ada");

The default GNAT scheme is actually implemented by providing the following default pragmas internally:

    pragma Source_File_Name
      (Spec_File_Name => "*.ads", Dot_Replacement => "-");
    pragma Source_File_Name
      (Body_File_Name => "*.adb", Dot_Replacement => "-");

Our final example implements a scheme typically used with one of the Ada 83 compilers, where the separator character for subunits was "__" (two underscores), specs were identified by adding ``_.ADA'`, bodies by adding ``.ADA'`, and subunits by adding ``.SEP'`. All file names were upper case. Child units were not present of course since this was an Ada 83 compiler, but it seems reasonable to extend this scheme to use the same double underscore separator for child units.

    pragma Source_File_Name
      (Spec_File_Name => "*_.ADA",
       Dot_Replacement => "__",
       Casing = Uppercase);
    pragma Source_File_Name
      (Body_File_Name => "*.ADA",
       Dot_Replacement => "__",
       Casing = Uppercase);
    pragma Source_File_Name
      (Subunit_File_Name => "*.SEP",
       Dot_Replacement => "__",
       Casing = Uppercase);

## Generating Object Files

An Ada program consists of a set of source files, and the first step in compiling the program is to generate the corresponding object files. These are generated by compiling a subset of these source files. The files you need to compile are the following:

- If a package spec has no body, compile the package spec to produce the object file for the package.
- If a package has both a spec and a body, compile the body to produce the object file for the package. The source file for the package spec need not be compiled in this case because there is only one object file, which contains the code for both the spec and body of the package.
- For a subprogram, compile the subprogram body to produce the object file for the subprogram. The spec, if one is present, is as usual in a separate file, and need not be compiled.
- In the case of subunits, only compile the parent unit. A single object file is generated for the entire subunit tree, which includes all the subunits.
- Compile child units independently of their parent units (though, of course, the spec of all the ancestor unit must be present in order to compile a child unit).
- Compile generic units in the same manner as any other units. The object files in this case are small dummy files that contain at most the flag used for elaboration checking. This is because GNAT always handles generic instantiation by means of macro expansion. However, it is still necessary to compile generic units, for dependency checking and elaboration purposes.

The preceding rules describe the set of files that must be compiled to generate the object files for a program. Each object file has the same name as the corresponding source file, except that the extension is ``.o'` as usual.

You may wish to compile other files for the purpose of checking their syntactic and semantic correctness. For example, in the case where a package has a separate spec and body, you would not normally compile the spec. However, it is convenient in practice to compile the spec to make sure it is error-free before compiling clients of this spec, because such compilations will fail if there is an error in the spec.

GNAT provides an option for compiling such files purely for the purposes of checking correctness; such compilations are not required as part of the process of building a program. To compile a file in this checking mode, use the `-gnatc` switch.

## Source Dependencies

A given object file clearly depends on the source file which is compiled to produce it. Here we are using depends in the sense of a typical `make` utility; in other words, an object file depends on a source file if changes to the source file require the object file to be recompiled. In addition to this basic dependency, a given object may depend on additional source files as follows:

- If a file being compiled `with`'s a unit X, the object file depends on the file containing the spec of unit X. This includes files that are `with`'ed implicitly either because they are parents of `with`'ed child units or they are run-time units required by the language constructs used in a particular unit.
- If a file being compiled instantiates a library level generic unit, the object file depends on both the spec and body files for this generic unit.
- If a file being compiled instantiates a generic unit defined within a package, the object file depends on the body file for the package as well as the spec file.
- If a file being compiled contains a call to a subprogram for which pragma `Inline` applies and inlining is activated with the `-gnatn` switch, the object file depends on the file containing the body of this subprogram as well as on the file containing the spec. Note that for inlining to actually occur as a result of the use of this switch, it is necessary to compile in optimizing mode. The use of `-gnatN` activates a more extensive inlining optimization that is performed by the front end of the compiler. This inlining does not require that the code generation be optimized. Like `-gnatn`, the use of this switch generates additional dependencies.
- If an object file O depends on the proper body of a subunit through inlining or instantiation, it depends on the parent unit of the subunit. This means that any modification of the parent unit or one of its subunits affects the compilation of O.
- The object file for a parent unit depends on all its subunit body files.
- The previous two rules meant that for purposes of computing dependencies and recompilation, a body and all its subunits are treated as an indivisible whole. These rules are applied transitively: if unit `A` `with`'s unit `B`, whose elaboration calls an inlined procedure in package `C`, the object file for unit `A` will depend on the body of `C`, in file ``c.adb'`. The set of dependent files described by these rules includes all the files on which the unit is semantically dependent, as described in the Ada 95 Language Reference Manual. However, it is a superset of what the ARM describes, because it includes generic, inline, and subunit dependencies. An object file must be recreated by recompiling the corresponding source file if any of the source files on which it depends are modified. For example, if the `make` utility is used to control compilation, the rule for an Ada object file must mention all the source files on which the object file depends, according to the above definition. The determination of the necessary recompilations is done automatically when one uses `gnatmake`.

## The Ada Library Information Files

Each compilation actually generates two output files. The first of these is the normal object file that has a ``.o'` extension. The second is a text file containing full dependency information. It has the same name as the source file, but an ``.ali'` extension. This file is known as the Ada Library Information (``ali'`) file. The following information is contained in the ``ali'` file.

- Version information (indicates which version of GNAT was used to compile the unit(s) in question)
- Main program information (including priority and time slice settings, as well as the wide character encoding used during compilation).
- List of arguments used in the `gcc` command for the compilation
- Attributes of the unit, including configuration pragmas used, an indication of whether the compilation was successful, exception model used etc.
- A list of relevant restrictions applying to the unit (used for consistency) checking.
- Categorization information (e.g. use of pragma `Pure`).
- Information on all `with`'ed units, including presence of `Elaborate` or `Elaborate_All` pragmas.
- Information from any `Linker_Options` pragmas used in the unit
- Information on the use of `Body_Version` or `Version` attributes in the unit.
- Dependency information. This is a list of files, together with time stamp and checksum information. These are files on which the unit depends in the sense that recompilation is required if any of these units are modified.
- Cross-reference data. Contains information on all entities referenced in the unit. Used by tools like `gnatxref` and `gnatfind` to provide cross-reference information.

For a full detailed description of the format of the ``ali'` file, see the source of the body of unit `Lib.Writ`, contained in file ``lib-writ.adb'` in the GNAT compiler sources.

## Binding an Ada Program

When using languages such as C and C++, once the source files have been compiled the only remaining step in building an executable program is linking the object modules together. This means that it is possible to link an inconsistent version of a program, in which two units have included different versions of the same header.

The rules of Ada do not permit such an inconsistent program to be built. For example, if two clients have different versions of the same package, it is illegal to build a program containing these two clients. These rules are enforced by the GNAT binder, which also determines an elaboration order consistent with the Ada rules.

The GNAT binder is run after all the object files for a program have been created. It is given the name of the main program unit, and from this it determines the set of units required by the program, by reading the corresponding ALI files. It generates error messages if the program is inconsistent or if no valid order of elaboration exists.

If no errors are detected, the binder produces a main program, in Ada by default, that contains calls to the elaboration procedures of those compilation unit that require them, followed by a call to the main program. This Ada program is compiled to generate the object file for the main program. The name of the Ada file is ``b~xxx.adb'` (with the corresponding spec ``b~xxx.ads'`) where xxx is the name of the main program unit.

Finally, the linker is used to build the resulting executable program, using the object from the main program from the bind step as well as the object files for the Ada units of the program.

## Mixed Language Programming

- Interfacing to C
- Calling Conventions

### Interfacing to C

There are two ways to build a program that contains some Ada files and some other language files depending on whether the main program is in Ada or not. If the main program is in Ada, you should proceed as follows:

1. Compile the other language files to generate object files. For instance:

       gcc -c file1.c
       gcc -c file2.c

2. Compile the Ada units to produce a set of object files and ALI files. For instance:

       gnatmake -c my_main.adb

3. Run the Ada binder on the Ada main program. For instance:

       gnatbind my_main.ali

4. Link the Ada main program, the Ada objects and the other language objects. For instance:

       gnatlink my_main.ali file1.o file2.o

The three last steps can be grouped in a single command:

    gnatmake my_main.adb -largs file1.o file2.o

If the main program is in some language other than Ada, Then you may have more than one entry point in the Ada subsystem. You must use a special option of the binder to generate callable routines to initialize and finalize the Ada units (see section Binding with Non-Ada Main Programs). Calls to the initialization and finalization routines must be inserted in the main program, or some other appropriate point in the code. The call to initialize the Ada units must occur before the first Ada subprogram is called, and the call to finalize the Ada units must occur after the last Ada subprogram returns. You use the same procedure for building the program as described previously. In this case, however, the binder only places the initialization and finalization subprograms into file ``b~xxx.adb'` instead of the main program. So, if the main program is not in Ada, you should proceed as follows:

1. Compile the other language files to generate object files. For instance:

       gcc -c file1.c
       gcc -c file2.c

2. Compile the Ada units to produce a set of object files and ALI files. For instance:

       gnatmake -c entry_point1.adb
       gnatmake -c entry_point2.adb

3. Run the Ada binder on the Ada main program. For instance:

       gnatbind -n entry_point1.ali entry_point2.ali

4. Link the Ada main program, the Ada objects and the other language objects. You only need to give the last entry point here. For instance:

       gnatlink entry_point2.ali file1.o file2.o

### Calling Conventions

GNAT follows standard calling sequence conventions and will thus interface to any other language that also follows these conventions. The following Convention identifiers are recognized by GNAT:

- Ada. This indicates that the standard Ada calling sequence will be used and all Ada data items may be passed without any limitations in the case where GNAT is used to generate both the caller and callee. It is also possible to mix GNAT generated code and code generated by another Ada compiler. In this case, the data types should be restricted to simple cases, including primitive types. Whether complex data types can be passed depends on the situation. Probably it is safe to pass simple arrays, such as arrays of integers or floats. Records may or may not work, depending on whether both compilers lay them out identically. Complex structures involving variant records, access parameters, tasks, or protected types, are unlikely to be able to be passed. Note that in the case of GNAT running on a platform that supports DEC Ada 83, a higher degree of compatibility can be guaranteed, and in particular records are layed out in an identical manner in the two compilers. Note also that if output from two different compilers is mixed, the program is responsible for dealing with elaboration issues. Probably the safest approach is to write the main program in the version of Ada other than GNAT, so that it takes care of its own elaboration requirements, and then call the GNAT-generated adainit procedure to ensure elaboration of the GNAT components. Consult the documentation of the other Ada compiler for further details on elaboration. However, it is not possible to mix the tasking run time of GNAT and DEC Ada 83, All the tasking operations must either be entirely within GNAT compiled sections of the program, or entirely within DEC Ada 83 compiled sections of the program.
- Asm. Equivalent to Ada.
- Assembler. Equivalent to Ada.
- COBOL. Data will be passed according to the conventions described in section B.4 of the Ada 95 Reference Manual.
- C. Data will be passed according to the conventions described in section B.3 of the Ada 95 Reference Manual.
- CPP. This stands for C++. For most purposes this is identical to C. See the separate description of the specialized GNAT pragmas relating to C++ interfacing for further details.
- Fortran. Data will be passed according to the conventions described in section B.5 of the Ada 95 Reference Manual.
- Intrinsic. This applies to an intrinsic operation, as defined in the Ada 95 Reference Manual. If a a pragma Import (Intrinsic) applies to a subprogram, this means that the body of the subprogram is provided by the compiler itself, usually by means of an efficient code sequence, and that the user does not supply an explicit body for it. In an application program, the pragma can only be applied to the following two sets of names, which the GNAT compiler recognizes.
  - Rotate_Left, Rotate_Right, Shift_Left, Shift_Right, Shift_Right_- Arithmetic. The corresponding subprogram declaration must have two formal parameters. The first one must be a signed integer type or a modular type with a binary modulus, and the second parameter must be of type Natural. The return type must be the same as the type of the first argument. The size of this type can only be 8, 16, 32, or 64.
  - binary arithmetic operators: "+", "-", "*", "/" The corresponding operator declaration must have parameters and result type that have the same root numeric type (for example, all three are long_float types). This simplifies the definition of operations that use type checking to perform dimensional checks:

        type Distance is new Long_Float;
        type Time     is new Long_Float;
        type Velocity is new Long_Float;
        function "/" (D : Distance; T : Time)
          return Velocity;
        pragma Import (Intrinsic, "/");

    This common idiom is often programmed with a generic definition and an explicit body. The pragma makes it simpler to introduce such declarations. It incurs no overhead in compilation time or code size, because it is implemented as a single machine instruction.
- Stdcall. This is relevant only to NT/Win95 implementations of GNAT, and specifies that the Stdcall calling sequence will be used, as defined by the NT API.
- Stubbed. This is a special convention that indicates that the compiler should provide a stub body that raises `Program_Error`.

## Building Mixed Ada & C++ Programs

Building a mixed application containing both Ada and C++ code may be a challenge for the unaware programmer. As a matter of fact, this interfacing has not been standardized in the Ada 95 reference manual due to the immaturity and lack of standard of C++ at the time. This section gives a few hints that should make this task easier. In particular the first section addresses the differences with interfacing with C. The second section looks into the delicate problem of linking the complete application from its Ada and C++ parts. The last section give some hints on how the GNAT run time can be adapted in order to allow inter-language dispatching with a new C++ compiler.

- Interfacing to C++
- Linking a Mixed C++ & Ada Program
- A Simple Example
- Adapting the Run Time to a New C++ Compiler

### Interfacing to C++

GNAT supports interfacing with C++ compilers generating code that is compatible with the standard Application Binary Interface of the given platform.

Interfacing can be done at 3 levels: simple data, subprograms and classes. In the first 2 cases, GNAT offer a specific Convention CPP that behaves exactly like Convention C. Usually C++ mangle names of subprograms and currently GNAT does not provide any help to solve the demangling problem. This problem can be addressed in 2 ways:

- by modifying the C++ code in order to force a C convention using the extern "C" syntax.
- by figuring out the mangled name and use it as the Link_Name argument of the pragma import.

Interfacing at the class level can be achieved by using the GNAT specific pragmas such as `CPP_Class` and `CPP_Virtual`. See the GNAT Reference Manual for additional information.

### Linking a Mixed C++ & Ada Program

Usually the linker of the C++ development system must be used to link mixed applications because most C++ systems will resolve elaboration issues (such as calling constructors on global class instances) transparently during the link phase. GNAT has been adapted to ease the use of a foreign linker for the last phase. Three cases can be considered:

1. Using GNAT and G++ (GNU C++ compiler) from the same GCC installation. The c++ linker can simply be called by using the c++ specific driver called `c++`. Note that this setup is not very common because it may request recompiling the whole GCC tree from sources and it does not allow to upgrade easily to a new version of one compiler for one of the two languages without taking the risk of destabilizing the other.

       $ c++ -c file1.C
       $ c++ -c file2.C
       $ gnatmake ada_unit -largs file1.o file2.o --LINK=c++

2. Using GNAT and G++ from 2 different GCC installations. If both compilers are on the PATH, the same method can be used. It is important to be aware that environment variables such as C_INCLUDE_PATH, GCC_EXEC_PREFIX, BINUTILS_ROOT or GCC_ROOT will affect both compilers at the same time and thus may make one of the 2 compilers operate improperly if they are set for the other. In particular it is important that the link command has access to the proper gcc library ``libgcc.a'`, that is to say the one that is part of the C++ compiler installation. The implicit link command as suggested in the gnatmake command from the former example can be replaced by an explicit link command with full verbosity in order to verify which library is used:

       $ gnatbind ada_unit
       $ gnatlink -v -v ada_unit file1.o file2.o --LINK=c++

   If there is a problem due to interfering environment variables, it can be workaround by using an intermediate script. The following example shows the proper script to use when GNAT has not been installed at its default location and g++ has been installed at its default location:

       $ gnatlink -v -v ada_unit file1.o file2.o --LINK=./my_script
       $ cat ./my_script
       #!/bin/sh
       unset BINUTILS_ROOT
       unset GCC_ROOT
       c++ $*

3. Using a non GNU C++ compiler. The same set of command as previously described can be used to insure that the c++ linker is used. Nonetheless, the Ada code may implicitly depend on the gcc library. The latter can be located thanks to gnatls: it is to be found on the last directory of the object path. It must then be explicitly mentioned in the link command :

       $ gnatls -v
       $ Gdir=<the last directory on the object path>
       $ gnatlink ada_unit file1.o file2.o -L$Gdir -lgcc
             --LINK=<cpp_linker>

### A Simple Example

The following example, provided as part of the GNAT examples, show how to achieve procedural interfacing between Ada and C++ in both directions. The C++ class A has 2 methods. The first method is exported to Ada by the means of an extern C wrapper function. THe second method calls an Ada subprogram. On the Ada side, The C++ calss is modelized by a limited record with a layout comparable to the C++ class. The Ada subprogram, in turn, calls the c++ method. So from the C++ main program the code goes back and forth between the 2 languages.

Here are the compilation commands for native configurations:

    $ gnatmake -c simple_cpp_interface
    $ c++ -c cpp_main.C
    $ c++ -c ex7.C
    $ gnatbind -n simple_cpp_interface
    $ gnatlink simple_cpp_interface -o cpp_main --LINK=$(CPLUSPLUS)
          -lstdc++ ex7.o cpp_main.o

Here are the corresponding sources:

    //cpp_main.C

    #include "ex7.h"

    extern "C" {
      void adainit (void);
      void adafinal (void);
      void method1 (A *t);
    }

    void method1 (A *t)
    {
      t->method1 ();
    }

    int main ()
    {
      A obj;
      adainit ();
      obj.method2 (3030);
      adafinal ();
    }

    //ex7.h

    class Origin {
     public:
      int o_value;
    };
    class A : public Origin {
     public:
      void method1 (void);
      virtual void method2 (int v);
      A();
      int   a_value;
    };

    //ex7.C

    #include "ex7.h"
    #include <stdio.h>

    extern "C" { void ada_method2 (A *t, int v);}

    void A::method1 (void)
    {
      a_value = 2020;
      printf ("in A::method1, a_value = %d \n",a_value);

    }

    void A::method2 (int v)
    {
       ada_method2 (this, v);
       printf ("in A::method2, a_value = %d \n",a_value);

    }

    A::A(void)
    {
       a_value = 1010;
      printf ("in A::A, a_value = %d \n",a_value);
    }

    -- Ada sources
    package body Simple_Cpp_Interface is

       procedure Ada_Method2 (This : in out A; V : Integer) is
       begin
          Method1 (This);
          This.A_Value := V;
       end Ada_Method2;

    end Simple_Cpp_Interface;

    package Simple_Cpp_Interface is
       type A is limited
          record
             O_Value : Integer;
             A_Value : Integer;
          end record;
       pragma Convention (C, A);

       procedure Method1 (This : in out A);
       pragma Import (C, Method1);

       procedure Ada_Method2 (This : in out A; V : Integer);
       pragma Export (C, Ada_Method2);

    end Simple_Cpp_Interface;

### Adapting the Run Time to a New C++ Compiler

GNAT offers the capability to derive Ada 95 tagged types directly from preexisting C++ classes and . See "Interfacing with C++" in the GNAT reference manual. The mechanism used by GNAT for achieving such a goal has been made user configurable through a GNAT library unit `Interfaces.CPP`. The default version of this file is adapted to the GNU c++ compiler. Internal knowledge of the virtual table layout used by the new C++ compiler is needed to configure properly this unit. The Interface of this unit is known by the compiler and cannot be changed except for the value of the constants defining the characteristics of the virtual table: CPP_DT_Prologue_Size, CPP_DT_Entry_Size, CPP_TSD_Prologue_Size, CPP_TSD_Entry_Size. Read comments in the source of this unit for more details.

## Comparison between GNAT and C/C++ Compilation Models

The GNAT model of compilation is close to the C and C++ models. You can think of Ada specs as corresponding to header files in C. As in C, you don't need to compile specs; they are compiled when they are used. The Ada `with` is similar in effect to the `#include` of a C header.

One notable difference is that, in Ada, you may compile specs separately to check them for semantic and syntactic accuracy. This is not always possible with C headers because they are fragments of programs that have less specific syntactic or semantic rules.

The other major difference is the requirement for running the binder, which performs two important functions. First, it checks for consistency. In C or C++, the only defense against assembling inconsistent programs lies outside the compiler, in a makefile, for example. The binder satisfies the Ada requirement that it be impossible to construct an inconsistent program when the compiler is used in normal mode.

The other important function of the binder is to deal with elaboration issues. There are also elaboration issues in C++ that are handled automatically. This automatic handling has the advantage of being simpler to use, but the C++ programmer has no control over elaboration. Where `gnatbind` might complain there was no valid order of elaboration, a C++ compiler would simply construct a program that malfunctioned at run time.

## Comparison between GNAT and Conventional Ada Library Models

This section is intended to be useful to Ada programmers who have previously used an Ada compiler implementing the traditional Ada library model, as described in the Ada 95 Language Reference Manual. If you have not used such a system, please go on to the next section.

In GNAT, there is no library in the normal sense. Instead, the set of source files themselves acts as the library. Compiling Ada programs does not generate any centralized information, but rather an object file and a ALI file, which are of interest only to the binder and linker. In a traditional system, the compiler reads information not only from the source file being compiled, but also from the centralized library. This means that the effect of a compilation depends on what has been previously compiled. In particular:

- When a unit is `with`'ed, the unit seen by the compiler corresponds to the version of the unit most recently compiled into the library.
- Inlining is effective only if the necessary body has already been compiled into the library.
- Compiling a unit may obsolete other units in the library.

In GNAT, compiling one unit never affects the compilation of any other units because the compiler reads only source files. Only changes to source files can affect the results of a compilation. In particular:

- When a unit is `with`'ed, the unit seen by the compiler corresponds to the source version of the unit that is currently accessible to the compiler.
- Inlining requires the appropriate source files for the package or subprogram bodies to be available to the compiler. Inlining is always effective, independent of the order in which units are complied.
- Compiling a unit never affects any other compilations. The editing of sources may cause previous compilations to be out of date if they depended on the source file being modified.

The most important result of these differences is that order of compilation is never significant in GNAT. There is no situation in which one is required to do one compilation before another. What shows up as order of compilation requirements in the traditional Ada library becomes, in GNAT, simple source dependencies; in other words, there is only a set of rules saying what source files must be present when a file is compiled.

# Compiling Using `gcc`

This chapter discusses how to compile Ada programs using the `gcc` command. It also describes the set of switches that can be used to control the behavior of the compiler.

- Compiling Programs
- Switches for gcc
- Search Paths and the Run-Time Library (RTL)
- Order of Compilation Issues
- Examples

## Compiling Programs

The first step in creating an executable program is to compile the units of the program using the `gcc` command. You must compile the following files:

- the body file (``.adb'`) for a library level subprogram or generic subprogram
- the spec file (``.ads'`) for a library level package or generic package that has no body
- the body file (``.adb'`) for a library level package or generic package that has a body

You need not compile the following files

- the spec of a library unit which has a body
- subunits

because they are compiled as part of compiling related units. GNAT package specs when the corresponding body is compiled, and subunits when the parent is compiled. If you attempt to compile any of these files, you will get one of the following error messages (where fff is the name of the file you compiled):

    No code generated for file fff (package spec)
    No code generated for file fff (subunit)

The basic command for compiling a file containing an Ada unit is

    $ gcc -c [switches] `file name'

where file name is the name of the Ada file (usually having an extension ``.ads'` for a spec or ``.adb'` for a body). You specify the `-c` switch to tell `gcc` to compile, but not link, the file. The result of a successful compilation is an object file, which has the same name as the source file but an extension of ``.o'` and an Ada Library Information (ALI) file, which also has the same name as the source file, but with ``.ali'` as the extension. GNAT creates these two output files in the current directory, but you may specify a source file in any directory using an absolute or relative path specification containing the directory information.

`gcc` is actually a driver program that looks at the extensions of the file arguments and loads the appropriate compiler. For example, the GNU C compiler is ``cc1'`, and the Ada compiler is ``gnat1'`. These programs are in directories known to the driver program (in some configurations via environment variables you set), but need not be in your path. The `gcc` driver also calls the assembler and any other utilities needed to complete the generation of the required object files.

It is possible to supply several file names on the same `gcc` command. This causes `gcc` to call the appropriate compiler for each file. For example, the following command lists three separate files to be compiled:

    $ gcc -c x.adb y.adb z.c

calls `gnat1` (the Ada compiler) twice to compile ``x.adb'` and ``y.adb'`, and `cc1` (the C compiler) once to compile ``z.c'`. The compiler generates three object files ``x.o'`, ``y.o'` and ``z.o'` and the two ALI files ``x.ali'` and ``y.ali'` from the Ada compilations. Any switches apply to all the files listed, except for `-gnatx` switches, which apply only to Ada compilations.

## Switches for `gcc`

The `gcc` command accepts switches that control the compilation process. These switches are fully described in this section. First we briefly list all the switches, in alphabetical order, then we describe the switches in more detail in functionally grouped sections.

- Output and Error Message Control
- Debugging and Assertion Control
- Run-Time Checks
- Stack Overflow Checking
- Run-Time Control
- Style Checking
- Using gcc for Syntax Checking
- Using gcc for Semantic Checking
- Compiling Ada 83 Programs
- Character Set Control
- File Naming Control
- Subprogram Inlining Control
- Auxiliary Output Control
- Debugging Control

`-b target`
  Compile your program to run on target, which is the name of a system configuration. You must have a GNAT cross-compiler built if target is not the same as your host system.
`-Bdir`
  Load compiler executables (for example, `gnat1`, the Ada compiler) from dir instead of the default location. Only use this switch when multiple versions of the GNAT compiler are available. See the `gcc` manual page for further details. You would normally use the `-b` or `-V` switch instead.
`-c`
  Compile. Always use this switch when compiling Ada programs. Note: for some other languages when using `gcc`, notably in the case of C and C++, it is possible to use use `gcc` without a `-c` switch to compile and link in one step. In the case of GNAT, you cannot use this approach, because the binder must be run and `gcc` cannot be used to run the GNAT binder.
`-g`
  Generate debugging information. This information is stored in the object file and copied from there to the final executable file by the linker, where it can be read by the debugger. You must use the `-g` switch if you plan on using the debugger.
`-Idir`
  Direct GNAT to search the dir directory for source files needed by the current compilation (see section Search Paths and the Run-Time Library (RTL)).
`-I-`
  Except for the source file named in the command line, do not look for source files in the directory containing the source file named in the command line (see section Search Paths and the Run-Time Library (RTL)).
`-o file`
  This switch is used in `gcc` to redirect the generated object file and its associated ALI file. Beware of this switch with GNAT, because it may cause the object file and ALI file to have different names which in turn may confuse the binder and the linker.
`-O[n]`
  n controls the optimization level.
  n = 0
    No optimization, the default setting if no `-O` appears
  n = 1
    Normal optimization, the default if you specify `-O` without an operand.
  n = 2
    Extensive optimization
  n = 3
    Extensive optimization with automatic inlining. This applies only to inlining within a unit. For details on control of inter-unit inlining see See section Subprogram Inlining Control.
`-S`
  Used in place of `-c` to cause the assembler source file to be generated, using ``.s'` as the extension, instead of the object file. This may be useful if you need to examine the generated assembly code.
`-v`
  Show commands generated by the `gcc` driver. Normally used only for debugging purposes or if you need to be sure what version of the compiler you are executing.
`-V ver`
  Execute ver version of the compiler. This is the `gcc` version, not the GNAT version.
`-funwind-tables`
  This switch causes the object files to be generated with unwind table information. This is required for use of zero cost exception handling, of for use of the trace capabilities in the GNAT library.
`-gnata`
  Assertions enabled. `Pragma Assert` and `pragma Debug` to be activated.
`-gnatb`
  Generate brief messages to ``stderr'` even if verbose mode set.
`-gnatc`
  Check syntax and semantics only (no code generation attempted).
`-gnatC`
  Compress debug information and external symbol name table entries.
`-gnatD`
  Output expanded source files for source level debugging. This switch also suppress generation of cross-reference information (see -gnatx).
`-gnatE`
  Full dynamic elaboration checks.
`-gnatf`
  Full errors. Multiple errors per line, all undefined references.
`-gnatF`
  Externals names are folded to all uppercase.
`-gnatg`
  Internal GNAT implementation mode. This should not be used for applications programs, it is intended only for use by the compiler and its run-time library. For documentation, see the GNAT sources.
`-gnatG`
  List generated expanded code in source form.
`-gnatic`
  Identifier character set (c=1/2/3/4/8/p/f/n/w).
`-gnath`
  Output usage information. The output is written to ``stdout'`.
`-gnatkn`
  Limit file names to n (1-999) characters (`k` = krunch).
`-gnatl`
  Output full source listing with embedded error messages.
`-gnatmn`
  Limit number of detected errors to n (1-999).
`-gnatn`
  Activate inlining across unit boundaries for subprograms for which pragma `inline` is specified.
`-gnatN`
  Activate front end inlining.
`-fno-inline`
  Suppresses all inlining, even if other optimization or inlining switches are set.
`-fstack-check`
  Activates stack checking. See separate section on stack checking for details of the use of this option.
`-gnato`
  Enable numeric overflow checking (which is not normally enabled by default).
`-gnatp`
  Suppress all checks.
`-gnatq`
  Don't quit; try semantics, even if parse errors.
`-gnatQ`
  Don't quit; generate ``ali'` and tree files even if illegalities.
`-gnatP`
  Enable polling. This is required on some systems (notably Windows NT) to obtain asynchronous abort and asynchronous transfer of control capability. See the description of pragma Polling in the GNAT Reference Manual for full details.
`-gnatR0/1/2`
  Output representation information for declared types and objects.
`-gnats`
  Syntax check only.
`-gnatt`
  Tree output file to be generated.
`-gnatT nnn`
  Set time slice to specified number of microseconds
`-gnatu`
  List units for this compilation.
`-gnatU`
  Tag all error messages with the unique string "error:"
`-gnatv`
  Verbose mode. Full error output with source lines to ``stdout'`.
`-gnatwm`
  Warning mode (m=`s,e,l` for suppress, treat as error, elaboration warnings).
`-gnatWe`
  Wide character encoding method (e=n/h/u/s/e/8).
`-gnatx`
  Suppress generation of cross-reference information.
`-gnatwm`
  Warning mode
`-gnaty`
  Enable built-in style checks. See separate section describing this feature.
`-gnatzm`
  Distribution stub generation and compilation (m=r/c for receiver/caller stubs).
`-gnat83`
  Enforce Ada 83 restrictions.
`-pass-exit-codes`
  Catch exit codes from the compiler and use the most meaningful as exit status.

You may combine a sequence of GNAT switches into a single switch. For example, the combined switch

    -gnatcfi3

is equivalent to specifying the following sequence of switches:

    -gnatc -gnatf -gnati3

### Output and Error Message Control

The standard default format for error messages is called "brief format." Brief format messages are written to ``stderr'` (the standard error file) and have the following form:

    @leftskip=.7cm
    e.adb:3:04: Incorrect spelling of keyword "function"
    e.adb:4:20: ";" should be "is"

The first integer after the file name is the line number in the file, and the second integer is the column number within the line. `glide` can parse the error messages and point to the referenced character. The following switches provide control over the error message format:

`-gnatv`
  The v stands for verbose. The effect of this setting is to write long-format error messages to ``stdout'` (the standard output file. The same program compiled with the `-gnatv` switch would generate:

      3. funcion X (Q : Integer)
         |
      >>> Incorrect spelling of keyword "function"
      4. return Integer;
                       |
      >>> ";" should be "is"

  The vertical bar indicates the location of the error, and the ``>>>'` prefix can be used to search for error messages. When this switch is used the only source lines output are those with errors.
`-gnatl`
  The `l` stands for list. This switch causes a full listing of the file to be generated. The output might look as follows:

       1. procedure E is
       2.    V : Integer;
       3.    funcion X (Q : Integer)
             |
          >>> Incorrect spelling of keyword "function"
       4.     return Integer;
                            |
          >>> ";" should be "is"
       5.    begin
       6.       return Q + Q;
       7.    end;
       8. begin
       9.    V := X + X;
      10.end E;

  When you specify the `-gnatv` or `-gnatl` switches and standard output is redirected, a brief summary is written to ``stderr'` (standard error) giving the number of error messages and warning messages generated.
`-gnatU`
  This switch forces all error messages to be preceded by the unique string "error:". This means that error messages take a few more characters in space, but allows easy searching for and identification of error messages.
`-gnatb`
  The `b` stands for brief. This switch causes GNAT to generate the brief format error messages to ``stderr'` (the standard error file) as well as the verbose format message or full listing (which as usual is written to ``stdout'` (the standard output file).
`-gnatmn`
  The `m` stands for maximum. n is a decimal integer in the range of 1 to 999 and limits the number of error messages to be generated. For example, using `-gnatm2` might yield

      @leftskip=.7cm
      e.adb:3:04: Incorrect spelling of keyword "function"
      e.adb:5:35: missing ".."
      fatal error: maximum errors reached
      compilation abandoned

`-gnatf`
  The `f` stands for full. Normally, the compiler suppresses error messages that are likely to be redundant. This switch causes all error messages to be generated. In particular, in the case of references to undefined variables. If a given variable is referenced several times, the normal format of messages is

      @leftskip=.7cm
      e.adb:7:07: "V" is undefined (more references follow)

  where the parenthetical comment warns that there are additional references to the variable `V`. Compiling the same program with the `-gnatf` switch yields

      e.adb:7:07: "V" is undefined
      e.adb:8:07: "V" is undefined
      e.adb:8:12: "V" is undefined
      e.adb:8:16: "V" is undefined
      e.adb:9:07: "V" is undefined
      e.adb:9:12: "V" is undefined

`-gnatq`
  The `q` stands for quit (really "don't quit"). In normal operation mode, the compiler first parses the program and determines if there are any syntax errors. If there are, appropriate error messages are generated and compilation is immediately terminated. This switch tells GNAT to continue with semantic analysis even if syntax errors have been found. This may enable the detection of more errors in a single run. On the other hand, the semantic analyzer is more likely to encounter some internal fatal error when given a syntactically invalid tree.
`-gnatQ`
  In normal operation mode, the ``ali'` file is not generated if any illegalities are detected in the program. The use of `-gnatQ` forces generation of the ``ali'` file. This file is marked as being in error, so it cannot be used for binding purposes, but it does contain reasonably complete cross-reference information, and thus may be useful for use by tools (e.g. semantic browing tools or integrated development environments) that are driven from the ``ali'` file. In addition, if `-gnatt` is also specified, then the tree file is generated even if there are illegalities. It may be useful in this case to also specify `-gnatq` to ensure that full semantic processing occurs. The resulting tree file can be processed by ASIS, for the purpose of providing partial information about illegal units, but if the error causes the tree to be badly malformed, then ASIS may crash during the analysis. In addition to error messages, which correspond to illegalities as defined in the Ada 95 Reference Manual, the compiler detects two kinds of warning situations. First, the compiler considers some constructs suspicious and generates a warning message to alert you to a possible error. Second, if the compiler detects a situation that is sure to raise an exception at run time, it generates a warning message. The following shows an example of warning messages:

      @leftskip=.2cm
      e.adb:4:24: warning: creation of object may raise Storage_Error
      e.adb:10:17: warning: static value out of range
      e.adb:10:17: warning: "Constraint_Error" will be raised at run time

  GNAT considers a large number of situations as appropriate for the generation of warning messages. As always, warnings are not definite indications of errors. For example, if you do an out-of-range assignment with the deliberate intention of raising a `Constraint_Error` exception, then the warning that may be issued does not indicate an error. Some of the situations for which GNAT issues warnings (at least some of the time) are:
  - Possible infinitely recursive calls
  - Out-of-range values being assigned
  - Possible order of elaboration problems
  - Unreachable code
  - Variables that are never assigned a value
  - Variables that are referenced before being initialized
  - Task entries with no corresponding Accept statement
  - Duplicate Accepts for the same task entry in a select
  - Objects that take too much storage
  - Unchecked conversion between types of differing sizes
  - Missing return statements along some execution paths in a function
  - Incorrect pragmas
  - Incorrect external names
  - Allocation from empty storage pool
  - Potentially blocking operations in protected types
  - Suspicious parenthesization of expressions
  - Mismatching bounds in an aggregate
  - Attempt to return local value by reference
  - Unrecognized pragmas
  - Premature instantiation of a generic body
  - Attempt to pack aliased components
  - Out of bounds array subscripts
  - Wrong length on string assignment
  - Violations of style rules if style checking is enabled
  The following switches are available to control the handling of warning messages:
  `-gnatwa (activate all optional errors)`
    This swich activates all optional warning messages, see remaining list in this section for details on optional warning messages that can be individually controlled. The one exception is that `-gnatwa` does not activate warnings for hiding variables (`-gnatwh`), so if this warning is required, it must be explicitly set.
  `-gnatwA (suppress all optional errors)`
    This swich suppresses all optional warning messages, see remaining list in this section for details on optional warning messages that can be individually controlled.
  `-gnatwb (activate warnings on biased rounding)`
    If a static floating-point expression has a value that is exactly half way between two adjacent machine numbers, then the rules of Ada (Ada Reference Manual, section 4.9(38)) require that this rounding be done away from zero, even if the normal unbiased rounding rules at run time would require rounding towards zero. This warning message alerts you to such instances where compile-time rounding and run-time rounding are not equivalent. If it is important to get proper run-time rounding, then you can force this by making one of the operands into a variable. The default is that such warnings are not generated. Note that `-gnatwa` does not affect the setting of this warning option.
  `-gnatwB (suppress warnings on biased rounding)`
    This switch disables warnings on biased rounding.
  `-gnatwc (activate warnings on conditionals)`
    This switch activates warnings for conditional expressions used in tests that are known to be True or False at compile time. The default is that such warnings are not generated. This warning can also be turned on using `-gnatwa`.
  `-gnatwC (suppress warnings on conditionals)`
    This switch suppresses warnings for conditional expressions used in tests that are known to be True or False at compile time.
  `-gnatwe (treat warnings as errors)`
    This switch causes warning messages to be treated as errors. The warning string still appears, but the warning messages are counted as errors, and prevent the generation of an object file.
  `-gnatwh (activate warnings on hiding)`
    This switch activates warnings on hiding declarations. A declaration is considered hiding if it is for a non-overloadable entity, and it declares an entity with the same name as some other entity that is directly or use-visible. The default is that such warnings are not generated. Note that `-gnatwa` does not affect the setting of this warning option.
  `-gnatwH (suppress warnings on hiding)`
    This switch suppresses warnings on hiding declarations.
  `-gnatwi (activate warnings on implementation units).`
    This switch activates warnings for a `with` of an internal GNAT implementation unit, defined as any unit from the `Ada`, `Interfaces`, `GNAT`, or `System` hierarchies that is not documented in either the Ada Reference Manual or the GNAT Programmer's Reference Manual. Such units are intended only for internal implementation purposes and should not be `with`'ed by user programs. The default is that such warnings are generated This warning can also be turned on using `-gnatwa`.
  `-gnatwI (disable warnings on implementation units).`
    This switch disables warnings for a `with` of an internal GNAT implementation unit.
  `-gnatwl (activate warnings on elaboration pragmas)`
    This swich activates warnings on missing pragma Elaborate_All statements. See the section in this guide on elaboration checking for details on when such pragma should be used. The default is that such warnings are not generated. This warning can also be turned on using `-gnatwa`.
  `-gnatwL (suppress warnings on elaboration pragmas)`
    This swich suppresses warnings on missing pragma Elaborate_All statements. See the section in this guide on elaboration checking for details on when such pragma should be used.
  `-gnatwo (activate warnings on address clause overlays)`
    This swich activates warnings for possibly unintended initialization effects of defining address clauses that cause one variable to overlap another. The default is that such warnings are generated. This warning can also be turned on using `-gnatwa`.
  `-gnatwO (suppress warnings on address clause overlays)`
    This swich suppresses warnings on possibly unintended initialization effects of defining address clauses that cause one variable to overlap another.
  `-gnatwp (activate warnings on ineffective pragma Inlines)`
    This swich activates warnings for failure of front end inlining (activated by `-gnatN`) to inline a particular call. There are many reasons for not being able to inline a call, including most commonly that the call is too complex to inline. This warning can also be turned on using `-gnatwa`.
  `-gnatwP (suppress warnings on ineffective pragma Inlines)`
    This swich suppresses warnings on ineffective pragma Inlines. If the inlining mechanism cannot inline a call, it will simply ignore the request silently.
  `-gnatwr (activate warnings on redundant constructs)`
    This switch activates warnings for redundant constructs. In particular assignments of a variable to itself, and a type conversion that converts an object to its own type. The default is that such warnings are not generated.
  `-gnatwR (suppress warnings on redundant constructs)`
    This switch suppresses warnings for redundant constructs.
  `-gnatws (suppress all warnings)`
    This switch completely suppresses the output of all warning messages from the GNAT front end. Note that it does not suppress warnings from the `gcc` back end. To suppress these back end warnings as well, use the switch `-w` in addition to `-gnatws`.
  `-gnatwu (activate warnings on unused entities)`
    This switch activates warnings to be generated for entities that are defined but not referenced, and for units that are `with`'ed and not referenced. In the case of packages, a warning is also generated if no entities in the package are referenced. This means that if the package is referenced but the only references are in `use` clauses or `renames` declarations, a warning is still generated. A warning is also generated for a generic package that is `with`'ed but never instantiated. In the case where a package or subprogram body is compiled, and there is a `with` on the corresponding spec that is only referenced in the body, a warning is also generated, noting that the `with` can be moved to the body. The default is that such warnings are not generated. This warning can also be turned on using `-gnatwa`.
  `-gnatwU (suppress warnings on unused entities)`
    This switch suppresses warnings for unused entities and packages. A string of warning parameters can be used in the same parameter. For example:

        -gnatwaLe

    Would turn on all optional warnings except for elaboration pragma warnings, and also specify that warnings should be treated as errors.
  `-w`
    This switch suppresses warnings from the `gcc` backend. It may be used in conjunction with `-gnatws` to ensure that all warnings are suppressed during the entire compilation process.

### Debugging and Assertion Control

`-gnata`
  The pragmas `Assert` and `Debug` normally have no effect and are ignored. This switch, where ``a'` stands for assert, causes `Assert` and `Debug` pragmas to be activated. The pragmas have the form:

         pragma Assert (Boolean-expression [,
                            static-string-expression])
         pragma Debug (procedure call)

  The `Assert` pragma causes Boolean-expression to be tested. If the result is `True`, the pragma has no effect (other than possible side effects from evaluating the expression). If the result is `False`, the exception `Assert_Error` declared in the package `System.Assertions` is raised (passing static-string-expression, if present, as the message associated with the exception). If no string expression is given the default is a string giving the file name and line number of the pragma. The `Debug` pragma causes procedure to be called. Note that `pragma Debug` may appear within a declaration sequence, allowing debugging procedures to be called between declarations.

### Style Checking

The -gnatyx switch causes the compiler to enforce specified style rules. A limited set of style rules has been used in writing the GNAT sources themselves. This switch allows user programs to activate all or some of these checks. If the source program fails a specified style check, an appropriate warning message is given, preceded by the character sequence "(style)". The string x is a sequence of letters or digits indicating the particular style checks to be performed. The following checks are defined:

`1-9 (specify indentation level)`
  If a digit from 1-9 appears in the string after `-gnaty` then proper indentation is checked, with the digit indicating the indentation level required. The general style of required indentation is as specified by the examples in the Ada Reference Manual. Full line comments must be aligned with the `--` starting on a column that is a multiple of the alignment level.
`a (check attribute casing)`
  If the letter a appears in the string after `-gnaty` then attribute names, including the case of keywords such as `digits` used as attributes names, must be written in mixed case, that is, the initial letter and any letter following an underscore must be uppercase. All other letters must be lowercase.
`b (blanks not allowed at statement end)`
  If the letter b appears in the string after `-gnaty` then trailing blanks are not allowed at the end of statements. The purpose of this rule, together with h (no horizontal tabs), is to enforce a canonical format for the use of blanks to separate source tokens.
`c (check comments)`
  If the letter c appears in the string after `-gnaty` then comments must meet the following set of rules:
  - The "--" that starts the column must either start in column one, or else at least one blank must precede this sequence.
  - Comments that follow other tokens on a line must have at least one blank following the "--" at the start of the comment.
  - Full line comments must have two blanks following the "--" that starts the comment, with the following exceptions.
  - A line consisting only of the "--" characters, possibly preceded by blanks is permitted.
  - A comment starting with "--x" where x is a special character is permitted. This alows proper processing of the output generated by specialized tools including `gnatprep` (where --! is used) and the SPARK annnotation language (where --# is used). For the purposes of this rule, a special character is defined as being in one of the ASCII ranges 16#21#..16#2F# or 16#3A#..16#3F#.
  - A line consisting entirely of minus signs, possibly preceded by blanks, is permitted. This allows the construction of box comments where lines of minus signs are used to form the top and bottom of the box.
  - If a comment starts and ends with "--" is permitted as long as at least one blank follows the initial "--". Together with the preceding rule, this allows the construction of box comments, as shown in the following example:

        ---------------------------
        -- This is a box comment --
        -- with two text lines.  --
        ---------------------------

`e (check end/exit labels)`
  If the letter e appears in the string after `-gnaty` then optional labels on `end` statements ending subprograms and on `exit` statements exiting named loops, are required to be present.
`f (no form feeds or vertical tabs)`
  If the letter f appears in the string after `-gnaty` then neither form feeds nor vertical tab characters are not permitted in the source text.
`h (no horizontal tabs)`
  If the letter h appears in the string after `-gnaty` then horizontal tab characters are not permitted in the source text. Together with the b (no blanks at end of line) check, this enforces a canonical form for the use of blanks to separate source tokens.
`i (check if-then layout)`
  If the letter i appears in the string after `-gnaty`, then the keyword `then` must appear either on the same line as corresponding `if`, or on a line on its own, lined up under the `if` with at least one non-blank line in between containing all or part of the condition to be tested.
`k (check keyword casing)`
  If the letter k appears in the string after `-gnaty` then all keywords must be in lower case (with the exception of keywords such as `digits` used as attribute names to which this check does not apply).
`l (check layout)`
  If the letter l appears in the string after `-gnaty` then layout of statement and declaration constructs must follow the recommendations in the Ada Reference Manual, as indicated by the form of the syntax rules. For example an `else` keyword must be lined up with the corresponding `if` keyword. There are two respects in which the style rule enforced by this check option are more liberal than those in the Ada Reference Manual. First in the case of record declarations, it is permissible to put the `record` keyword on the same line as the `type` keyword, and then the `end` in `end record` must line up under `type`. For example, either of the following two layouts is acceptable:

      type q is record
         a : integer;
         b : integer;
      end record;

      type q is
         record
            a : integer;
            b : integer;
         end record;

  Second, in the case of a block statement, a permitted alternative is to put the block label on the same line as the `declare` or `begin` keyword, and then line the `end` keyword up under the block label. For example both the following are permitted:

      Block : declare
         A : Integer := 3;
      begin
         Proc (A, A);
      end Block;

      Block :
         declare
            A : Integer := 3;
         begin
            Proc (A, A);
         end Block;

  The same alternative format is allowed for loops. For example, both of the following are permitted:

      Clear : while J < 10 loop
         A (J) := 0;
      end loop Clear;

      Clear :
         while J < 10 loop
            A (J) := 0;
         end loop Clear;

`m (check maximum line length)`
  If the letter m appears in the string after `-gnaty` then the length of source lines must not exceed 79 characters, including any trailing blanks. The value of 79 allows convenient display on an 80 character wide device or window, allowing for possible special treatment of 80 character lines.
`Mnnn (set maximum line length)`
  If the sequence Mnnn, where nnn is a decimal number, appears in the string after `-gnaty` then the length of lines must not exceed the given value.
`n (check casing of entities in Standard)`
  If the letter n appears in the string after `-gnaty` then any identifier from Standard must be cased to match the presentation in the Ada Reference Manual (for example, `Integer` and `ASCII.NUL`).
`o (check order of subprogram bodies)`
  If the letter o appears in the string after `-gnaty` then all subprogram bodies in a given scope (e.g. a package body) must be in alphabetical order. The ordering rule uses normal Ada rules for comparing strings, ignoring casing of letters, except that if there is a trailing numeric suffix, then the value of this suffix is used in the ordering (e.g. Junk2 comes before Junk10).
`p (check pragma casing)`
  If the letter p appears in the string after `-gnaty` then pragma names must be written in mixed case, that is, the initial letter and any letter following an underscore must be uppercase. All other letters must be lowercase.
`r (check references)`
  If the letter r appears in the string after `-gnaty` then all identifier references must be cased in the same way as the corresponding declaration. No specific casing style is imposed on identifiers. The only requirement is for consistency of references with declarations.
`s (check separate specs)`
  If the letter s appears in the string after `-gnaty` then separate declarations ("specs") are required for subprograms (a body is not allowed to serve as its own declaration). The only exception is that parameterless library level procedures are not required to have a separate declaration. This exception covers the most frequent form of main program procedures.
`t (check token spacing)`
  If the letter t appears in the string after `-gnaty` then the following token spacing rules are enforced:
  - The keywords `abs` and `not` must be followed by a space.
  - The token `=>` must be surrounded by spaces.
  - The token `<>` must be preceded by a space or a left parenthesis.
  - Binary operators other than `**` must be surrounded by spaces. There is no restriction on the layout of the `**` binary operator.
  - Colon must be surrounded by spaces.
  - Colon-equal (assignment) must be surrounded by spaces.
  - Comma must be the first non-blank character on the line, or be immediately preceded by a non-blank character, and must be followed by a space.
  - Left parenthesis must be preceded by a space, and must not be followed by a space (it can be at the end of a line).
  - A right parenthesis must either be the first non-blank character on a line, or it must be preceded by a non-blank character.
  - A semicolon must not be preceded by a space, and must not be followed by a non-blank character.
  - A unary plus or minus may not be followed by a space.
  - A vertical bar must be surrounded by spaces.
  In the above rules, appearing in column one is always permitted, that is, counts as meeting either a requirement for a required preceding space, or as meeting a requirement for no preceding space. Appearing at the end of a line is also always permitted, that is, counts as meeting either a requirement for a following space, or as meeting a requirement for no following space.

If any of these style rules is violated, a message is generated giving details on the violation. The initial characters of such messages are always "(style)". Note that these messages are treated as warning messages, so they normally do not prevent the generation of an object file. The `-gnatwe` switch can be used to treat warning messages, including style messages, as fatal errors.

The switch `-gnaty` on its own (that is not followed by any letters or digits), is equivalent to `gnaty3abcefhiklmprst`, that is all checking options are enabled with the exception of -gnatyo, with an indentation level of 3. This is the standard checking option that is used for the GNAT sources.

### Run-Time Checks

If you compile with the default options, GNAT will insert many run-time checks into the compiled code, including code that performs range checking against constraints, but not arithmetic overflow checking for integer operations (including division by zero) or checks for access before elaboration on subprogram calls. All other run-time checks, as required by the Ada 95 Reference Manual, are generated by default. The following `gcc` switches refine this default behavior:

`-gnatp`
  Suppress all run-time checks as though `pragma Suppress (all_checks`) had been present in the source. Use this switch to improve the performance of the code at the expense of safety in the presence of invalid data or program bugs.
`-gnato`
  Enables overflow checking for integer operations. This causes GNAT to generate slower and larger executable programs by adding code to check for both overflow and division by zero (resulting in raising `Constraint_Error` as required by standard Ada semantics). These overflow checks correspond to situations in which the true value of the result of an operation may be outside the base range of the result type. The following example shows the distinction:

      X1 : Integer := Integer'Last;
      X2 : Integer range 1 .. 5 := 5;
      ...
      X1 := X1 + 1;   -- -gnato required to catch the Constraint_Error
      X2 := X2 + 1;   -- range check, -gnato has no effect here

  Here the first addition results in a value that is outside the base range of Integer, and hence requires an overflow check for detection of the constraint error. The second increment operation results in a violation of the explicit range constraint, and such range checks are always performed. Basically the compiler can assume that in the absence of the `-gnato` switch that any value of type `xxx` is in range of the base type of `xxx`. Note that the `-gnato` switch does not affect the code generated for any floating-point operations; it applies only to integer semantics). For floating-point, GNAT has the `Machine_Overflows` attribute set to `False` and the normal mode of operation is to generate IEEE NaN and infinite values on overflow or invalid operations (such as dividing 0.0 by 0.0). The reason that we distinguish overflow checking from other kinds of range constraint checking is that a failure of an overflow check can generate an incorrect value, but cannot cause erroneous behavior. This is unlike the situation with a constraint check on an array subscript, where failure to perform the check can result in random memory description, or the range check on a case statement, where failure to perform the check can cause a wild jump. Note again that `-gnato` is off by default, so overflow checking is not performed in default mode. This means that out of the box, with the default settings, GNAT does not do all the checks expected from the language description in the Ada Reference Manual. If you want all constraint checks to be performed, as described in this Manual, then you must explicitly use the -gnato switch either on the `gnatmake` or `gcc` command.
`-gnatE`
  Enables dynamic checks for access-before-elaboration on subprogram calls and generic instantiations. For full details of the effect and use of this switch, See section Compiling Using `gcc`.

The setting of these switches only controls the default setting of the checks. You may modify them using either `Suppress` (to remove checks) or `Unsuppress` (to add back suppressed checks) pragmas in the program source.

### Stack Overflow Checking

For most operating systems, `gcc` does not perform stack overflow checking by default. This means that if the main environment task or some other task exceeds the available stack space, then unpredictable behavior will occur.

To activate stack checking, compile all units with the gcc option `-fstack-check`. For example:

    gcc -c -fstack-check package1.adb

Units compiled with this option will generate extra instructions to check that any use of the stack (for procedure calls or for declaring local variables in declare blocks) do not exceed the available stack space. If the space is exceeded, then a `Storage_Error` exception is raised.

For declared tasks, the stack size is always controlled by the size given in an applicable `Storage_Size` pragma (or is set to the default size if no pragma is used.

For the environment task, the stack size depends on system defaults and is unknown to the compiler. The stack may even dynamically grow on some systems, precluding the normal Ada semantics for stack overflow. In the worst case, unbounded stack usage, causes unbounded stack expansion resulting in the system running out of virtual memory.

The stack checking may still work correctly if a fixed size stack is allocated, but this cannot be guaranteed. To ensure that a clean exception is signalled for stack overflow, set the environment variable `GNAT_STACK_LIMIT` to indicate the maximum stack area that can be used, as in:

    SET GNAT_STACK_LIMIT 1600

The limit is given in kilobytes, so the above declaration would set the stack limit of the environment task to 1.6 megabytes. Note that the only purpose of this usage is to limit the amount of stack used by the environment task. If it is necessary to increase the amount of stack for the environment task, then this is an operating systems issue, and must be addressed with the appropriate operating systems commands.

### Run-Time Control

`-gnatT nnn`
  The `gnatT` switch can be used to specify the time-slicing value to be used for task switching between equal priority tasks. The value `nnn` is given in microseconds as a decimal integer. Setting the time-slicing value is only effective if the underlying thread control system can accomodate time slicing. Check the documentation of your operating system for details. Note that the time-slicing value can also be set by use of pragma `Time_Slice` or by use of the `t` switch in the gnatbind step. The pragma overrides a command line argument if both are present, and the `t` switch for gnatbind overrides both the pragma and the `gcc` command line switch.

### Using `gcc` for Syntax Checking

`-gnats`
  The `s` stands for syntax. Run GNAT in syntax checking only mode. For example, the command

      $ gcc -c -gnats x.adb

  compiles file ``x.adb'` in syntax-check-only mode. You can check a series of files in a single command , and can use wild cards to specify such a group of files. Note that you must specify the `-c` (compile only) flag in addition to the `-gnats` flag. . You may use other switches in conjunction with `-gnats`. In particular, `-gnatl` and `-gnatv` are useful to control the format of any generated error messages. The output is simply the error messages, if any. No object file or ALI file is generated by a syntax-only compilation. Also, no units other than the one specified are accessed. For example, if a unit `X` `with`'s a unit `Y`, compiling unit `X` in syntax check only mode does not access the source file containing unit `Y`. Normally, GNAT allows only a single unit in a source file. However, this restriction does not apply in syntax-check-only mode, and it is possible to check a file containing multiple compilation units concatenated together. This is primarily used by the `gnatchop` utility (see section Renaming Files Using `gnatchop`).

### Using `gcc` for Semantic Checking

`-gnatc`
  The `c` stands for check. Causes the compiler to operate in semantic check mode, with full checking for all illegalities specified in the Ada 95 Reference Manual, but without generation of any object code (no object file is generated). Because dependent files must be accessed, you must follow the GNAT semantic restrictions on file structuring to operate in this mode:
  - The needed source files must be accessible (see section Search Paths and the Run-Time Library (RTL)).
  - Each file must contain only one compilation unit.
  - The file name and unit name must match (see section File Naming Rules).
  The output consists of error messages as appropriate. No object file is generated. An ``ALI'` file is generated for use in the context of cross-reference tools, but this file is marked as not being suitable for binding (since no object file is generated). The checking corresponds exactly to the notion of legality in the Ada 95 Reference Manual. Any unit can be compiled in semantics-checking-only mode, including units that would not normally be compiled (subunits, and specifications where a separate body is present).

### Compiling Ada 83 Programs

`-gnat83`
  Although GNAT is primarily an Ada 95 compiler, it accepts this switch to specify that an Ada 83 program is to be compiled in Ada83 mode. If you specify this switch, GNAT rejects most Ada 95 extensions and applies Ada 83 semantics where this can be done easily. It is not possible to guarantee this switch does a perfect job; for example, some subtle tests, such as are found in earlier ACVC tests (that have been removed from the ACVC suite for Ada 95), may not compile correctly. However, for most purposes, using this switch should help to ensure that programs that compile correctly under the `-gnat83` switch can be ported easily to an Ada 83 compiler. This is the main use of the switch. With few exceptions (most notably the need to use `<>` on unconstrained generic formal parameters, the use of the new Ada 95 keywords, and the use of packages with optional bodies), it is not necessary to use the `-gnat83` switch when compiling Ada 83 programs, because, with rare exceptions, Ada 95 is upwardly compatible with Ada 83. This means that a correct Ada 83 program is usually also a correct Ada 95 program.

### Character Set Control

`-gnatic`
  Normally GNAT recognizes the Latin-1 character set in source program identifiers, as described in the Ada 95 Reference Manual. This switch causes GNAT to recognize alternate character sets in identifiers. c is a single character indicating the character set, as follows:
  `1`
    Latin-1 identifiers
  `2`
    Latin-2 letters allowed in identifiers
  `3`
    Latin-3 letters allowed in identifiers
  `4`
    Latin-4 letters allowed in identifiers
  `p`
    IBM PC letters (code page 437) allowed in identifiers
  `8`
    IBM PC letters (code page 850) allowed in identifiers
  `f`
    Full upper-half codes allowed in identifiers
  `n`
    No upper-half codes allowed in identifiers
  `w`
    Wide-character codes allowed in identifiers
  See section Foreign Language Representation, for full details on the implementation of these character sets.
`-gnatWe`
  Specify the method of encoding for wide characters. e is one of the following:
  `h`
    Hex encoding (brackets coding also recognized)
  `u`
    Upper half encoding (brackets encoding also recognized)
  `s`
    Shift/JIS encoding (brackets encoding also recognized)
  `e`
    EUC encoding (brackets encoding also recognized)
  `8`
    UTF-8 encoding (brackets encoding also recognized)
  `b`
    Brackets encoding only (default value)
  For full details on the these encoding methods see See section Wide Character Encodings. Note that brackets coding is always accepted, even if one of the other options is specified, so for example `-gnatW8` specifies that both brackets and `UTF-8` encodings will be recognized. The units that are with'ed directly or indirectly will be scanned using the specified representation scheme, and so if one of the non-brackets scheme is used, it must be used consistently throughout the program. However, since brackets encoding is always recognized, it may be conveniently used in standard libraries, allowing these libraries to be used with any of the available coding schemes. scheme. If no `-gnatW?` parameter is present, then the default representation is Brackets encoding only. Note that the wide character representation that is specified (explicitly or by default) for the main program also acts as the default encoding used for Wide_Text_IO files if not specifically overridden by a WCEM form parameter.

### File Naming Control

`-gnatkn`
  Activates file name "krunching". n, a decimal integer in the range 1-999, indicates the maximum allowable length of a file name (not including the ``.ads'` or ``.adb'` extension). The default is not to enable file name krunching. For the source file naming rules, See section File Naming Rules.

### Subprogram Inlining Control

`-gnatn`
  The `n` here is intended to suggest the first syllable of the word "inline". GNAT recognizes and processes `Inline` pragmas. However, for the inlining to actually occur, optimization must be enabled. To enable inlining across unit boundaries, this is, inlining a call in one unit of a subprogram declared in a `with`'ed unit, you must also specify this switch. In the absence of this switch, GNAT does not attempt inlining across units and does not need to access the bodies of subprograms for which `pragma Inline` is specified if they are not in the current unit. If you specify this switch the compiler will access these bodies, creating an extra source dependency for the resulting object file, and where possible, the call will be inlined. For further details on when inlining is possible see See section Inlining of Subprograms.
`-gnatN`
  The front end inlining activated by this switch is generally more extensive, and quite often more effective than the standard `-gnatn` inlining mode. It will also generate additional dependencies.

### Auxiliary Output Control

`-gnatt`
  Cause GNAT to write the internal tree for a unit to a file (with the extension ``.adt'`. This not normally required, but is used by separate analysis tools. Typically these tools do the necessary compilations automatically, so you should never have to specify this switch in normal operation.
`-gnatu`
  Print a list of units required by this compilation on ``stdout'`. The listing includes all units on which the unit being compiled depends either directly or indirectly.
`-pass-exit-codes`
  If this switch is not used, the exit code returned by `gcc` when compiling multiple files indicates whether all source files have been successfully used to generate object files or not. When `-pass-exit-codes` is used, `gcc` exits with an extended exit status and allows an integrated development environment to better react to a compilation failure. Those exit status are:
  5
    There was an error in at least one source file.
  3
    At least one source file did not generate an object file.
  2
    The compiler died unexpectedly (internal error for example).
  0
    An object file has been generated for every source file.

### Debugging Control

`-gnatdx`
  Activate internal debugging switches. x is a letter or digit, or string of letters or digits, which specifies the type of debugging outputs desired. Normally these are used only for internal development or system debugging purposes. You can find full documentation for these switches in the body of the `Debug` unit in the compiler source file ``debug.adb'`.
`-gnatG`
  This switch causes the compiler to generate auxiliary output containing a pseudo-source listing of the generated expanded code. Like most Ada compilers, GNAT works by first transforming the high level Ada code into lower level constructs. For example, tasking operations are transformed into calls to the tasking run-time routines. A unique capability of GNAT is to list this expanded code in a form very close to normal Ada source. This is very useful in understanding the implications of various Ada usage on the efficiency of the generated code. There are many cases in Ada (e.g. the use of controlled types), where simple Ada statements can generate a lot of run-time code. By using `-gnatG` you can identify these cases, and consider whether it may be desirable to modify the coding approach to improve efficiency. The format of the output is very similar to standard Ada source, and is easily understood by an Ada programmer. The following special syntactic additions correspond to low level features used in the generated code that do not have any exact analogies in pure Ada source form. The following is a partial list of these special constructions. See the specification of package `Sprint` in file ``sprint.ads'` for a full list.
  `new xxx [storage_pool = yyy]`
    Shows the storage pool being used for an allocator.
  `at end procedure-name;`
    Shows the finalization (cleanup) procedure for a scope.
  `(if expr then expr else expr)`
    Conditional expression equivalent to the `x?y:z` construction in C.
  `target^(source)`
    A conversion with floating-point truncation instead of rounding.
  `target?(source)`
    A conversion that bypasses normal Ada semantic checking. In particular enumeration types and fixed-point types are treated simply as integers.
  `target?^(source)`
    Combines the above two cases.
  `x #/ y`
  `x #mod y`
  `x #* y`
  `x #rem y`
    A division or multiplication of fixed-point values which are treated as integers without any kind of scaling.
  `free expr [storage_pool = xxx]`
    Shows the storage pool associated with a `free` statement.
  `freeze typename [actions]`
    Shows the point at which typename is frozen, with possible associated actions to be performed at the freeze point.
  `reference itype`
    Reference (and hence definition) to internal type itype.
  `function-name! (arg, arg, arg)`
    Intrinsic function call.
  `labelname : label`
    Declaration of label labelname.
  `expr && expr && expr ... && expr`
    A multiple concatenation (same effect as expr & expr & expr, but handled more efficiently).
  `[constraint_error]`
    Raise the `Constraint_Error` exception.
  `expression'reference`
    A pointer to the result of evaluating expression.
  `target-type!(source-expression)`
    An unchecked conversion of source-expression to target-type.
  `[numerator/denominator]`
    Used to represent internal real literals (that) have no exact representation in base 2-16 (for example, the result of compile time evaluation of the expression 1.0/27.0).
  `-gnatD`
    This switch is used in conjunction with `-gnatG` to cause the expanded source, as described above to be written to files with names ``xxx.dg'`, where ``xxx'` is the normal file name, for example, if the source file name is ``hello.adb'`, then a file ``hello.adb.dg'` will be written. The debugging information generated by the `gcc` `-g` switch will refer to the generated ``xxx.dg'` file. This allows you to do source level debugging using the generated code which is sometimes useful for complex code, for example to find out exactly which part of a complex construction raised an exception. This switch also suppress generation of cross-reference information (see -gnatx).
  `-gnatC`
    In the generated debugging information, and also in the case of long external names, the compiler uses a compression mechanism if the name is very long. This compression method uses a checksum, and avoids trouble on some operating systems which have difficulty with very long names. The `-gnatC` switch forces this compression approach to be used on all external names and names in the debugging information tables. This reduces the size of the generated executable, at the expense of making the naming scheme more complex. The compression only affects the qualification of the name. Thus a name in the source:

        Very_Long_Package.Very_Long_Inner_Package.Var

    would normally appear in these tables as:

        very_long_package__very_long_inner_package__var

    but if the `-gnatC` switch is used, then the name appears as

        XCb7e0c705__var

    Here b7e0c705 is a compressed encoding of the qualification prefix. The GNAT Ada aware version of GDB understands these encoded prefixes, so if this debugger is used, the encoding is largely hidden from the user of the compiler.
`-gnatRx`
  This switch controls output from the compiler of a listing showing representation information for declared types and objects. For `-gnatR0`, no information is output (equivalent to omitting the `-gnatR` switch). For `-gnatR1` (which is the default, so `-gnatR` with no parameter has the same effect), size and alignment information is listed for declared array and record types. For `-gnatR2`, size and alignment information is listed for all declared types and objects.
`-gnatx`
  Normally the compiler generates full cross-referencing information in the ``ALI'` file. This information is used by a number of tools, including `gnatfind` and `gnatxref`. The -gnatx switch suppresses this information. This saves some space and may slightly speed up compilation, but means that these tools cannot be used.

## Search Paths and the Run-Time Library (RTL)

With the GNAT source-based library system, the compiler must be able to find source files for units that are needed by the unit being compiled. Search paths are used to guide this process.

The compiler compiles one source file whose name must be given explicitly on the command line. In other words, no searching is done for this file. To find all other source files that are needed (the most common being the specs of units), the compiler examines the following directories, in the following order:

1. The directory containing the source file of the main unit being compiled (the file name on the command line).
2. Each directory named by an `-I` switch given on the `gcc` command line, in the order given.
3. Each of the directories listed in the value of the `ADA_INCLUDE_PATH` environment variable. Construct this value exactly as the `PATH` environment variable: a list of directory names separated by colons.
4. The content of the "ada_source_path" file which is part of the GNAT installation tree and is used to store standard libraries such as the GNAT Run Time Library (RTL) source files. section Installing an Ada Library

Specifying the switch `-I-` inhibits the use of the directory containing the source file named in the command line. You can still have this directory on your search path, but in this case it must be explicitly requested with a `-I` switch.

Specifying the switch `-nostdinc` inhibits the search of the default location for the GNAT Run Time Library (RTL) source files.

The compiler outputs its object files and ALI files in the current working directory. Caution: The object file can be redirected with the `-o` switch; however, `gcc` and `gnat1` have not been coordinated on this so the ALI file will not go to the right place. Therefore, you should avoid using the `-o` switch.

The packages `Ada`, `System`, and `Interfaces` and their children make up the GNAT RTL, together with the simple `System.IO` package used in the "Hello World" example. The sources for these units are needed by the compiler and are kept together in one directory. Not all of the bodies are needed, but all of the sources are kept together anyway. In a normal installation, you need not specify these directory names when compiling or binding. Either the environment variables or the built-in defaults cause these files to be found.

In addition to the language-defined hierarchies (System, Ada and Interfaces), the GNAT distribution provides a fourth hierarchy, consisting of child units of GNAT. This is a collection of generally useful routines. See the GNAT Reference Manual for further details.

Besides simplifying access to the RTL, a major use of search paths is in compiling sources from multiple directories. This can make development environments much more flexible.

## Order of Compilation Issues

If, in our earlier example, there was a spec for the `hello` procedure, it would be contained in the file ``hello.ads'`; yet this file would not have to be explicitly compiled. This is the result of the model we chose to implement library management. Some of the consequences of this model are as follows:

- There is no point in compiling specs (except for package specs with no bodies) because these are compiled as needed by clients. If you attempt a useless compilation, you will receive an error message. It is also useless to compile subunits because they are compiled as needed by the parent.
- There are no order of compilation requirements: performing a compilation never obsoletes anything. The only way you can obsolete something and require recompilations is to modify one of the source files on which it depends.
- There is no library as such, apart from the ALI files (see section The Ada Library Information Files, for information on the format of these files). For now we find it convenient to create separate ALI files, but eventually the information therein may be incorporated into the object file directly.
- When you compile a unit, the source files for the specs of all units that it `with`'s, all its subunits, and the bodies of any generics it instantiates must be available (reachable by the search-paths mechanism described above), or you will receive a fatal error message.

## Examples

The following are some typical Ada compilation command line examples:

`$ gcc -c xyz.adb`
  Compile body in file ``xyz.adb'` with all default options.
`$ gcc -c -O2 -gnata xyz-def.adb`
  Compile the child unit package in file ``xyz-def.adb'` with extensive optimizations, and pragma `Assert`/`Debug` statements enabled.
`$ gcc -c -gnatc abc-def.adb`
  Compile the subunit in file ``abc-def.adb'` in semantic-checking-only mode.

# Binding Using `gnatbind`

- Running gnatbind
- Generating the Binder Program in C
- Consistency-Checking Modes
- Binder Error Message Control
- Elaboration Control
- Output Control
- Binding with Non-Ada Main Programs
- Binding Programs with No Main Subprogram
- Summary of Binder Switches
- Command-Line Access
- Search Paths for gnatbind
- Examples of gnatbind Usage

This chapter describes the GNAT binder, `gnatbind`, which is used to bind compiled GNAT objects. The `gnatbind` program performs four separate functions:

1. Checks that a program is consistent, in accordance with the rules in Chapter 10 of the Ada 95 Reference Manual. In particular, error messages are generated if a program uses inconsistent versions of a given unit.
2. Checks that an acceptable order of elaboration exists for the program and issues an error message if it cannot find an order of elaboration that satisfies the rules in Chapter 10 of the Ada 95 Language Manual.
3. Generates a main program incorporating the given elaboration order. This program is a small Ada package (body and spec) that must be subsequently compiled using the GNAT compiler. The necessary compilation step is usually performed automatically by `gnatlink`. The two most important functions of this program are to call the elaboration routines of units in an appropriate order and to call the main program.
4. Determines the set of object files required by the given main program. This information is output in the forms of comments in the generated C program, to be read by the `gnatlink` utility used to link the Ada application.

## Running `gnatbind`

The form of the `gnatbind` command is

    $ gnatbind [switches] mainprog[.ali] [switches]

where mainprog.adb is the Ada file containing the main program unit body. If no switches are specified, `gnatbind` constructs an Ada package in two files whose names are ``b~ada_main.ads'`, and ``b~ada_main.adb'`. For example, if given the parameter ``hello.ali'`, for a main program contained in file ``hello.adb'`, the binder output files would be ``b~hello.ads'` and ``b~hello.adb'`.

When doing consistency checking, the binder takes any source files it can locate into consideration. For example, if the binder determines that the given main program requires the package `Pack`, whose ``.ali'` file is ``pack.ali'` and whose corresponding source spec file is ``pack.ads'`, it attempts to locate the source file ``pack.ads'` (using the same search path conventions as previously described for the `gcc` command). If it can locate this source file, it checks that the time stamps or source checksums of the source and its references to in ``ali'` files match. In other words, any ``ali'` files that mentions this spec must have resulted from compiling this version of the source file (or in the case where the source checksums match, a version close enough that the difference does not matter).

The effect of this consistency checking, which includes source files, is that the binder ensures that the program is consistent with the latest version of the source files that can be located at bind time. Editing a source file without compiling files that depend on the source file cause error messages to be generated by the binder.

For example, suppose you have a main program ``hello.adb'` and a package `P`, from file ``p.ads'` and you perform the following steps:

1. Enter `gcc -c hello.adb` to compile the main program.
2. Enter `gcc -c p.ads` to compile package `P`.
3. Edit file ``p.ads'`.
4. Enter `gnatbind hello`.

At this point, the file ``p.ali'` contains an out-of-date time stamp because the file ``p.ads'` has been edited. The attempt at binding fails, and the binder generates the following error messages:

    error: "hello.adb" must be recompiled ("p.ads" has been modified)
    error: "p.ads" has been modified and must be recompiled

Now both files must be recompiled as indicated, and then the bind can succeed, generating a main program. You need not normally be concerned with the contents of this file, but it is similar to the following which is the binder file generated for a simple "hello world" program.

    @leftskip=0cm
    --  The package is called Ada_Main unless this name is actually used
    --  as a unit name in the partition, in which case some other unique
    --  name is used.

    with System;
    package Ada_Main is

       --  The main program saves the parameters (argument count,
       --  argument values, environment pointer) in global variables
       --  for later access by other units including
       --  Ada.Command_Line.

       gnat_argc : Integer;
       gnat_argv : System.Address;
       gnat_envp : System.Address;

       --  The actual variables are stored in a library routine. This
       --  is useful for some shared library situations, where there
       --  are problems if variables are not in the library.

       pragma Import (C, gnat_argc);
       pragma Import (C, gnat_argv);
       pragma Import (C, gnat_envp);

       --  The exit status is similarly an external location

       gnat_exit_status : Integer;
       pragma Import (C, gnat_exit_status);

       --  This is the generated adafinal routine that performs
       --  finalization at the end of execution. In the case where
       --  Ada is the main program, this main program makes a call
       --  to adafinal at program termination.

       procedure adafinal;
       pragma Export (C, adafinal);

       --  This is the generated adainit routine that performs
       --  initialization at the start of execution. In the case
       --  where Ada is the main program, this main program makes
       --  a call to adainit at program startup.

       procedure adainit;
       pragma Export (C, adainit);

       --  This routine is called at the start of execution. It is
       --  a dummy routine that is used by the debugger to breakpoint
       --  at the start of execution.

       procedure Break_Start;
       pragma Import (C, Break_Start, "__gnat_break_start");

       --  This is the actual generated main program (it would be
       --  suppressed if the no main program swich were used). As
       --  required by standard system conventions, this program has
       --  the external name main.

       function main
         (argc : Integer;
          argv : System.Address;
          envp : System.Address)
          return Integer;
       pragma Export (C, main, "main");

       --  The following set of constants give the version
       --  identification values for every unit in the bound
       --  partition. This identification is computed from all
       --  dependent semantic units, and corresponds to the
       --  string that would be returned by use of the
       --  Body_Version or Version attributes.

       u00001 : constant Integer := 16#425FD0AF#;
       u00002 : constant Integer := 16#077A2651#;
       u00003 : constant Integer := 16#08ADDC9E#;
       u00004 : constant Integer := 16#1D370323#;
       u00005 : constant Integer := 16#3043D77B#;
       u00006 : constant Integer := 16#2359F9ED#;
       u00007 : constant Integer := 16#0CA940CF#;
       u00008 : constant Integer := 16#69BA6A59#;
       u00009 : constant Integer := 16#156A40CF#;
       u00010 : constant Integer := 16#033DABE0#;
       u00011 : constant Integer := 16#6AB38FEA#;
       u00012 : constant Integer := 16#7AAA368C#;
       u00013 : constant Integer := 16#7D13B305#;
       u00014 : constant Integer := 16#62D2B79D#;
       u00015 : constant Integer := 16#2E865F1E#;
       u00016 : constant Integer := 16#6379D875#;
       u00017 : constant Integer := 16#72D6A51D#;
       u00018 : constant Integer := 16#6E88E3D7#;
       u00019 : constant Integer := 16#45C8383C#;
       u00020 : constant Integer := 16#385E7AC2#;
       u00021 : constant Integer := 16#08FE4C1F#;
       u00022 : constant Integer := 16#23B87757#;
       u00023 : constant Integer := 16#3A4BFD9A#;
       u00024 : constant Integer := 16#4C9F3930#;
       u00025 : constant Integer := 16#2F1EB794#;
       u00026 : constant Integer := 16#0E2A461A#;
       u00027 : constant Integer := 16#5570D114#;
       u00028 : constant Integer := 16#501FA6BF#;
       u00029 : constant Integer := 16#57692181#;
       u00030 : constant Integer := 16#7C25DE96#;
       u00031 : constant Integer := 16#521B9399#;
       u00032 : constant Integer := 16#689CC1B9#;
       u00033 : constant Integer := 16#0357E00A#;
       u00034 : constant Integer := 16#1345CFE9#;
       u00035 : constant Integer := 16#343244DE#;
       u00036 : constant Integer := 16#6725DC79#;
       u00037 : constant Integer := 16#2DAF477E#;
       u00038 : constant Integer := 16#4F0184F2#;
       u00039 : constant Integer := 16#0A0669D8#;
       u00040 : constant Integer := 16#26610831#;
       u00041 : constant Integer := 16#0B5A4DF9#;
       u00042 : constant Integer := 16#1D4F93E8#;
       u00043 : constant Integer := 16#30B2EC3D#;
       u00044 : constant Integer := 16#34054F96#;
       u00045 : constant Integer := 16#6598BA3E#;
       u00046 : constant Integer := 16#2C9C021D#;
       u00047 : constant Integer := 16#177A51F6#;
       u00048 : constant Integer := 16#1CBC39CD#;
       u00049 : constant Integer := 16#5461BB3E#;
       u00050 : constant Integer := 16#03F36D98#;
       u00051 : constant Integer := 16#208D3EF6#;
       u00052 : constant Integer := 16#33AF4230#;
       u00053 : constant Integer := 16#0B97C6BF#;
       u00054 : constant Integer := 16#34B32999#;

       --  The following Export pragms export the version numbers
       --  with symbolic --  names ending in B (for body) or S
       --  (for spec) so that they can be located in a link. The
       --  information provided here is sufficient to track down
       --  the exact versions of units used in a given build.

       pragma Export (C, u00001, "helloB");
       pragma Export (C, u00002, "system__standard_libraryB");
       pragma Export (C, u00003, "system__standard_libraryS");
       pragma Export (C, u00004, "systemS");
       pragma Export (C, u00005, "system__exceptionsS");
       pragma Export (C, u00006, "adaS");
       pragma Export (C, u00007, "ada__exceptionsB");
       pragma Export (C, u00008, "ada__exceptionsS");
       pragma Export (C, u00009, "gnatS");
       pragma Export (C, u00010, "gnat__heap_sort_aB");
       pragma Export (C, u00011, "gnat__heap_sort_aS");
       pragma Export (C, u00012, "system__exception_tableB");
       pragma Export (C, u00013, "system__exception_tableS");
       pragma Export (C, u00014, "gnat__htableB");
       pragma Export (C, u00015, "gnat__htableS");
       pragma Export (C, u00016, "system__machine_codeS");
       pragma Export (C, u00017, "system__secondary_stackB");
       pragma Export (C, u00018, "system__secondary_stackS");
       pragma Export (C, u00019, "system__parametersB");
       pragma Export (C, u00020, "system__parametersS");
       pragma Export (C, u00021, "system__soft_linksB");
       pragma Export (C, u00022, "system__soft_linksS");
       pragma Export (C, u00023, "system__stack_checkingB");
       pragma Export (C, u00024, "system__stack_checkingS");
       pragma Export (C, u00025, "system__storage_elementsB");
       pragma Export (C, u00026, "system__storage_elementsS");
       pragma Export (C, u00027, "text_ioS");
       pragma Export (C, u00028, "ada__text_ioB");
       pragma Export (C, u00029, "ada__text_ioS");
       pragma Export (C, u00030, "ada__streamsS");
       pragma Export (C, u00031, "ada__tagsB");
       pragma Export (C, u00032, "ada__tagsS");
       pragma Export (C, u00033, "interfacesS");
       pragma Export (C, u00034, "interfaces__c_streamsB");
       pragma Export (C, u00035, "interfaces__c_streamsS");
       pragma Export (C, u00036, "system__file_ioB");
       pragma Export (C, u00037, "system__file_ioS");
       pragma Export (C, u00038, "ada__finalizationB");
       pragma Export (C, u00039, "ada__finalizationS");
       pragma Export (C, u00040, "system__finalization_rootB");
       pragma Export (C, u00041, "system__finalization_rootS");
       pragma Export (C, u00042, "system__stream_attributesB");
       pragma Export (C, u00043, "system__stream_attributesS");
       pragma Export (C, u00044, "ada__io_exceptionsS");
       pragma Export (C, u00045, "system__unsigned_typesS");
       pragma Export (C, u00046, "system__finalization_implementationB");
       pragma Export (C, u00047, "system__finalization_implementationS");
       pragma Export (C, u00048, "system__string_ops_concat_3B");
       pragma Export (C, u00049, "system__string_ops_concat_3S");
       pragma Export (C, u00050, "system__string_opsB");
       pragma Export (C, u00051, "system__string_opsS");
       pragma Export (C, u00052, "system__file_control_blockS");
       pragma Export (C, u00053, "ada__finalization__list_controllerB");
       pragma Export (C, u00054, "ada__finalization__list_controllerS");

    end Ada_Main;

    --  The following source file name pragmas allow the generated file
    --  names to be unique for different main programs. They are needed
    --  since the package name will always be Ada_Main.

    pragma Source_File_Name (Ada_Main, Spec_File_Name => "b~hello.ads");
    pragma Source_File_Name (Ada_Main, Body_File_Name => "b~hello.adb");

    package body Ada_Main is

    --  Generated package body for Ada_Main starts here

       -------------
       -- adainit --
       -------------

       procedure adainit is

          --  Set_Globals is a library routine that stores away the
          --  value of the indicated set of global values in global
          --  variables within the library.

          procedure Set_Globals
            (Main_Priority            : Integer;
             Time_Slice_Value         : Integer;
             WC_Encoding              : Character;
             Locking_Policy           : Character;
             Queuing_Policy           : Character;
             Task_Dispatching_Policy  : Character;
             Adafinal                 : System.Address;
             Unreserve_All_Interrupts : Boolean;
             Exception_Tracebacks     : Boolean);
          pragma Import (C, Set_Globals, "__gnat_set_globals");

          --  SDP_Table_Build is a library routine used to build the
          --  exception tables. See unit Ada.Exceptions in files
          --  a-except.ads/adb for full details of how zero cost
          --  exception handling works. This procedure, the call to
          --  it, and the two following tables are all omitted if the
          --  build is in longjmp/setjump exception mode.

          procedure SDP_Table_Build
            (SDP_Addresses   : System.Address;
             SDP_Count       : Natural;
             Elab_Addresses  : System.Address;
             Elab_Addr_Count : Natural);
          pragma Import (C, SDP_Table_Build, "__gnat_SDP_Table_Build");

          --  Table of Unit_Exception_Table addresses. Used for zero
          --  cost exception handling to build the top level table.

          ST : aliased constant array (1 .. 21) of System.Address := (
            Hello'UET_Address,
            Ada.Exceptions'UET_Address,
            Gnat.Heap_Sort_A'UET_Address,
            System.Exception_Table'UET_Address,
            System.Secondary_Stack'UET_Address,
            System.Parameters'UET_Address,
            System.Soft_Links'UET_Address,
            System.Stack_Checking'UET_Address,
            Ada.Text_Io'UET_Address,
            Ada.Streams'UET_Address,
            Ada.Tags'UET_Address,
            Interfaces.C_Streams'UET_Address,
            System.File_Io'UET_Address,
            Ada.Finalization'UET_Address,
            System.Finalization_Root'UET_Address,
            System.Stream_Attributes'UET_Address,
            System.Finalization_Implementation'UET_Address,
            System.String_Ops_Concat_3'UET_Address,
            System.String_Ops'UET_Address,
            System.File_Control_Block'UET_Address,
            Ada.Finalization.List_Controller'UET_Address);

          --  Table of addresses of elaboration routines. Used for
          --  zero cost exception handling to make sure these
          --  addresses are included in the top level procedure
          --  address table.

          EA : aliased constant array (1 .. 22) of System.Address := (
            adainit'Code_Address,
            adafinal'Code_Address,
            Ada.Exceptions'Elab_Spec'Address,
            System.Exceptions'Elab_Spec'Address,
            Interfaces.C_Streams'Elab_Spec'Address,
            System.Exception_Table'Elab_Body'Address,
            Ada.Io_Exceptions'Elab_Spec'Address,
            Ada.Tags'Elab_Spec'Address,
            Ada.Tags'Elab_Body'Address,
            Ada.Streams'Elab_Spec'Address,
            System.Stack_Checking'Elab_Spec'Address,
            System.Soft_Links'Elab_Body'Address,
            System.Secondary_Stack'Elab_Body'Address,
            Ada.Exceptions'Elab_Body'Address,
            System.Finalization_Root'Elab_Spec'Address,
            System.Finalization_Implementation'Elab_Spec'Address,
            Ada.Finalization'Elab_Spec'Address,
            Ada.Finalization.List_Controller'Elab_Spec'Address,
            System.File_Control_Block'Elab_Spec'Address,
            System.File_Io'Elab_Body'Address,
            Ada.Text_Io'Elab_Spec'Address,
            Ada.Text_Io'Elab_Body'Address);

       --  Start of processing for adainit

       begin
          --  Call SDP_Table_Build to build the top level procedure
          --  table for zero cost exception handling (omitted in
          --  longjmp/setjump mode).

          SDP_Table_Build (ST'Address, 21, EA'Address, 22);

          --  Call Set_Globals to record various information for
          --  this partition.  The values are derived by the binder
          --  from information stored in the ali files by the compiler.

          Set_Globals
            (Main_Priority            => -1,
             --  Priority of main program, -1 if no pragma Priority used

             Time_Slice_Value         => -1,
             --  Time slice from Time_Slice pragma, -1 if none used

             WC_Encoding              => 'b',
             --  Wide_Character encoding used, default is brackets

             Locking_Policy           => ' ',
             --  Locking_Policy used, default of space means not
             --  specified, otherwise it is the first character of
             --  the policy name.

             Queuing_Policy           => ' ',
             --  Queuing_Policy used, default of space means not
             --  specified, otherwise it is the first character of
             --  the policy name.

             Task_Dispatching_Policy  => ' ',
             --  Task_Dispatching_Policy used, default of space means
             --  not specified, otherwise first character of the
             --  policy name.

             Adafinal                 => adafinal'Address,
             --  Address of generated ada final routine

             Unreserve_All_Interrupts => False,
             --  Set true if pragma Unreserve_All_Interrupts was used

             Exception_Tracebacks     => False);
             --  Indicates if exception tracebacks are enabled

          --  Now we have the elaboration calls for all units in the
          --  partition. The all is commented out if the given unit
          --  has no elaboration code. We retain the commented out call
          --  to indicate the full order chosen. The Elab_Spec and
          --  Elab_Body attributes generate references to the implicit
          --  elaboration procedures generated by the compiler for each
          --  unit that requires elaboration.

          -- System'Elab_Spec;
          -- Ada'Elab_Spec;
          -- Gnat'Elab_Spec;
          -- Gnat.Heap_Sort_A'Elab_Spec;
          -- Gnat.Htable'Elab_Spec;
          -- Gnat.Htable'Elab_Body;
          -- Interfaces'Elab_Spec;
          -- System.Machine_Code'Elab_Spec;
          -- System.Parameters'Elab_Spec;
          -- System.Standard_Library'Elab_Spec;
             Ada.Exceptions'Elab_Spec;
             System.Exceptions'Elab_Spec;
          -- System.Parameters'Elab_Body;
          -- Gnat.Heap_Sort_A'Elab_Body;
             Interfaces.C_Streams'Elab_Spec;
          -- Interfaces.C_Streams'Elab_Body;
          -- System.Exception_Table'Elab_Spec;
             System.Exception_Table'Elab_Body;
             Ada.Io_Exceptions'Elab_Spec;
          -- System.Storage_Elements'Elab_Spec;
          -- System.Storage_Elements'Elab_Body;
          -- System.Secondary_Stack'Elab_Spec;
             Ada.Tags'Elab_Spec;
             Ada.Tags'Elab_Body;
             Ada.Streams'Elab_Spec;
             System.Stack_Checking'Elab_Spec;
          -- System.Soft_Links'Elab_Spec;
             System.Soft_Links'Elab_Body;
          -- System.Stack_Checking'Elab_Body;
             System.Secondary_Stack'Elab_Body;
             Ada.Exceptions'Elab_Body;
          -- System.Standard_Library'Elab_Body;
          -- System.String_Ops'Elab_Spec;
          -- System.String_Ops'Elab_Body;
          -- System.String_Ops_Concat_3'Elab_Spec;
          -- System.String_Ops_Concat_3'Elab_Body;
          -- System.Unsigned_Types'Elab_Spec;
          -- System.Stream_Attributes'Elab_Spec;
          -- System.Stream_Attributes'Elab_Body;
             System.Finalization_Root'Elab_Spec;
          -- System.Finalization_Root'Elab_Body;
             System.Finalization_Implementation'Elab_Spec;
          -- System.Finalization_Implementation'Elab_Body;
             Ada.Finalization'Elab_Spec;
          -- Ada.Finalization'Elab_Body;
             Ada.Finalization.List_Controller'Elab_Spec;
          -- Ada.Finalization.List_Controller'Elab_Body;
             System.File_Control_Block'Elab_Spec;
          -- System.File_Io'Elab_Spec;
             System.File_Io'Elab_Body;
             Ada.Text_Io'Elab_Spec;
             Ada.Text_Io'Elab_Body;
          -- Text_Io'Elab_Spec;
          -- hello'elab_body;
          null;
       end adainit;

       --------------
       -- adafinal --
       --------------

       procedure adafinal is

          --  The actual finalization is performed by calling the
          --  library routine in System.Finalization_Implementation.

          procedure do_finalize;
          pragma Import
            (C, do_finalize,
             "system__finalization_implementation__finalize_global_list");

       begin
          do_finalize;
       end adafinal;

       ----------
       -- main --
       ----------

       --  main is actually a function, as in the ANSI C standard,
       --  defined to return the exit status. The three parameters
       --  are the argument count, argument values and environment
       --  pointer.

       function main
         (argc : Integer;
          argv : System.Address;
          envp : System.Address)
          return Integer
       is
          --  The initialize routine performs low level system
          --  initialization using a standard library routine which
          --  sets up signal handling and performs any other
          --  required setup. The routine can be found in file
          --  a-init.c.

          procedure initialize;
          pragma Import (C, initialize, "__gnat_initialize");

          --  The finalize routine performs low level system
          --  finalization using a standard library routine. The
          --  routine is found in file a-final.c and in the standard
          --  distribution is a dummy routine that does nothing, so
          --  really this is a hook for special user finalization.

          procedure finalize;
          pragma Import (C, finalize, "__gnat_finalize");

          --  We get to the main program of the partition by using
          --  pragma Import because if we try to with the unit and
          --  call it Ada style, then not only do we waste time
          --  recompiling it, but also, we don't really know the right
          --  switches (e.g. identifier character set) to be used
          --  to compile it.

          procedure Ada_Main_Program;
          pragma Import (Ada, Ada_Main_Program, "_ada_hello");

       --  Start of processing for main

       begin
          --  Save global variables

          gnat_argc := argc;
          gnat_argv := argv;
          gnat_envp := envp;

          --  Call low level system initialization

          Initialize;

          --  Call our generated Ada initialization routine

          adainit;

          --  This is the point at which we want the debugger to get
          --  control

          Break_Start;

          --  Now we call the main program of the partition

          Ada_Main_Program;

          --  Perform Ada finalization

          adafinal;

          --  Perform low level system finalization

          Finalize;

          --  Return the proper exit status
          return (gnat_exit_status);
       end;

    --  This section is entirely comments, so it has no effect on the
    --  compilation of the Ada_Main package. It provides the list of
    --  object files and linker options, as well as some standard
    --  libraries needed for the link. The gnatlink utility parses
    --  this b~hello.adb file to read these comment lines to generate
    --  the appropriate command line arguments for the call to the
    --  system linker. The BEGIN/END lines are used for sentinels for
    --  this parsing operation.

    --  The exact file names will of course depend on the environment,
    --  host/target and location of files on the host system.

    -- BEGIN Object file/option list
       --   ./system.o
       --   ./ada.o
       --   ./gnat.o
       --   ./g-htable.o
       --   ./interfac.o
       --   ./s-maccod.o
       --   ./s-except.o
       --   ./s-parame.o
       --   ./g-hesora.o
       --   ./i-cstrea.o
       --   ./s-exctab.o
       --   ./a-ioexce.o
       --   ./s-stoele.o
       --   ./a-tags.o
       --   ./a-stream.o
       --   ./s-soflin.o
       --   ./s-stache.o
       --   ./s-secsta.o
       --   ./a-except.o
       --   ./s-stalib.o
       --   ./s-strops.o
       --   ./s-sopco3.o
       --   ./s-unstyp.o
       --   ./s-stratt.o
       --   ./s-finroo.o
       --   ./s-finimp.o
       --   ./a-finali.o
       --   ./a-filico.o
       --   ./s-ficobl.o
       --   ./s-fileio.o
       --   ./a-textio.o
       --   ./text_io.o
       --   ./hello.o
       --   -L./
    ...  Target/System specific library search directives
       --   -lgnat
    -- END Object file/option list

    end Ada_Main;

The Ada code in the above example is exactly what is generated by the binder. We have added comments to more clearly indicate the function of each part of the generated `Ada_Main` package.

The code is standard Ada in all respects, and can be processed by any tools that handle Ada. In particular, it is possible to use the debugger in Ada mode to debug the generated Ada_Main package. For example, suppose that for reasons that you do not understand, your program is blowing up during elaboration of the body of `Ada.Text_IO`. To chase this bug down, you can place a breakpoint on the call:

    Ada.Text_Io'Elab_Body;

and trace the elaboration routine for this package to find out where the problem might be (more usually of course you would be debugging elaboration code in your own application).

## Generating the Binder Program in C

In most normal usage, the default mode of `gnatbind` which is to generate the main package in Ada, as described in the previous section. In particular, this means that any Ada programmer can read and understand the generated main program. It can also be debugged just like any other Ada code provided the `-g` switch is used for `gnatbind` and `gnatlink`.

However for some purposes it may be convenient to generate the main program in C rather than Ada. This may for example be helpful when you are generating a mixed language program with the main program in C. The GNAT compiler itself is an example. The use of the `-C` switch for both `gnatbind` and `gnatlink` will cause the program to be generated in C (and compiled using the gnu C compiler). The following shows the C code generated for the same "Hello World" program:

    extern int gnat_argc;
    extern char **gnat_argv;
    extern char **gnat_envp;
    extern int gnat_exit_status;
    void adafinal ();

    void adainit ()
    {
       extern void *__gnat_hello__SDP;
       extern void *__gnat_ada__exceptions__SDP;
       extern void *__gnat_gnat__heap_sort_a__SDP;
       extern void *__gnat_system__exception_table__SDP;
       extern void *__gnat_system__secondary_stack__SDP;
       extern void *__gnat_system__parameters__SDP;
       extern void *__gnat_system__soft_links__SDP;
       extern void *__gnat_system__stack_checking__SDP;
       extern void *__gnat_ada__text_io__SDP;
       extern void *__gnat_ada__streams__SDP;
       extern void *__gnat_ada__tags__SDP;
       extern void *__gnat_interfaces__c_streams__SDP;
       extern void *__gnat_system__file_io__SDP;
       extern void *__gnat_ada__finalization__SDP;
       extern void *__gnat_system__finalization_root__SDP;
       extern void *__gnat_system__stream_attributes__SDP;
       extern void *__gnat_system__finalization_implementation__SDP;
       extern void *__gnat_system__string_ops_concat_3__SDP;
       extern void *__gnat_system__string_ops__SDP;
       extern void *__gnat_system__file_control_block__SDP;
       extern void *__gnat_ada__finalization__list_controller__SDP;

       void **st[21] = {
         &__gnat_hello__SDP,
         &__gnat_ada__exceptions__SDP,
         &__gnat_gnat__heap_sort_a__SDP,
         &__gnat_system__exception_table__SDP,
         &__gnat_system__secondary_stack__SDP,
         &__gnat_system__parameters__SDP,
         &__gnat_system__soft_links__SDP,
         &__gnat_system__stack_checking__SDP,
         &__gnat_ada__text_io__SDP,
         &__gnat_ada__streams__SDP,
         &__gnat_ada__tags__SDP,
         &__gnat_interfaces__c_streams__SDP,
         &__gnat_system__file_io__SDP,
         &__gnat_ada__finalization__SDP,
         &__gnat_system__finalization_root__SDP,
         &__gnat_system__stream_attributes__SDP,
         &__gnat_system__finalization_implementation__SDP,
         &__gnat_system__string_ops_concat_3__SDP,
         &__gnat_system__string_ops__SDP,
         &__gnat_system__file_control_block__SDP,
         &__gnat_ada__finalization__list_controller__SDP};

       extern void ada__exceptions___elabs ();
       extern void system__exceptions___elabs ();
       extern void interfaces__c_streams___elabs ();
       extern void system__exception_table___elabb ();
       extern void ada__io_exceptions___elabs ();
       extern void ada__tags___elabs ();
       extern void ada__tags___elabb ();
       extern void ada__streams___elabs ();
       extern void system__stack_checking___elabs ();
       extern void system__soft_links___elabb ();
       extern void system__secondary_stack___elabb ();
       extern void ada__exceptions___elabb ();
       extern void system__finalization_root___elabs ();
       extern void system__finalization_implementation___elabs ();
       extern void ada__finalization___elabs ();
       extern void ada__finalization__list_controller___elabs ();
       extern void system__file_control_block___elabs ();
       extern void system__file_io___elabb ();
       extern void ada__text_io___elabs ();
       extern void ada__text_io___elabb ();

       void (*ea[22]) () = {
         adainit,
         adafinal,
         ada__exceptions___elabs,
         system__exceptions___elabs,
         interfaces__c_streams___elabs,
         system__exception_table___elabb,
         ada__io_exceptions___elabs,
         ada__tags___elabs,
         ada__tags___elabb,
         ada__streams___elabs,
         system__stack_checking___elabs,
         system__soft_links___elabb,
         system__secondary_stack___elabb,
         ada__exceptions___elabb,
         system__finalization_root___elabs,
         system__finalization_implementation___elabs,
         ada__finalization___elabs,
         ada__finalization__list_controller___elabs,
         system__file_control_block___elabs,
         system__file_io___elabb,
         ada__text_io___elabs,
         ada__text_io___elabb};

       __gnat_SDP_Table_Build (&st, 21, ea, 22);
       __gnat_set_globals (
          -1,      /* Main_Priority              */
          -1,      /* Time_Slice_Value           */
          'b',     /* WC_Encoding                */
          ' ',     /* Locking_Policy             */
          ' ',     /* Queuing_Policy             */
          ' ',     /* Tasking_Dispatching_Policy */
          adafinal,/* Finalization routine address */
          0,       /* Unreserve_All_Interrupts */
          0);      /* Exception_Tracebacks */

    /* system___elabs (); */
    /* ada___elabs (); */
    /* gnat___elabs (); */
    /* gnat__heap_sort_a___elabs (); */
    /* gnat__htable___elabs (); */
    /* gnat__htable___elabb (); */
    /* interfaces___elabs (); */
    /* system__machine_code___elabs (); */
    /* system__parameters___elabs (); */
    /* system__standard_library___elabs (); */
       ada__exceptions___elabs ();
       system__exceptions___elabs ();
    /* system__parameters___elabb (); */
    /* gnat__heap_sort_a___elabb (); */
       interfaces__c_streams___elabs ();
    /* interfaces__c_streams___elabb (); */
    /* system__exception_table___elabs (); */
       system__exception_table___elabb ();
       ada__io_exceptions___elabs ();
    /* system__storage_elements___elabs (); */
    /* system__storage_elements___elabb (); */
    /* system__secondary_stack___elabs (); */
       ada__tags___elabs ();
       ada__tags___elabb ();
       ada__streams___elabs ();
       system__stack_checking___elabs ();
    /* system__soft_links___elabs (); */
       system__soft_links___elabb ();
    /* system__stack_checking___elabb (); */
       system__secondary_stack___elabb ();
       ada__exceptions___elabb ();
    /* system__standard_library___elabb (); */
    /* system__string_ops___elabs (); */
    /* system__string_ops___elabb (); */
    /* system__string_ops_concat_3___elabs (); */
    /* system__string_ops_concat_3___elabb (); */
    /* system__unsigned_types___elabs (); */
    /* system__stream_attributes___elabs (); */
    /* system__stream_attributes___elabb (); */
       system__finalization_root___elabs ();
    /* system__finalization_root___elabb (); */
       system__finalization_implementation___elabs ();
    /* system__finalization_implementation___elabb (); */
       ada__finalization___elabs ();
    /* ada__finalization___elabb (); */
       ada__finalization__list_controller___elabs ();
    /* ada__finalization__list_controller___elabb (); */
       system__file_control_block___elabs ();
    /* system__file_io___elabs (); */
       system__file_io___elabb ();
       ada__text_io___elabs ();
       ada__text_io___elabb ();
    /* text_io___elabs (); */
    /* hello___elabb (); */
    }
    void adafinal () {
       system__finalization_implementation__finalize_global_list ();
    }
    int main (argc, argv, envp)
        int argc;
        char **argv;
        char **envp;
    {
       gnat_argc = argc;
       gnat_argv = argv;
       gnat_envp = envp;

       __gnat_initialize();
       adainit();
       __gnat_break_start();

       _ada_hello ();

       adafinal();
       __gnat_finalize();
       exit (gnat_exit_status);
    }
    unsigned helloB = 0x425FD0AF;
    unsigned system__standard_libraryB = 0x077A2651;
    unsigned system__standard_libraryS = 0x08ADDC9E;
    unsigned systemS = 0x1D370323;
    unsigned system__exceptionsS = 0x3043D77B;
    unsigned adaS = 0x2359F9ED;
    unsigned ada__exceptionsB = 0x0CA940CF;
    unsigned ada__exceptionsS = 0x69BA6A59;
    unsigned gnatS = 0x156A40CF;
    unsigned gnat__heap_sort_aB = 0x033DABE0;
    unsigned gnat__heap_sort_aS = 0x6AB38FEA;
    unsigned system__exception_tableB = 0x7AAA368C;
    unsigned system__exception_tableS = 0x7D13B305;
    unsigned gnat__htableB = 0x62D2B79D;
    unsigned gnat__htableS = 0x2E865F1E;
    unsigned system__machine_codeS = 0x6379D875;
    unsigned system__secondary_stackB = 0x72D6A51D;
    unsigned system__secondary_stackS = 0x6E88E3D7;
    unsigned system__parametersB = 0x45C8383C;
    unsigned system__parametersS = 0x385E7AC2;
    unsigned system__soft_linksB = 0x08FE4C1F;
    unsigned system__soft_linksS = 0x23B87757;
    unsigned system__stack_checkingB = 0x3A4BFD9A;
    unsigned system__stack_checkingS = 0x4C9F3930;
    unsigned system__storage_elementsB = 0x2F1EB794;
    unsigned system__storage_elementsS = 0x0E2A461A;
    unsigned text_ioS = 0x5570D114;
    unsigned ada__text_ioB = 0x501FA6BF;
    unsigned ada__text_ioS = 0x57692181;
    unsigned ada__streamsS = 0x7C25DE96;
    unsigned ada__tagsB = 0x521B9399;
    unsigned ada__tagsS = 0x689CC1B9;
    unsigned interfacesS = 0x0357E00A;
    unsigned interfaces__c_streamsB = 0x1345CFE9;
    unsigned interfaces__c_streamsS = 0x343244DE;
    unsigned system__file_ioB = 0x6725DC79;
    unsigned system__file_ioS = 0x2DAF477E;
    unsigned ada__finalizationB = 0x4F0184F2;
    unsigned ada__finalizationS = 0x0A0669D8;
    unsigned system__finalization_rootB = 0x26610831;
    unsigned system__finalization_rootS = 0x0B5A4DF9;
    unsigned system__stream_attributesB = 0x1D4F93E8;
    unsigned system__stream_attributesS = 0x30B2EC3D;
    unsigned ada__io_exceptionsS = 0x34054F96;
    unsigned system__unsigned_typesS = 0x6598BA3E;
    unsigned system__finalization_implementationB = 0x2C9C021D;
    unsigned system__finalization_implementationS = 0x177A51F6;
    unsigned system__string_ops_concat_3B = 0x1CBC39CD;
    unsigned system__string_ops_concat_3S = 0x5461BB3E;
    unsigned system__string_opsB = 0x03F36D98;
    unsigned system__string_opsS = 0x208D3EF6;
    unsigned system__file_control_blockS = 0x33AF4230;
    unsigned ada__finalization__list_controllerB = 0x0B97C6BF;
    unsigned ada__finalization__list_controllerS = 0x34B32999;

    /* BEGIN Object file/option list
    ./system.o
    ./ada.o
    ./gnat.o
    ./g-htable.o
    ./interfac.o
    ./s-maccod.o
    ./s-except.o
    ./s-parame.o
    ./g-hesora.o
    ./i-cstrea.o
    ./s-exctab.o
    ./a-ioexce.o
    ./s-stoele.o
    ./a-tags.o
    ./a-stream.o
    ./s-soflin.o
    ./s-stache.o
    ./s-secsta.o
    ./a-except.o
    ./s-stalib.o
    ./s-strops.o
    ./s-sopco3.o
    ./s-unstyp.o
    ./s-stratt.o
    ./s-finroo.o
    ./s-finimp.o
    ./a-finali.o
    ./a-filico.o
    ./s-ficobl.o
    ./s-fileio.o
    ./a-textio.o
    ./text_io.o
    ./hello.o
    -L./
    ...  Target/System specific library search directives
    -lgnat
       END Object file/option list */

Here again, the C code is exactly what is generated by the binder. The functions of the various parts of this code correspond in an obvious manner with the commented Ada code shown in the example in the previous section.

## Consistency-Checking Modes

As described in the previous section, by default `gnatbind` checks that object files are consistent with one another and are consistent with any source files it can locate. The following switches control binder access to sources.

`-s`
  Require source files to be present. In this mode, the binder must be able to locate all source files that are referenced, in order to check their consistency. In normal mode, if a source file cannot be located it is simply ignored. If you specify this switch, a missing source file is an error.
`-x`
  Exclude source files. In this mode, the binder only checks that ALI files are consistent with one another. Source files are not accessed. The binder runs faster in this mode, and there is still a guarantee that the resulting program is self-consistent. If a source file has been edited since it was last compiled, and you specify this switch, the binder will not detect that the object file is out of date with respect to the source file. Note that this is the mode that is automatically used by `gnatmake` because in this case the checking against sources has already been performed by `gnatmake` in the course of compilation (i.e. before binding).

## Binder Error Message Control

The following switches provide control over the generation of error messages from the binder:

`-v`
  Verbose mode. In the normal mode, brief error messages are generated to ``stderr'`. If this switch is present, a header is written to ``stdout'` and any error messages are directed to ``stdout'`. All that is written to ``stderr'` is a brief summary message.
`-b`
  Generate brief error messages to ``stderr'` even if verbose mode is specified. This is relevant only when used with the `-v` switch.
`-mn`
  Limits the number of error messages to n, a decimal integer in the range 1-999. The binder terminates immediately if this limit is reached.
`-Mxxx`
  Renames the generated main program from `main` to `xxx`. This is useful in the case of some cross-building environments, where the actual main program is separate from the one generated by `gnatbind`.
`-ws`
  Suppress all warning messages.
`-we`
  Treat any warning messages as fatal errors.
`-t`
  The binder performs a number of consistency checks including:
  - Check that time stamps of a given source unit are consistent
  - Check that checksums of a given source unit are consistent
  - Check that consistent versions of `GNAT` were used for compilation
  - Check consistency of configuration pragmas as required
  Normally failure of such checks, in accordance with the consistency requirements of the Ada Reference Manual, causes error messages to be generated which abort the binder and prevent the output of a binder file and subsequent link to obtain an executable. The `-t` switch converts these error messages into warnings, so that binding and linking can continue to completion even in the presence of such errors. The result may be a failed link (due to missing symbols), or a non-functional executable which has undefined semantics. This means that `-t` should be used only in unusual situations, with extreme care.

## Elaboration Control

The following switches provide additional control over the elaboration order. For full details see See section Elaboration Order Handling in GNAT.

`-f`
  Instructs the binder to ignore directives from the compiler about implied `Elaborate_All` pragmas, and to use full Ada 95 Reference Manual semantics in an attempt to find a legal elaboration order, even if it seems likely that this order will cause an elaboration exception.
`-p`
  Normally the binder attempts to choose an elaboration order that is likely to minimize the likelihood of an elaboration order error resulting in raising a `Program_Error` exception. This switch reverses the action of the binder, and requests that it deliberately choose an order that is likely to maximize the likelihood of an elaboration error. This is useful in ensuring portability and avoiding dependence on accidental fortuitous elaboration ordering. Normally it only makes sense to use the `-p` switch if dynamic elaboration checking is used (`-gnatE` switch used for compilation). This is because in the default static elaboration mode, all necessary `Elaborate_All` pragmas are implicitly inserted. These implicit pragmas are still respected by the binder in `-p` mode, so a safe elaboration order is assured.

## Output Control

The following switches allow additional control over the output generated by the binder.

`-A`
  Generate binder program in Ada (default). The binder program is named ``b~mainprog.adb'` by default. This can be changed with `-o` `gnatbind` option.
`-C`
  Generate binder program in C. The binder program is named ``b_mainprog.c'`. This can be changed with `-o` `gnatbind` option.
`-e`
  Output complete list of elaboration-order dependencies, showing the reason for each dependency. This output can be rather extensive but may be useful in diagnosing problems with elaboration order. The output is written to ``stdout'`.
`-h`
  Output usage information. The output is written to ``stdout'`.
`-l`
  Output chosen elaboration order. The output is written to ``stdout'`.
`-O`
  Output full names of all the object files that must be linked to provide the Ada component of the program. The output is written to ``stdout'`. This list includes the files explicitly supplied and referenced by the user as well as implicitly referenced run-time unit files. The latter are omitted if the corresponding units reside in shared libraries. The directory names for the run-time units depend on the system configuration.
`-o file`
  Set name of output file to file instead of the normal ``b~mainprog.adb'` default. Note that file denote the Ada binder generated body filename. In C mode you would normally give file an extension of ``.c'` because it will be a C source program. Note that if this option is used, then linking must be done manually. It is not possible to use gnatlink in this case, since it cannot locate the binder file.
`-c`
  Check only. Do not generate the binder output file. In this mode the binder performs all error checks but does not generate an output file.

## Binding with Non-Ada Main Programs

In our description so far we have assumed that the main program is in Ada, and that the task of the binder is to generate a corresponding function `main` that invokes this Ada main program. GNAT also supports the building of executable programs where the main program is not in Ada, but some of the called routines are written in Ada and compiled using GNAT (see section Mixed Language Programming). The following switch is used in this situation:

`-n`
  No main program. The main program is not in Ada.

In this case, most of the functions of the binder are still required, but instead of generating a main program, the binder generates a file containing the following callable routines:

`adainit`
  You must call this routine to initialize the Ada part of the program by calling the necessary elaboration routines. A call to `adainit` is required before the first call to an Ada subprogram. Note that it is assumed that the basic execution environment must be setup to be appropriate for Ada execution at the point where the first Ada subprogram is called. In particular, if the Ada code will do any floating-point operations, then the FPU must be setup in an appropriate manner. For the case of the x86, for example, full precision mode is required. The procedure GNAT.Float_Control.Reset may be used to ensure that the FPU is in the right state.
`adafinal`
  You must call this routine to perform any library-level finalization required by the Ada subprograms. A call to `adafinal` is required after the last call to an Ada subprogram, and before the program terminates.

If the `-n` switch is given, more than one ALI file may appear on the command line for `gnatbind`. The normal closure calculation is performed for each of the specified units. Calculating the closure means finding out the set of units involved by tracing `with` references. The reason it is necessary to be able to specify more than one ALI file is that a given program may invoke two or more quite separate groups of Ada units.

The binder takes the name of its output file from the last specified ALI file, unless overridden by the use of the `\-o file\/OUTPUT=file\`. The output is an Ada unit in source form that can be compiled with GNAT unless the -C switch is used in which case the output is a C source file, which must be compiled using the C compiler. This compilation occurs automatically as part of the `gnatlink` processing.

Currently the GNAT run time requires a FPU using 80 bits mode precision. Under targets where this is not the default it is required to call GNAT.Float_Control.Reset before using floating point numbers (this include float computation, float input and output) in the Ada code. A side effect is that this could be the wrong mode for the foreign code where floating point computation could be broken after this call.

## Binding Programs with No Main Subprogram

It is possible to have an Ada program which does not have a main subprogram. This program will call the elaboration routines of all the packages, then the finalization routines.

The following switch is used to bind programs organized in this manner:

`-z`
  Normally the binder checks that the unit name given on the command line corresponds to a suitable main subprogram. When this switch is used, a list of ALI files can be given, and the execution of the program consists of elaboration of these units in an appropriate order.

## Summary of Binder Switches

The following are the switches available with `gnatbind`:

`-aO`
  Specify directory to be searched for ALI files.
`-aI`
  Specify directory to be searched for source file.
`-A`
  Generate binder program in Ada (default)
`-b`
  Generate brief messages to ``stderr'` even if verbose mode set.
`-c`
  Check only, no generation of binder output file.
`-C`
  Generate binder program in C
`-e`
  Output complete list of elaboration-order dependencies.
`-E`
  Store tracebacks in exception occurrences when the target supports it. This is the default with the zero cost exception mechanism. This option is currently only supported on Solaris, Linux and Windows ix86. Under Solaris and Linux you need to use explicitly the `gcc` flag `-funwind-tables` when compiling every file in your application. See also the packages `GNAT.Traceback` and `GNAT.Traceback.Symbolic`. Under Windows there is no specific option to use to enable this feature but you must not use `-fomit-frame-pointer` `gcc` option.
`-f`
  On some targets, the command line length is limited, and `gnatlink` will generate a separate file for the linker if the list of object files is too long. The `-f` flag forces this file to be generated even if the limit is not exceeded. This is useful in some cases to deal with special situations where the command line length is exceeded.
`-h`
  Output usage (help) information
`-I`
  Specify directory to be searched for source and ALI files.
`-I-`
  Do not look for sources in the current directory where `gnatbind` was invoked, and do not look for ALI files in the directory containing the ALI file named in the `gnatbind` command line.
`-l`
  Output chosen elaboration order.
`-Mxyz`
  Rename generated main program from main to xyz
`-mn`
  Limit number of detected errors to n (1-999).
`-n`
  No main program.
`-nostdinc`
  Do not look for sources in the system default directory.
`-nostdlib`
  Do not look for library files in the system default directory.
`-o file`
  Name the output file file (default is ``b~xxx.adb'`). Note that if this option is used, then linking must be done manually, gnatlink cannot be used.
`-O`
  Output object list.
`-p`
  Pessimistic (worst-case) elaboration order
`-s`
  Require all source files to be present.
`-static`
  Link against a static GNAT run time.
`-shared`
  Link against a shared GNAT run time when available.
`-t`
  Tolerate time stamp and other consistency errors
`-Tn`
  Set the time slice value to n microseconds. A value of zero means no time slicing and also indicates to the tasking run time to match as close as possible to the annex D requirements of the RM.
`-v`
  Verbose mode. Write error messages, header, summary output to ``stdout'`.
`-wx`
  Warning mode (x=s/e for suppress/treat as error)
`-x`
  Exclude source files (check object consistency only).
`-z`
  No main subprogram.

You may obtain this listing by running the program `gnatbind` with no arguments.

## Command-Line Access

The package `Ada.Command_Line` provides access to the command-line arguments and program name. In order for this interface to operate correctly, the two variables

    int gnat_argc;
    char **gnat_argv;

are declared in one of the GNAT library routines. These variables must be set from the actual `argc` and `argv` values passed to the main program. With no `n` present, `gnatbind` generates the C main program to automatically set these variables. If the `n` switch is used, there is no automatic way to set these variables. If they are not set, the procedures in `Ada.Command_Line` will not be available, and any attempt to use them will raise `Constraint_Error`. If command line access is required, your main program must set `gnat_argc` and `gnat_argv` from the `argc` and `argv` values passed to it.

## Search Paths for `gnatbind`

The binder takes the name of an ALI file as its argument and needs to locate source files as well as other ALI files to verify object consistency.

For source files, it follows exactly the same search rules as `gcc` (see section Search Paths and the Run-Time Library (RTL)). For ALI files the directories searched are:

1. The directory containing the ALI file named in the command line, unless the switch `-I-` is specified.
2. All directories specified by `-I` switches on the `gnatbind` command line, in the order given.
3. Each of the directories listed in the value of the `ADA_OBJECTS_PATH` environment variable. Construct this value exactly as the `PATH` environment variable: a list of directory names separated by colons.
4. The content of the "ada_object_path" file which is part of the GNAT installation tree and is used to store standard libraries such as the GNAT Run Time Library (RTL) unless the switch `-nostdlib` is specified. section Installing an Ada Library

In the binder the switch `-I` is used to specify both source and library file paths. Use `-aI` instead if you want to specify source paths only, and `-aO` if you want to specify library paths only. This means that for the binder `-I`dir is equivalent to `-aI`dir `-aO`dir. The binder generates the bind file (a C language source file) in the current working directory.

The packages `Ada`, `System`, and `Interfaces` and their children make up the GNAT Run-Time Library, together with the package GNAT and its children, which contain a set of useful additional library functions provided by GNAT. The sources for these units are needed by the compiler and are kept together in one directory. The ALI files and object files generated by compiling the RTL are needed by the binder and the linker and are kept together in one directory, typically different from the directory containing the sources. In a normal installation, you need not specify these directory names when compiling or binding. Either the environment variables or the built-in defaults cause these files to be found.

Besides simplifying access to the RTL, a major use of search paths is in compiling sources from multiple directories. This can make development environments much more flexible.

## Examples of `gnatbind` Usage

This section contains a number of examples of using the GNAT binding utility `gnatbind`.

`gnatbind hello`
  The main program `Hello` (source program in ``hello.adb'`) is bound using the standard switch settings. The generated main program is ``b~hello.adb'`. This is the normal, default use of the binder.
`gnatbind hello -o mainprog.adb`
  The main program `Hello` (source program in ``hello.adb'`) is bound using the standard switch settings. The generated main program is ``mainprog.adb'` with the associated spec in ``mainprog.ads'`. Note that you must specify the body here not the spec, in the case where the output is in Ada. Note that if this option is used, then linking must be done manually, since gnatlink will not be able to find the generated file.
`gnatbind main -C -o mainprog.c -x`
  The main program `Main` (source program in ``main.adb'`) is bound, excluding source files from the consistency checking, generating the file ``mainprog.c'`.
`gnatbind -x main_program -C -o mainprog.c`
  This command is exactly the same as the previous example. Switches may appear anywhere in the command line, and single letter switches may be combined into a single switch.
`gnatbind -n math dbase -C -o ada-control.c`
  The main program is in a language other than Ada, but calls to subprograms in packages `Math` and `Dbase` appear. This call to `gnatbind` generates the file ``ada-control.c'` containing the `adainit` and `adafinal` routines to be called before and after accessing the Ada units.

# Linking Using `gnatlink`

This chapter discusses `gnatlink`, a utility program used to link Ada programs and build an executable file. This is a simple program that invokes the UNIX linker (via the `gcc` command) with a correct list of object files and library references. `gnatlink` automatically determines the list of files and references for the Ada part of a program. It uses the binder file generated by the binder to determine this list.

- Running gnatlink
- Switches for gnatlink
- Setting Stack Size from gnatlink
- Setting Heap Size from gnatlink

## Running `gnatlink`

The form of the `gnatlink` command is

    $ gnatlink [switches] mainprog[.ali] [non-Ada objects]
          [linker options]

``mainprog.ali'` references the ALI file of the main program. The ``.ali'` extension of this file can be omitted. From this reference, `gnatlink` locates the corresponding binder file ``b~mainprog.adb'` and, using the information in this file along with the list of non-Ada objects and linker options, constructs a UNIX linker command file to create the executable.

The arguments following ``mainprog.ali'` are passed to the linker uninterpreted. They typically include the names of object files for units written in other languages than Ada and any library references required to resolve references in any of these foreign language units, or in `pragma Import` statements in any Ada units.

linker options is an optional list of linker specific switches. The default linker called by gnatlink is gcc which in turn calls the appropriate system linker usually called ld. Standard options for the linker such as `-lmy_lib` or `-Ldir` can be added as is. For options that are not recognized by gcc as linker options, the gcc switches `-Xlinker` or `-Wl,` shall be used. Refer to the GCC documentation for details. Here is an example showing how to generate a linker map assuming that the underlying linker is GNU ld:

    $ gnatlink my_prog -Wl,-Map,MAPFILE

Using linker options it is possible to set the program stack and heap size. See see section Setting Stack Size from `gnatlink` and see section Setting Heap Size from `gnatlink`.

`gnatlink` determines the list of objects required by the Ada program and prepends them to the list of objects passed to the linker. `gnatlink` also gathers any arguments set by the use of `pragma Linker_Options` and adds them to the list of arguments presented to the linker.

## Switches for `gnatlink`

The following switches are available with the `gnatlink` utility:

`-A`
  The binder has generated code in Ada. This is the default.
`-C`
  If instead of generating a file in Ada, the binder has generated one in C, then the linker needs to know about it. Use this switch to signal to `gnatlink` that the binder has generated C code rather than Ada code.
`-g`
  The option to include debugging information causes the Ada bind file (in other words, ``b~mainprog.adb'`) to be compiled with `-g`. In addition, the binder does not delete the ``b~mainprog.adb'`, ``b~mainprog.o'` and ``b~mainprog.ali'` files. Without `-g`, the binder removes these files by default. The same procedure apply if a C bind file was generated using `-C` `gnatbind` option, in this case the filenames are ``b_mainprog.c'` and ``b_mainprog.o'`.
`-n`
  Do not compile the file generated by the binder. This may be used when a link is rerun with different options, but there is no need to recompile the binder file.
`-v`
  Causes additional information to be output, including a full list of the included object files. This switch option is most useful when you want to see what set of object files are being used in the link step.
`-v -v`
  Very verbose mode. Requests that the compiler operate in verbose mode when it compiles the binder file, and that the system linker run in verbose mode.
`-o exec-name`
  exec-name specifies an alternate name for the generated executable program. If this switch is omitted, the executable has the same name as the main unit. For example, `gnatlink try.ali` creates an executable called ``try'`.
`-b target`
  Compile your program to run on target, which is the name of a system configuration. You must have a GNAT cross-compiler built if target is not the same as your host system.
`-Bdir`
  Load compiler executables (for example, `gnat1`, the Ada compiler) from dir instead of the default location. Only use this switch when multiple versions of the GNAT compiler are available. See the `gcc` manual page for further details. You would normally use the `-b` or `-V` switch instead.
`--GCC=compiler_name`
  Program used for compiling the binder file. The default is `gcc`'. You need to use quotes around compiler_name if `compiler_name` contains spaces or other separator characters. As an example `--GCC="foo -x -y"` will instruct `gnatlink` to use `foo -x -y` as your compiler. Note that switch `-c` is always inserted after your command name. Thus in the above example the compiler command that will be used by `gnatlink` will be `foo -c -x -y`. If several `--GCC=compiler_name` are used, only the last compiler_name is taken into account. However, all the additional switches are also taken into account. Thus, `--GCC="foo -x -y" --GCC="bar -z -t"` is equivalent to `--GCC="bar -x -y -z -t"`.
`--LINK=name`
  name is the name of the linker to be invoked. This is especially useful in mixed language programs since languages such as c++ require their own linker to be used. When this switch is omitted, the default name for the linker is (``gcc'`). When this switch is used, the specified linker is called instead of (``gcc'`) with exactly the same parameters that would have been passed to (``gcc'`) so if the desired linker requires different parameters it is necessary to use a wrapper script that massages the parameters before invoking the real linker. It may be useful to control the exact invocation by using the verbose switch.

## Setting Stack Size from `gnatlink`

It is possible to specify the program stack size from `gnatlink`. Assuming that the underlying linker is GNU ld there is two ways to do so:

- using `-Xlinker` linker option

      $ gnatlink hello -Xlinker --stack=0x10000,0x1000

  This set the stack reserve size to 0x10000 bytes and the stack commit size to 0x1000 bytes.
- using `-Wl` linker option

      $ gnatlink hello -Wl,--stack=0x1000000

  This set the stack reserve size to 0x1000000 bytes. Note that with `-Wl` option it is not possible to set the stack commit size because the coma is a separator for this option.

## Setting Heap Size from `gnatlink`

It is possible to specify the program heap size from `gnatlink`. Assuming that the underlying linker is GNU ld there is two ways to do so:

- using `-Xlinker` linker option

      $ gnatlink hello -Xlinker --heap=0x10000,0x1000

  This set the heap reserve size to 0x10000 bytes and the heap commit size to 0x1000 bytes.
- using `-Wl` linker option

      $ gnatlink hello -Wl,--heap=0x1000000

  This set the heap reserve size to 0x1000000 bytes. Note that with `-Wl` option it is not possible to set the heap commit size because the coma is a separator for this option.

# The GNAT Make Program `gnatmake`

- Running gnatmake
- Switches for gnatmake
- Mode Switches for gnatmake
- Notes on the Command Line
- How gnatmake Works
- Examples of gnatmake Usage

A typical development cycle when working on an Ada program consists of the following steps:

1. Edit some sources to fix bugs.
2. Add enhancements.
3. Compile all sources affected.
4. Rebind and relink.
5. Test.

The third step can be tricky, because not only do the modified files have to be compiled, but any files depending on these files must also be recompiled. The dependency rules in Ada can be quite complex, especially in the presence of overloading, `use` clauses, generics and inlined subprograms.

`gnatmake` automatically takes care of the third and fourth steps of this process. It determines which sources need to be compiled, compiles them, and binds and links the resulting object files.

Unlike some other Ada make programs, the dependencies are always accurately recomputed from the new sources. The source based approach of the GNAT compilation model makes this possible. This means that if changes to the source program cause corresponding changes in dependencies, they will always be tracked exactly correctly by `gnatmake`.

## Running `gnatmake`

The form of the `gnatmake` command is

    $ gnatmake [switches] file_name [mode_switches]

The only required argument is file_name, which specifies the compilation unit that is the main program. If `switches` are present, they can be placed before of after file_name. If mode_switches are present, they must always be placed after file_name and all `switches`.

If you are using standard file extensions (.adb and .ads), then the extension may be omitted from the file_name argument. However, if you are using non-standard extensions, then it is required that the extension be given. A relative or absolute directory path can be specified in file_name, in which case, the input source file will be searched for in the specified directory only. Otherwise, the input source file will first be searched in the directory where `gnatmake` was invoked and if it is not found, it will be search on the source path of the compiler as described in section Search Paths and the Run-Time Library (RTL).

All `gnatmake` output (except when you specify `-M`) is to ``stderr'`. The output produced by the `-M` switch is send to ``stdout'`.

## Switches for `gnatmake`

You may specify any of the following switches to `gnatmake`:

`--GCC=compiler_name`
  Program used for compiling. The default is `gcc`'. You need to use quotes around compiler_name if `compiler_name` contains spaces or other separator characters. As an example `--GCC="foo -x -y"` will instruct `gnatmake` to use `foo -x -y` as your compiler. Note that switch `-c` is always inserted after your command name. Thus in the above example the compiler command that will be used by `gnatmake` will be `foo -c -x -y`. If several `--GCC=compiler_name` are used, only the last compiler_name is taken into account. However, all the additional switches are also taken into account. Thus, `--GCC="foo -x -y" --GCC="bar -z -t"` is equivalent to `--GCC="bar -x -y -z -t"`.
`--GNATBIND=binder_name`
  Program used for binding. The default is `gnatbind`'. You need to use quotes around binder_name if binder_name contains spaces or other separator characters. As an example `--GNATBIND="bar -x -y"` will instruct `gnatmake` to use `bar -x -y` as your binder. Binder switches that are normally appended by `gnatmake` to `gnatbind`' are now appended to the end of `bar -x -y`.
`--GNATLINK=linker_name`
  Program used for linking. The default is `gnatlink`'. You need to use quotes around linker_name if linker_name contains spaces or other separator characters. As an example `--GNATLINK="lan -x -y"` will instruct `gnatmake` to use `lan -x -y` as your linker. Linker switches that are normally appended by `gnatmake` to `gnatlink`' are now appended to the end of `lan -x -y`.
`-a`
  Consider all files in the make process, even the GNAT internal system files (for example, the predefined Ada library files), as well as any locked files. Locked files are files whose ALI file is write-protected. By default, `gnatmake` does not check these files, because the assumption is that the GNAT internal files are properly up to date, and also that any write protected ALI files have been properly installed. Note that if there is an installation problem, such that one of these files is not up to date, it will be properly caught by the binder. You may have to specify this switch if you are working on GNAT itself. `-a` is also useful in conjunction with `-f` if you need to recompile an entire application, including run-time files, using special configuration pragma settings, such as a non-standard `Float_Representation` pragma. By default `gnatmake -a` compiles all GNAT internal files with `gcc -c -gnatg` rather than `gcc -c`.
`-c`
  Compile only. Do not perform binding and linking. If the root unit specified by file_name is not a main unit, this is the default. Otherwise `gnatmake` will attempt binding and linking unless all objects are up to date and the executable is more recent than the objects.
`-f`
  Force recompilations. Recompile all sources, even though some object files may be up to date, but don't recompile predefined or GNAT internal files or locked files (files with a write-protected ALI file), unless the `-a` switch is also specified.
`-i`
  In normal mode, `gnatmake` compiles all object files and ALI files into the current directory. If the `-i` switch is used, then instead object files and ALI files that already exist are overwritten in place. This means that once a large project is organized into separate directories in the desired manner, then `gnatmake` will automatically maintain and update this organization. If no ALI files are found on the Ada object path (section Search Paths and the Run-Time Library (RTL)), the new object and ALI files are created in the directory containing the source being compiled. If another organization is desired, where objects and sources are kept in different directories, a useful technique is to create dummy ALI files in the desired directories. When detecting such a dummy file, `gnatmake` will be forced to recompile the corresponding source file, and it will be put the resulting object and ALI files in the directory where it found the dummy file.
`-jn`
  Use n processes to carry out the (re)compilations. On a multiprocessor machine compilations will occur in parallel. In the event of compilation errors, messages from various compilations might get interspersed (but `gnatmake` will give you the full ordered list of failing compiles at the end). If this is problematic, rerun the make process with n set to 1 to get a clean list of messages.
`-k`
  Keep going. Continue as much as possible after a compilation error. To ease the programmer's task in case of compilation errors, the list of sources for which the compile fails is given when `gnatmake` terminates.
`-m`
  Specifies that the minimum necessary amount of recompilations be performed. In this mode `gnatmake` ignores time stamp differences when the only modifications to a source file consist in adding/removing comments, empty lines, spaces or tabs. This means that if you have changed the comments in a source file or have simply reformatted it, using this switch will tell gnatmake not to recompile files that depend on it (provided other sources on which these files depend have undergone no semantic modifications).
`-M`
  Check if all objects are up to date. If they are, output the object dependences to ``stdout'` in a form that can be directly exploited in a ``Makefile'`. By default, each source file is prefixed with its (relative or absolute) directory name. This name is whatever you specified in the various `-aI` and `-I` switches. If you use `gnatmake -M` `-q` (see below), only the source file names, without relative paths, are output. If you just specify the `-M` switch, dependencies of the GNAT internal system files are omitted. This is typically what you want. If you also specify the `-a` switch, dependencies of the GNAT internal files are also listed. Note that dependencies of the objects in external Ada libraries (see switch `-aL`dir in the following list) are never reported.
`-n`
  Don't compile, bind, or link. Checks if all objects are up to date. If they are not, the full name of the first file that needs to be recompiled is printed. Repeated use of this option, followed by compiling the indicated source file, will eventually result in recompiling all required units.
`-o exec_name`
  Output executable name. The name of the final executable program will be exec_name. If the `-o` switch is omitted the default name for the executable will be the name of the input file in appropriate form for an executable file on the host system.
`-q`
  Quiet. When this flag is not set, the commands carried out by `gnatmake` are displayed.
`-s`
  Recompile if compiler switches have changed since last compilation. All compiler switches but -I and -o are taken into account in the following way: orders between different first letter" switches are ignored, but orders between same switches are taken into account. For example, `-O -O2` is different than `-O2 -O`, but `-g -O` is equivalent to `-O -g`.
`-u`
  Unique. Recompile at most the main file. It implies -c. Combined with -f, it is equivalent to calling the compiler directly.
`-v`
  Verbose. Displays the reason for all recompilations `gnatmake` decides are necessary.
`-z`
  No main subprogram. Bind and link the program even if the unit name given on the command line is a package name. The resulting executable will execute the elaboration routines of the package and its closure, then the finalization routines.
``gcc` switches`
  The switch `-g` or any uppercase switch (other than `-A`, or `-L`) or any switch that is more than one character is passed to `gcc` (e.g. `-O`, `-gnato,` etc.)

Source and library search path switches:

`-aIdir`
  When looking for source files also look in directory dir. The order in which source files search is undertaken is described in section Search Paths and the Run-Time Library (RTL).
`-aLdir`
  Consider dir as being an externally provided Ada library. Instructs `gnatmake` to skip compilation units whose ``.ali'` files have been located in directory dir. This allows you to have missing bodies for the units in dir. You still need to specify the location of the specs for these units by using the switches `-aIdir` or `-Idir`. Note: this switch is provided for compatibility with previous versions of `gnatmake`. The easier method of causing standard libraries to be excluded from consideration is to write-protect the corresponding ALI files.
`-aOdir`
  When searching for library and object files, look in directory dir. The order in which library files are searched is described in section Search Paths for `gnatbind`.
`-Adir`
  Equivalent to `-aLdir -aIdir`.
`-Idir`
  Equivalent to `-aOdir -aIdir`.
`-I-`
  Do not look for source files in the directory containing the source file named in the command line. Do not look for ALI or object files in the directory where `gnatmake` was invoked.
`-Ldir`
  Add directory dir to the list of directories in which the linker will search for libraries. This is equivalent to `-largs -L`dir.
`-nostdinc`
  Do not look for source files in the system default directory.
`-nostdlib`
  Do not look for library files in the system default directory.

## Mode Switches for `gnatmake`

The mode switches (referred to as `mode_switches`) allow the inclusion of switches that are to be passed to the compiler itself, the binder or the linker. The effect of a mode switch is to cause all subsequent switches up to the end of the switch list, or up to the next mode switch, to be interpreted as switches to be passed on to the designated component of GNAT.

`-cargs switches`
  Compiler switches. Here switches is a list of switches that are valid switches for `gcc`. They will be passed on to all compile steps performed by `gnatmake`.
`-bargs switches`
  Binder switches. Here switches is a list of switches that are valid switches for `gcc`. They will be passed on to all bind steps performed by `gnatmake`.
`-largs switches`
  Linker switches. Here switches is a list of switches that are valid switches for `gcc`. They will be passed on to all link steps performed by `gnatmake`.

## Notes on the Command Line

This section contains some additional useful notes on the operation of the `gnatmake` command.

- If `gnatmake` finds no ALI files, it recompiles the main program and all other units required by the main program. This means that `gnatmake` can be used for the initial compile, as well as during subsequent steps of the development cycle.
- If you enter `gnatmake file.adb`, where ``file.adb'` is a subunit or body of a generic unit, `gnatmake` recompiles ``file.adb'` (because it finds no ALI) and stops, issuing a warning.
- In `gnatmake` the switch `-I` is used to specify both source and library file paths. Use `-aI` instead if you just want to specify source paths only and `-aO` if you want to specify library paths only.
- `gnatmake` examines both an ALI file and its corresponding object file for consistency. If an ALI is more recent than its corresponding object, or if the object file is missing, the corresponding source will be recompiled. Note that `gnatmake` expects an ALI and the corresponding object file to be in the same directory.
- `gnatmake` will ignore any files whose ALI file is write-protected. This may conveniently be used to exclude standard libraries from consideration and in particular it means that the use of the `-f` switch will not recompile these files unless `-a` is also specified.
- `gnatmake` has been designed to make the use of Ada libraries particularly convenient. Assume you have an Ada library organized as follows: obj-dir contains the objects and ALI files for of your Ada compilation units, whereas include-dir contains the specs of these units, but no bodies. Then to compile a unit stored in `main.adb`, which uses this Ada library you would just type

      $ gnatmake -aIinclude-dir  -aLobj-dir  main

- Using `gnatmake` along with the `-m (minimal recompilation)` switch provides an extremely powerful tool: you can freely update the comments/format of your source files without having to recompile everything. Note, however, that adding or deleting lines in a source files may render its debugging info obsolete. If the file in question is a spec, the impact is rather limited, as that debugging info will only be useful during the elaboration phase of your program. For bodies the impact can be more significant. In all events, your debugger will warn you if a source file is more recent than the corresponding object, and therefore obsolescence of debugging information will go unnoticed.

## How `gnatmake` Works

Generally `gnatmake` automatically performs all necessary recompilations and you don't need to worry about how it works. However, it may be useful to have some basic understanding of the `gnatmake` approach and in particular to understand how it uses the results of previous compilations without incorrectly depending on them.

First a definition: an object file is considered up to date if the corresponding ALI file exists and its time stamp predates that of the object file and if all the source files listed in the dependency section of this ALI file have time stamps matching those in the ALI file. This means that neither the source file itself nor any files that it depends on have been modified, and hence there is no need to recompile this file.

`gnatmake` works by first checking if the specified main unit is up to date. If so, no compilations are required for the main unit. If not, `gnatmake` compiles the main program to build a new ALI file that reflects the latest sources. Then the ALI file of the main unit is examined to find all the source files on which the main program depends, and `gnatmake` recursively applies the above procedure on all these files.

This process ensures that `gnatmake` only trusts the dependencies in an existing ALI file if they are known to be correct. Otherwise it always recompiles to determine a new, guaranteed accurate set of dependencies. As a result the program is compiled "upside down" from what may be more familiar as the required order of compilation in some other Ada systems. In particular, clients are compiled before the units on which they depend. The ability of GNAT to compile in any order is critical in allowing an order of compilation to be chosen that guarantees that `gnatmake` will recompute a correct set of new dependencies if necessary.

## Examples of `gnatmake` Usage

`gnatmake hello.adb`
  Compile all files necessary to bind and link the main program ``hello.adb'` (containing unit `Hello`) and bind and link the resulting object files to generate an executable file ``hello'`.
`gnatmake -q Main_Unit -cargs -O2 -bargs -l`
  Compile all files necessary to bind and link the main program unit `Main_Unit` (from file ``main_unit.adb'`). All compilations will be done with optimization level 2 and the order of elaboration will be listed by the binder. `gnatmake` will operate in quiet mode, not displaying commands it is executing.

# Renaming Files Using `gnatchop`

This chapter discusses how to handle files with multiple units by using the `gnatchop` utility. This utility is also useful in renaming files to meet the standard GNAT default file naming conventions.

- Handling Files with Multiple Units
- Operating gnatchop in Compilation Mode
- Command Line for gnatchop
- Switches for gnatchop
- Examples of gnatchop Usage

## Handling Files with Multiple Units

The basic compilation model of GNAT requires that a file submitted to the compiler have only one unit and there be a strict correspondence between the file name and the unit name.

The `gnatchop` utility allows both of these rules to be relaxed, allowing GNAT to process files which contain multiple compilation units and files with arbitrary file names. `gnatchop` reads the specified file and generates one or more output files, containing one unit per file. The unit and the file name correspond, as required by GNAT.

If you want to permanently restructure a set of "foreign" files so that they match the GNAT rules, and do the remaining development using the GNAT structure, you can simply use `gnatchop` once, generate the new set of files and work with them from that point on.

Alternatively, if you want to keep your files in the "foreign" format, perhaps to maintain compatibility with some other Ada compilation system, you can set up a procedure where you use `gnatchop` each time you compile, regarding the source files that it writes as temporary files that you throw away.

## Operating gnatchop in Compilation Mode

The basic function of `gnatchop` is to take a file with multiple units and split it into separate files. The boundary between files is reasonably clear, except for the issue of comments and pragmas. In default mode, the rule is that any pragmas between units belong to the previous unit, except that configuration pragmas always belong to the following unit. Any comments belong to the following unit. These rules almost always result in the right choice of the split point without needing to mark it explicitly and most users will find this default to be what they want. In this default mode it is incorrect to submit a file containing only configuration pragmas, or one that ends in configuration pragmas, to `gnatchop`.

However, using a special option to activate "compilation mode", `gnatchop` can perform another function, which is to provide exactly the semantics required by the RM for handling of configuration pragmas in a compilation. In the absence of configuration pragmas (at the main file level), this option has no effect, but it causes such configuration pragmas to be handled in a quite different manner.

First, in compilation mode, if `gnatchop` is given a file that consists of only configuration pragmas, then this file is appended to the ``gnat.adc'` file in the current directory. This behavior provides the required behavior described in the RM for the actions to be taken on submitting such a file to the compiler, namely that these pragmas should apply to all subsequent compilations in the same compilation environment. Using GNAT, the current directory, possibly containing a ``gnat.adc'` file is the representation of a compilation environment. For more information on the ``gnat.adc'` file, see the section on handling of configuration pragmas see section Handling of Configuration Pragmas.

Second, in compilation mode, if `gnatchop` is given a file that starts with configuration pragmas, and contains one or more units, then these configuration pragmas are prepended to each of the chopped files. This behavior provides the required behavior described in the RM for the actions to be taken on compiling such a file, namely that the pragmas apply to all units in the compilation, but not to subsequently compiled units.

Finally, if configuration pragmas appear between units, they are appended to the previous unit. This results in the previous unit being illegal, since the compiler does not accept configuration pragmas that follow a unit. This provides the required RM behavior that forbids configuration pragmas other than those preceding the first compilation unit of a compilation.

For most purposes, `gnatchop` will be used in default mode. The compilation mode described above is used only if you need exactly accurate behavior with respect to compilations, and you have files that contain multiple units and configuration pragmas. In this circumstance the use of `gnatchop` with the compilation mode switch provides the required behavior, and is for example the mode in which GNAT processes the ACVC tests.

## Command Line for `gnatchop`

The `gnatchop` command has the form:

    $ gnatchop switches file name [file name file name ...]
          [directory]

The only required argument is the file name of the file to be chopped. There are no restrictions on the form of this file name. The file itself contains one or more Ada units, in normal GNAT format, concatenated together. As shown, more than one file may be presented to be chopped.

When run in default mode, `gnatchop` generates one output file in the current directory for each unit in each of the files.

directory, if specified, gives the name of the directory to which the output files will be written. If it is not specified, all files are written to the current directory.

For example, given a file called ``hellofiles'` containing

    procedure hello;

    with Text_IO; use Text_IO;
    procedure hello is
    begin
       Put_Line ("Hello");
    end hello;

the command

    $ gnatchop hellofiles

generates two files in the current directory, one called ``hello.ads'` containing the single line that is the procedure spec, and the other called ``hello.adb'` containing the remaining text. The original file is not affected. The generated files can be compiled in the normal manner.

## Switches for `gnatchop`

`gnatchop` recognizes the following switches:

`-c`
  Causes `gnatchop` to operate in compilation mode, in which configuration pragmas are handled according to strict RM rules. See previous section for a full description of this mode.
`-gnatxxx`
  This passes the given `-gnatxxx` switch to `gnat` which is used to parse the given file. Not all `xxx` options make sense, but for example, the use of `-gnati2` allows `gnatchop` to process a source file that uses Latin-2 coding for identifiers.
`-h`
  Causes `gnatchop` to generate a brief help summary to the standard output file showing usage information.
`-kmm`
  Limit generated file names to the specified number `mm` of characters. This is useful if the resulting set of files is required to be interoperable with systems which limit the length of file names. No space is allowed between the `-k` and the numeric value. The numeric value may be omitted in which case a default of `-k8`, suitable for use with DOS-like file systems, is used. If no `-k` switch is present then there is no limit on the length of file names.
`-q`
  Causes output of informational messages indicating the set of generated files to be suppressed. Warnings and error messages are unaffected.
`-r`
  Generate `Source_Reference` pragmas. Use this switch if the output files are regarded as temporary and development is to be done in terms of the original unchopped file. This switch causes `Source_Reference` pragmas to be inserted into each of the generated files to refers back to the original file name and line number. The result is that all error messages refer back to the original unchopped file. In addition, the debugging information placed into the object file (when the `-g` switch of `gcc` or `gnatmake` is specified) also refers back to this original file so that tools like profilers and debuggers will give information in terms of the original unchopped file. If the original file to be chopped itself contains a `Source_Reference` pragma referencing a third file, then gnatchop respects this pragma, and the generated `Source_Reference` pragmas in the chopped file refer to the original file, with appropriate line numbers. This is particularly useful when `gnatchop` is used in conjunction with `gnatprep` to compile files that contain preprocessing statements and multiple units.
`-v`
  Causes `gnatchop` to operate in verbose mode. The version number and copyright notice are output, as well as exact copies of the gnat1 commands spawned to obtain the chop control information.
`-w`
  Overwrite existing file names. Normally `gnatchop` regards it as a fatal error if there is already a file with the same name as a file it would otherwise output, in other words if the files to be chopped contain duplicated units. This switch bypasses this check, and causes all but the last instance of such duplicated units to be skipped.

## Examples of `gnatchop` Usage

`gnatchop -w hello_s.ada ichbiah/files`
  Chops the source file ``hello_s.ada'`. The output files will be placed in the directory ``ichbiah/files'`, overwriting any files with matching names in that directory (no files in the current directory are modified).
`gnatchop archive`
  Chops the source file ``archive'` into the current directory. One useful application of `gnatchop` is in sending sets of sources around, for example in email messages. The required sources are simply concatenated (for example, using a UNIX `cat` command), and then `gnatchop` is used at the other end to reconstitute the original file names.
`gnatchop file1 file2 file3 direc`
  Chops all units in files ``file1'`, ``file2'`, ``file3'`, placing the resulting files in the directory ``direc'`. Note that if any units occur more than once anywhere within this set of files, an error message is generated, and no files are written. To override this check, use the `-w` switch, in which case the last occurrence in the last file will be the one that is output, and earlier duplicate occurrences for a given unit will be skipped.

# Configuration Pragmas

In Ada 95, configuration pragmas include those pragmas described as such in the Ada 95 Reference Manual, as well as implementation-dependent pragmas that are configuration pragmas. See the individual descriptions of pragmas in the GNAT Reference Manual for details on these additional GNAT-specific configuration pragmas. Most notably, the pragma `Source_File_Name`, which allows specifying non-default names for source files, is a configuration pragma.

- Handling of Configuration Pragmas
- The Configuration Pragmas File

## Handling of Configuration Pragmas

Configuration pragmas may either appear at the start of a compilation unit, in which case they apply only to that unit, or they may apply to all compilations performed in a given compilation environment.

GNAT also provides the `gnatchop` utility to provide an automatic way to handle configuration pragmas following the semantics for compilations (that is, files with multiple units), described in the RM. See section see section Operating gnatchop in Compilation Mode for details. However, for most purposes, it will be more convenient to edit the ``gnat.adc'` file that contains configuration pragmas directly, as described in the following section.

## The Configuration Pragmas File

In GNAT a compilation environment is defined by the current directory at the time that a compile command is given. This current directory is searched for a file whose name is ``gnat.adc'`. If this file is present, it is expected to contain one or more configuration pragmas that will be applied to the current compilation.

Configuration pragmas may be entered into the ``gnat.adc'` file either by running `gnatchop` on a source file that consists only of configuration pragmas, or more conveniently by direct editing of the ``gnat.adc'` file, which is a standard format source file.

# Elaboration Order Handling in GNAT

- Elaboration Code in Ada 95
- Checking the Elaboration Order in Ada 95
- Controlling the Elaboration Order in Ada 95
- Controlling Elaboration in GNAT - Internal Calls
- Controlling Elaboration in GNAT - External Calls
- Default Behavior in GNAT - Ensuring Safety
- Elaboration Issues for Library Tasks
- Mixing Elaboration Models
- What to Do If the Default Elaboration Behavior Fails
- Elaboration for Access-to-Subprogram Values
- Summary of Procedures for Elaboration Control
- Other Elaboration Order Considerations

This chapter describes the handling of elaboration code in Ada 95 and in GNAT, and discusses how the order of elaboration of program units can be controlled in GNAT, either automatically or with explicit programming features.

## Elaboration Code in Ada 95

Ada 95 provides rather general mechanisms for executing code at elaboration time, that is to say before the main program starts executing. Such code arises in three contexts:

Initializers for variables.
  Variables declared at the library level, in package specs or bodies, can require initialization that is performed at elaboration time, as in:

      Sqrt_Half : Float := Sqrt (0.5);

Package initialization code
  Code in a `BEGIN-END` section at the outer level of a package body is executed as part of the package body elaboration code.
Library level task allocators
  Tasks that are declared using task allocators at the library level start executing immediately and hence can execute at elaboration time.

Subprogram calls are possible in any of these contexts, which means that any arbitrary part of the program may be executed as part of the elaboration code. It is even possible to write a program which does all its work at elaboration time, with a null main program, although stylistically this would usually be considered an inappropriate way to structure a program.

An important concern arises in the context of elaboration code: we have to be sure that it is executed in an appropriate order. What we have is a series of elaboration code sections, potentially one section for each unit in the program. It is important that these execute in the correct order. Correctness here means that, taking the above example of the declaration of `Sqrt_Half`, if some other piece of elaboration code references `Sqrt_Half`, then it must run after the section of elaboration code that contains the declaration of `Sqrt_Half`.

There would never be any order of elaboration problem if we made a rule that whenever you `with` a unit, you must elaborate both the spec and body of that unit before elaborating the unit doing the `with`'ing:

    with Unit_1;
    package Unit_2 is ...

would require that both the body and spec of `Unit_1` be elaborated before the spec of `Unit_2`. However, a rule like that would be far too restrictive. In particular, it would make it impossible to have routines in separate packages that were mutually recursive.

You might think that a clever enough compiler could look at the actual elaboration code and determine an appropriate correct order of elaboration, but in the general case, this is not possible. Consider the following example.

In the body of `Unit_1`, we have a procedure `Func_1` that references the variable `Sqrt_1`, which is declared in the elaboration code of the body of `Unit_1`:

    Sqrt_1 : Float := Sqrt (0.1);

The elaboration code of the body of `Unit_1` also contains:

    if expression_1 = 1 then
       Q := Unit_2.Func_2;
    end if;

`Unit_2` is exactly parallel, it has a procedure `Func_2` that references the variable `Sqrt_2`, which is declared in the elaboration code of the body `Unit_2`:

    Sqrt_2 : Float := Sqrt (0.1);

The elaboration code of the body of `Unit_2` also contains:

    if expression_2 = 2 then
       Q := Unit_1.Func_1;
    end if;

Now the question is, which of the following orders of elaboration is acceptable:

    Spec of Unit_1
    Spec of Unit_2
    Body of Unit_1
    Body of Unit_2

or

    Spec of Unit_2
    Spec of Unit_1
    Body of Unit_2
    Body of Unit_1

If you carefully analyze the flow here, you will see that you cannot tell at compile time the answer to this question. If `expression_1` is not equal to 1, and `expression_2` is not equal to 2, then either order is acceptable, because neither of the function calls is executed. If both tests evaluate to true, then neither order is acceptable and in fact there is no correct order.

If one of the two expressions is true, and the other is false, then one of the above orders is correct, and the other is incorrect. For example, if `expression_1` = 1 and `expression_2` /= 2, then the call to `Func_2` will occur, but not the call to `Func_1.` This means that it is essential to elaborate the body of `Unit_1` before the body of `Unit_2`, so the first order of elaboration is correct and the second is wrong.

By making `expression_1` and `expression_2` depend on input data, or perhaps the time of day, we can make it impossible for the compiler or binder to figure out which of these expressions will be true, and hence it is impossible to guarantee a safe order of elaboration at run time.

## Checking the Elaboration Order in Ada 95

In some languages that involve the same kind of elaboration problems, e.g. Java and C++, the programmer is expected to worry about these ordering problems himself, and it is common to write a program in which an incorrect elaboration order gives surprising results, because it references variables before they are initialized. Ada 95 is designed to be a safe language, and a programmer-beware approach is clearly not sufficient. Consequently, the language provides three lines of defense:

Standard rules
  Some standard rules restrict the possible choice of elaboration order. In particular, if you `with` a unit, then its spec is always elaborated before the unit doing the `with`. Similarly, a parent spec is always elaborated before the child spec, and finally a spec is always elaborated before its corresponding body.
Dynamic elaboration checks
  Dynamic checks are made at run time, so that if some entity is accessed before it is elaborated (typically by means of a subprogram call) then the exception (`Program_Error`) is raised.
Elaboration control
  Facilities are provided for the programmer to specify the desired order of elaboration.

Let's look at these facilities in more detail. First, the rules for dynamic checking. One possible rule would be simply to say that the exception is raised if you access a variable which has not yet been elaborated. The trouble with this approach is that it could require expensive checks on every variable reference. Instead Ada 95 has two rules which are a little more restrictive, but easier to check, and easier to state:

Restrictions on calls
  A subprogram can only be called at elaboration time if its body has been elaborated. The rules for elaboration given above guarantee that the spec of the subprogram has been elaborated before the call, but not the body. If this rule is violated, then the exception `Program_Error` is raised.
Restrictions on instantiations
  A generic unit can only be instantiated if the body of the generic unit has been elaborated. Again, the rules for elaboration given above guarantee that the spec of the generic unit has been elaborated before the instantiation, but not the body. If this rule is violated, then the exception `Program_Error` is raised.

The idea is that if the body has been elaborated, then any variables it references must have been elaborated; by checking for the body being elaborated we guarantee that none of its references causes any trouble. As we noted above, this is a little too restrictive, because a subprogram that has no non-local references in its body may in fact be safe to call. However, it really would be unsafe to rely on this, because it would mean that the caller was aware of details of the implementation in the body. This goes against the basic tenets of Ada.

A plausible implementation can be described as follows. A Boolean variable is associated with each subprogram and each generic unit. This variable is initialized to False, and is set to True at the point body is elaborated. Every call or instantiation checks the variable, and raises `Program_Error` if the variable is False.

## Controlling the Elaboration Order in Ada 95

In the previous section we discussed the rules in Ada 95 which ensure that `Program_Error` is raised if an incorrect elaboration order is chosen. This prevents erroneous executions, but we need mechanisms to specify a correct execution and avoid the exception altogether. To achieve this, Ada 95 provides a number of features for controlling the order of elaboration. We discuss these features in this section.

First, there are several ways of indicating to the compiler that a given unit has no elaboration problems:

packages that do not require a body
  In Ada 95, a library package that does not require a body does not permit a body. This means that if we have a such a package, as in:

      package Definitions is
         generic
            type m is new integer;
         package Subp is
            type a is array (1 .. 10) of m;
            type b is array (1 .. 20) of m;
         end Subp;
      end Definitions;

  A package that `with`'s `Definitions` may safely instantiate `Definitions.Subp` because the compiler can determine that there definitely is no package body to worry about in this case
pragma Pure
  Places sufficient restrictions on a unit to guarantee that no call to any subprogram in the unit can result in an elaboration problem. This means that the compiler does not need to worry about the point of elaboration of such units, and in particular, does not need to check any calls to any subprograms in this unit.
pragma Preelaborate
  This pragma places slightly less stringent restrictions on a unit than does pragma Pure, but these restrictions are still sufficient to ensure that there are no elaboration problems with any calls to the unit.
pragma Elaborate_Body
  This pragma requires that the body of a unit be elaborated immediately after its spec. Suppose a unit `A` has such a pragma, and unit `B` does a `with` of unit `A`. Recall that the standard rules require the spec of unit `A` to be elaborated before the `with`'ing unit; given the pragma in `A`, we also know that the body of `A` will be elaborated before `B`, so that calls to `A` are safe and do not need a check.

Note that, unlike pragma `Pure` and pragma `Preelaborate`, the use of `Elaborate_Body` does not guarantee that the program is free of elaboration problems, because it may not be possible to satisfy the requested elaboration order. Let's go back to the example with `Unit_1` and `Unit_2`. If a programmer marks `Unit_1` as `Elaborate_Body`, and not `Unit_2,` then the order of elaboration will be:

    Spec of Unit_2
    Spec of Unit_1
    Body of Unit_1
    Body of Unit_2

Now that means that the call to `Func_1` in `Unit_2` need not be checked, it must be safe. But the call to `Func_2` in `Unit_1` may still fail if `Expression_1` is equal to 1, and the programmer must still take responsibility for this not being the case.

If all units carry a pragma `Elaborate_Body`, then all problems are eliminated, except for calls entirely within a body, which are in any case fully under programmer control. However, using the pragma everywhere is not always possible. In particular, for our `Unit_1`/`Unit_2` example, if we marked both of them as having pragma `Elaborate_Body`, then clearly there would be no possible elaboration order.

The above pragmas allow a server to guarantee safe use by clients, and clearly this is the preferable approach. Consequently a good rule in Ada 95 is to mark units as `Pure` or `Preelaborate` if possible, and if this is not possible, mark them as `Elaborate_Body` if possible. As we have seen, there are situations where neither of these three pragmas can be used. So we also provide methods for clients to control the order of elaboration of the servers on which they depend:

pragma Elaborate (unit)
  This pragma is placed in the context clause, after a `with` statement, and it requires that the body of the named unit be elaborated before the unit in which the pragma occurs. The idea is to use this pragma if the current unit calls at elaboration time, directly or indirectly, some subprogram in the named unit.
pragma Elaborate_All (unit)
  This is a stronger version of the Elaborate pragma. Consider the following example:

      Unit A with's unit B and calls B.Func in elab code
      Unit B with's unit C, and B.Func calls C.Func

  Now if we put a pragma `Elaborate (B)` in unit `A`, this ensures that the body of `B` is elaborated before the call, but not the body of `C`, so the call to `C.Func` could still cause `Program_Error` to be raised. The effect of a pragma `Elaborate_All` is stronger, it requires not only that the body of the named unit be elaborated before the unit doing the `with`, but also the bodies of all units that the named unit uses, following `with` links transitively. For example, if we put a pragma `Elaborate_All (B)` in unit `A`, then it requires not only that the body of `B` be elaborated before `A`, but also the body of `C`, because `B` `with`'s `C`.

We are now in a position to give a usage rule in Ada 95 for avoiding elaboration problems, at least if dynamic dispatching and access to subprogram values are not used. We will handle these cases separately later.

The rule is simple. If a unit has elaboration code that can directly or indirectly make a call to a subprogram in a `with`'ed unit, or instantiate a generic unit in a `with`'ed unit, then if the `with`'ed unit does not have pragma Pure, Preelaborate, or Elaborate_Body, then the client should have an Elaborate_All for the `with`'ed unit. By following this rule a client is assured that calls can be made without risk of an exception. If this rule is not followed, then a program may be in one of four states:

No order exists
  No order of elaboration exists which follows the rules, taking into account any Elaborate, Elaborate_All, or Elaborate_Body pragmas. In this case, an Ada 95 compiler must diagnose the situation at bind time, and refuse to build an executable program.
One or more orders exist, all incorrect
  One or more acceptable elaboration orders exists, and all of them generate an elaboration order problem. In this case, the binder can build an executable program, but `Program_Error` will be raised when the program is run.
Several orders exist, some right, some incorrect
  One or more acceptable elaboration orders exists, and some of them work, and some do not. The programmer has not controlled the order of elaboration, so the binder may or may not pick one of the correct orders, and the program may or may not raise an exception when it is run. This is the worst case, because it means that the program may fail when moved to another compiler, or even another version of the same compiler.
One or more orders exists, all correct
  One ore more acceptable elaboration orders exist, and all of them work. In this case the program runs successfully. This state of affairs can be guaranteed by following the rule we gave above, but may be true even if the rule is not followed.

Note that one additional advantage of following our Elaborate_All rule is that the program continues to stay in the ideal (all orders OK) state even if maintenance changes some bodies of some subprograms. Conversely, if a program that does not follow this rule happens to be safe at some point, this state of affairs may deteriorate silently as a result of maintenance changes.

## Controlling Elaboration in GNAT - Internal Calls

In the case of internal calls, i.e. calls within a single package, the programmer has full control over the order of elaboration, and it is up to the programmer to elaborate declarations in an appropriate order. For example writing:

    function One return Float;

    Q : Float := One;

    function One return Float is
    begin
         return 1.0;
    end One;

will obviously raise `Program_Error` at run time, because function One will be called before its body is elaborated. In this case GNAT will generate a warning that the call will raise `Program_Error`:

     1. procedure y is
     2.    function One return Float;
     3.
     4.    Q : Float := One;
                        |
        >>> warning: cannot call "One" before body is elaborated
        >>> warning: Program_Error will be raised at run time

     5.
     6.    function One return Float is
     7.    begin
     8.         return 1.0;
     9.    end One;
    10.
    11. begin
    12.    null;
    13. end;

Note that in this particular case, it is likely that the call is safe, because the function `One` does not access any global variables. Nevertheless in Ada 95, we do not want the validity of the check to depend on the contents of the body (think about the separate compilation case), so this is still wrong, as we discussed in the previous sections.

The error is easily corrected by rearranging the declarations so that the body of One appears before the declaration containing the call (note that in Ada 95, declarations can appear in any order, so there is no restriction that would prevent this reordering, and if we write:

    function One return Float;

    function One return Float is
    begin
         return 1.0;
    end One;

    Q : Float := One;

then all is well, no warning is generated, and no `Program_Error` exception will be raised. Things are more complicated when a chain of subprograms is executed:

    function A return Integer;
    function B return Integer;
    function C return Integer;

    function B return Integer is begin return A; end;
    function C return Integer is begin return B; end;

    X : Integer := C;

    function A return Integer is begin return 1; end;

Now the call to `C` at elaboration time in the declaration of `X` is correct, because the body of `C` is already elaborated, and the call to `B` within the body of `C` is correct, but the call to `A` within the body of `B` is incorrect, because the body of `A` has not been elaborated, so `Program_Error` will be raised on the call to `A`. In this case GNAT will generate a warning that `Program_Error` may be raised at the point of the call. Let's look at the warning:

     1. procedure x is
     2.    function A return Integer;
     3.    function B return Integer;
     4.    function C return Integer;
     5.
     6.    function B return Integer is begin return A; end;
                                                        |
        >>> warning: call to "A" before body is elaborated may
                     raise Program_Error
        >>> warning: "B" called at line 7
        >>> warning: "C" called at line 9

     7.    function C return Integer is begin return B; end;
     8.
     9.    X : Integer := C;
    10.
    11.    function A return Integer is begin return 1; end;
    12.
    13. begin
    14.    null;
    15. end;

Note that the message here says "may raise", instead of the direct case, where the message says "will be raised". That's because whether `A` is actually called depends in general on run-time flow of control. For example, if the body of `B` said

    function B return Integer is
    begin
       if some-condition-depending-on-input-data then
          return A;
       else
          return 1;
       end if;
    end B;

then we could not know until run time whether the incorrect call to A would actually occur, so `Program_Error` might or might not be raised. It is possible for a compiler to do a better job of analyzing bodies, to determine whether or not `Program_Error` might be raised, but it certainly couldn't do a perfect job (that would require solving the halting problem and is provably impossible), and because this is a warning anyway, it does not seem worth the effort to do the analysis. Cases in which it would be relevant are rare.

In practice, warnings of either of the forms given above will usually correspond to real errors, and should be examined carefully and eliminated. In the rare case where a warning is bogus, it can be suppressed by any of the following methods:

- Compile with the `-gnatws` switch set
- Suppress `Elaboration_Checks` for the called subprogram
- Use pragma `Warnings_Off` to turn warnings off for the call

For the internal elaboration check case, GNAT by default generates the necessary run-time checks to ensure that `Program_Error` is raised if any call fails an elaboration check. Of course this can only happen if a warning has been issued as described above. The use of pragma `Suppress (Elaboration_Checks)` may (but is not guaranteed) to suppress some of these checks, meaning that it may be possible (but is not guaranteed) for a program to be able to call a subprogram whose body is not yet elaborated, without raising a `Program_Error` exception.

## Controlling Elaboration in GNAT - External Calls

The previous section discussed the case in which the execution of a particular thread of elaboration code occurred entirely within a single unit. This is the easy case to handle, because a programmer has direct and total control over the order of elaboration, and furthermore, checks need only be generated in cases which are rare and which the compiler can easily detect. The situation is more complex when separate compilation is taken into account. Consider the following:

    package Math is
       function Sqrt (Arg : Float) return Float;
    end Math;

    package body Math is
       function Sqrt (Arg : Float) return Float is
       begin
             ...
       end Sqrt;
    end Math;
    with Math;
    package Stuff is
       X : Float := Math.Sqrt (0.5);
    end Stuff;

    with Stuff;
    procedure Main is
    begin
       ...
    end Main;

where `Main` is the main program. When this program is executed, the elaboration code must first be executed, and one of the jobs of the binder is to determine the order in which the units of a program are to be elaborated. In this case we have four units: the spec and body of `Math`, the spec of `Stuff` and the body of `Main`). In what order should the four separate sections of elaboration code be executed?

There are some restrictions in the order of elaboration that the binder can choose. In particular, if unit U has a `with` for a package `X`, then you are assured that the spec of `X` is elaborated before U , but you are not assured that the body of `X` is elaborated before U. This means that in the above case, the binder is allowed to choose the order:

    spec of Math
    spec of Stuff
    body of Math
    body of Main

but that's not good, because now the call to `Math.Sqrt` that happens during the elaboration of the `Stuff` spec happens before the body of `Math.Sqrt` is elaborated, and hence causes `Program_Error` exception to be raised. At first glance, one might say that the binder is misbehaving, because obviously you want to elaborate the body of something you `with` first, but that is not a general rule that can be followed in all cases. Consider

    package X is ...

    package Y is ...

    with X;
    package body Y is ...

    with Y;
    package body X is ...

This is a common arrangement, and, apart from the order of elaboration problems that might arise in connection with elaboration code, this works fine. A rule that says that you must first elaborate the body of anything you `with` cannot work in this case (the body of `X` `with`'s `Y`, which means you would have to elaborate the body of `Y` first, but that `with`'s `X`, which means you have to elaborate the body of `X` first, but ... and we have a loop that cannot be broken.

It is true that the binder can in many cases guess an order of elaboration that is unlikely to cause a `Program_Error` exception to be raised, and it tries to do so (in the above example of `Math/Stuff/Spec`, the GNAT binder will in fact always elaborate the body of `Math` right after its spec, so all will be well).

However, a program that blindly relies on the binder to be helpful can get into trouble, as we discussed in the previous sections, so GNAT provides a number of facilities for assisting the programmer in developing programs that are robust with respect to elaboration order.

## Default Behavior in GNAT - Ensuring Safety

The default behavior in GNAT ensures elaboration safety. In its default mode GNAT implements the rule we previously described as the right approach. Let's restate it:

If a unit has elaboration code that can directly or indirectly make a call to a subprogram in a `with`'ed unit, or instantiate a generic unit in a `with`'ed unit, then if the `with`'ed unit does not have pragma `Pure`, `Preelaborate`, or `Elaborate_Body`, then the client should have an `Elaborate_All` for the `with`'ed unit. By following this rule a client is assured that calls and instantiations can be made without risk of an exception.

In this mode GNAT traces all calls that are potentially made from elaboration code, and puts in any missing implicit `Elaborate_All` pragmas. The advantage of this approach is that no elaboration problems are possible if the binder can find an elaboration order that is consistent with these implicit `Elaborate_All` pragmas. The disadvantage of this approach is that no such order may exist.

If the binder does not generate any diagnostics, then it means that it has found an elaboration order that is guaranteed to be safe. However, the binder may still be relying on implicitly generated `Elaborate_All` pragmas so portability to other compilers than GNAT is not guaranteed.

If it is important to guarantee portability, then the compilations should use the `-gnatwl` (warn on elaboration problems) switch. This will cause warning messages to be generated indicating the missing `Elaborate_All` pragmas. Consider the following source program:

    with k;
    package j is
      m : integer := k.r;
    end;

where it is clear that there should be a pragma `Elaborate_All` for unit `k`. An implicit pragma will be generated, and it is likely that the binder will be able to honor it. However, it is safer to include the pragma explicitly in the source. If this unit is compiled with the `-gnatwl` switch, then the compiler outputs a warning:

    1. with k;
    2. package j is
    3.   m : integer := k.r;
                         |
       >>> warning: call to "r" may raise Program_Error
       >>> warning: missing pragma Elaborate_All for "k"

    4. end;

and these warnings can be used as a guide for supplying manually the missing pragmas.

This default mode is not strictly compatible with the Ada Reference Manual, and it is possible to construct programs which will compile using the dynamic model described there, but will run into a circularity using the safer static model we have described.

Of course any Ada compiler must be able to operate in a mode consistent with the requirements of the Ada Reference Manual, and in particular must have the capability of implementing the standard dynamic model of elaboration with run-time checks.

In GNAT, this standard mode can be achieved either by the use of the `-gnatE` switch on the compiler (`gcc` or `gnatmake`) command, or by the use of the configuration pragma:

    pragma Elaboration_Checks (RM);

Either approach will cause the unit affected to be compiled using the standard dynamic run-time elaboration checks described in the Ada Reference Manual. The static model is generally preferable, since it is clearly safer to rely on compile and link time checks rather than run-time checks. However, in the case of legacy code, it may be difficult to meet the requirements of the static model. This is issue is further discussed in the section "What to do if the Default Elaboration Behavior Fails".

Note that the static model provides a strict subset of the allowed behavior and programs of the Ada Reference Manual, so if you do adhere to the static model, you are assured that your program will work using the dynamic model.

## Elaboration Issues for Library Tasks

In this section we examine special elaboration issues that arise for programs that declare library level tasks.

Generally the model of execution of an Ada program is that all units are elaborated, and then execution of the program starts. However, the declaration of library tasks definitely does not fit this model. The reason for this is that library tasks start as soon as they are declared (more precisely, as soon as the statement part of the enclosing package body is reached), that is to say before elaboration of the program is complete. This means that if such a task calls a subprogram, or an entry in another task, the callee may or may not be elaborated yet, and in the standard Reference Manual model of dynamic elaboration checks, you can even get timing dependent Program_Error exceptions, since there can be a race between the elaboration code and the task code.

The static model of elaboration in GNAT seeks to avoid all such dynamic behavior, by being conservative, and the conservative approach in this particular case is to assume that all the code in a task body is potentially executed at elaboration time if a task is declared at the library level.

This can definitely result in unexpected circularities. Consider the following example

    package Decls is
      task Lib_Task is
         entry Start;
      end Lib_Task;

      type My_Int is new Integer;

      function Ident (M : My_Int) return My_Int;
    end Decls;

    with Utils;
    package body Decls is
      task body Lib_Task is
      begin
         accept Start;
         Utils.Put_Val (2);
      end Lib_Task;

      function Ident (M : My_Int) return My_Int is
      begin
         return M;
      end Ident;
    end Decls;

    with Decls;
    package Utils is
      procedure Put_Val (Arg : Decls.My_Int);
    end Utils;

    with Text_IO;
    package body Utils is
      procedure Put_Val (Arg : Decls.My_Int) is
      begin
         Text_IO.Put_Line (Decls.My_Int'Image (Decls.Ident (Arg)));
      end Put_Val;
    end Utils;

    with Decls;
    procedure Main is
    begin
       Decls.Lib_Task.Start;
    end;

If the above example is compiled in the default static elaboration mode, then a circularity occurs. The circularity comes from the call `Utils.Put_Val` in the task body of `Decls.Lib_Task`. Since this call occurs in elaboration code, we need an implicit pragma `Elaborate_All` for `Utils`. This means that not only must the spec and body of `Utils` be elaborated before the body of `Decls`, but also the spec and body of any unit that is `with'ed` by the body of `Utils` must also be elaborated before the body of `Decls`. This is the transitive implication of pragma `Elaborate_All` and it makes sense, because in general the body of `Put_Val` might have a call to something in a `with'ed` unit.

In this case, the body of Utils (actually its spec) `with's` `Decls`. Unfortunately this means that the body of `Decls` must be elaborated before itself, in case there is a call from the body of `Utils`.

Here is the exact chain of events we are worrying about:

1. In the body of `Decls` a call is made from within the body of a library task to a subprogram in the package `Utils`. Since this call may occur at elaboration time (given that the task is activated at elaboration time), we have to assume the worst, i.e. that the call does happen at elaboration time.
2. This means that the body and spec of `Util` must be elaborated before the body of `Decls` so that this call does not cause an access before elaboration.
3. Within the body of `Util`, specifically within the body of `Util.Put_Val` there may be calls to any unit `with`'ed by this package.
4. One such `with`'ed package is package `Decls`, so there might be a call to a subprogram in `Decls` in `Put_Val`. In fact there is such a call in this example, but we would have to assume that there was such a call even if it were not there, since we are not supposed to write the body of `Decls` knowing what is in the body of `Utils`; certainly in the case of the static elaboration model, the compiler does not know what is in other bodies and must assume the worst.
5. This means that the spec and body of `Decls` must also be elaborated before we elaborate the unit containing the call, but that unit is `Decls`! This means that the body of `Decls` must be elaborated before itself, and that's a circularity.

Indeed, if you add an explicit pragma Elaborate_All for `Utils` in the body of `Decls` you will get a true Ada Reference Manual circularity that makes the program illegal.

In practice, we have found that problems with the static model of elaboration in existing code often arise from library tasks, so we must address this particular situation.

Note that if we compile and run the program above, using the dynamic model of elaboration (that is to say use the `-gnatE` switch), then it compiles, binds, links, and runs, printing the expected result of 2. Therefore in some sense the circularity here is only apparent, and we need to capture the properties of this program that distinguish it from other library-level tasks that have real elaboration problems.

We have four possible answers to this question:

- Use the dynamic model of elaboration. If we use the `-gnatE` switch, then as noted above, the program works. Why is this? If we examine the task body, it is apparent that the task cannot proceed past the `accept` statement until after elaboration has been completed, because the corresponding entry call comes from the main program, not earlier. This is why the dynamic model works here. But that's really giving up on a precise analysis, and we prefer to take this approach only if we cannot solve the problem in any other manner. So let us examine two ways to reorganize the program to avoid the potential elaboration problem.
- Split library tasks into separate packages. Write separate packages, so that library tasks are isolated from other declarations as much as possible. Let us look at a variation on the above program.

      package Decls1 is
        task Lib_Task is
           entry Start;
        end Lib_Task;
      end Decls1;

      with Utils;
      package body Decls1 is
        task body Lib_Task is
        begin
           accept Start;
           Utils.Put_Val (2);
        end Lib_Task;
      end Decls1;

      package Decls2 is
        type My_Int is new Integer;
        function Ident (M : My_Int) return My_Int;
      end Decls2;

      with Utils;
      package body Decls2 is
        function Ident (M : My_Int) return My_Int is
        begin
           return M;
        end Ident;
      end Decls2;

      with Decls2;
      package Utils is
        procedure Put_Val (Arg : Decls2.My_Int);
      end Utils;

      with Text_IO;
      package body Utils is
        procedure Put_Val (Arg : Decls2.My_Int) is
        begin
           Text_IO.Put_Line (Decls2.My_Int'Image (Decls2.Ident (Arg)));
        end Put_Val;
      end Utils;

      with Decls1;
      procedure Main is
      begin
         Decls1.Lib_Task.Start;
      end;

  All we have done is to split `Decls` into two packages, one containing the library task, and one containing everything else. Now there is no cycle, and the program compiles, binds, links and executes using the default static model of elaboration.
- Declare separate task types. A significant part of the problem arises because of the use of the single task declaration form. This means that the elaboration of the task type, and the elaboration of the task itself (i.e. the creation of the task) happen at the same time. A good rule of style in Ada 95 is to always create explicit task types. By following the additional step of placing task objects in separate packages from the task type declaration, many elaboration problems are avoided. Here is another modified example of the example program:

      package Decls is
        task type Lib_Task_Type is
           entry Start;
        end Lib_Task_Type;

        type My_Int is new Integer;

        function Ident (M : My_Int) return My_Int;
      end Decls;

      with Utils;
      package body Decls is
        task body Lib_Task_Type is
        begin
           accept Start;
           Utils.Put_Val (2);
        end Lib_Task_Type;

        function Ident (M : My_Int) return My_Int is
        begin
           return M;
        end Ident;
      end Decls;

      with Decls;
      package Utils is
        procedure Put_Val (Arg : Decls.My_Int);
      end Utils;

      with Text_IO;
      package body Utils is
        procedure Put_Val (Arg : Decls.My_Int) is
        begin
           Text_IO.Put_Line (Decls.My_Int'Image (Decls.Ident (Arg)));
        end Put_Val;
      end Utils;

      with Decls;
      package Declst is
         Lib_Task : Decls.Lib_Task_Type;
      end Declst;

      with Declst;
      procedure Main is
      begin
         Declst.Lib_Task.Start;
      end;

  What we have done here is to replace the `task` declaration in package `Decls` with a `task type` declaration. Then we introduce a separate package `Declst` to contain the actual task object. This separates the elaboration issues for the `task type` declaration, which causes no trouble, from the elaboration issues of the task object, which is also unproblematic, since it is now independent of the elaboration of `Utils`. This separation of concerns also corresponds to a generally sound engineering principle of separating declarations from instances. This version of the program also compiles, binds, links, and executes, generating the expected output.
- Use No_Entry_Calls_In_Elaboration_Code restriction. The previous two approaches described how a program can be restructured to avoid the special problems caused by library task bodies. in practice, however, such restructuring may be difficult to apply to existing legacy code, so we must consider solutions that do not require massive rewriting. Let us consider more carefully why our original sample program works under the dynamic model of elaboration. The reason is that the code in the task body blocks immediately on the `accept` statement. Now of course there is nothing to prohibit elaboration code from making entry calls (for example from another library level task), so we cannot tell in isolation that the task will not execute the accept statement during elaboration. However, in practice it is very unusual to see elaboration code make any entry calls, and the pattern of tasks starting at elaboration time and then immediately blocking on `accept` or `select` statements is very common. What this means is that the compiler is being too pessimistic when it analyzes the whole package body as though it might be executed at elaboration time. If we know that the elaboration code contains no entry calls, (a very safe assumption most of the time, that could almost be made the default behavior), then we can compile all units of the program under control of the following configuration pragma:

      pragma Restrictions (No_Entry_Calls_In_Elaboration_Code);

  This pragma can be placed in the ``gnat.adc'` file in the usual manner. If we take our original unmodified program and compile it in the presence of a ``gnat.adc'` containing the above pragma, then once again, we can compile, bind, link, and execute, obtaining the expected result. In the presence of this pragma, the compiler does not trace calls in a task body, that appear after the first `accept` or `select` statement, and therefore does not report a potential circularity in the original program. The compiler will check to the extent it can that the above restriction is not violated, but it is not always possible to do a complete check at compile time, so it is important to use this pragma only if the stated restriction is in fact met, that is to say no task receives an entry call before elaboration of all units is completed.

## Mixing Elaboration Models

So far, we have assumed that the entire program is either compiled using the dynamic model or static model, ensuring consistencty. It is possible to mix the two models, but rules have to be followed if this mixing is done to ensure that elaboration checks are not omitted.

The basic rule is that a unit compiled with the static model cannot be `with'ed` by a unit compiled with the dynamic model. The reason for this is that in the static model, a unit assumes that its clients guarantee to use (the equivalent of) pragma `Elaborate_All` so that no elaboration checks are required in inner subprograms, and this assumption is violated if the client is compiled with dynamic checks.

The precise rule is as follows. A unit that is compiled with dynamic checks can only `with` a unit that meets at least one of the following criteria:

- The `with'ed` unit is itself compiled with dynamic elaboration checks (that is with the `-gnatE` switch.
- The `with'ed` unit is an internal GNAT implementation unit from the System, Interfaces, Ada, or GNAT hierarchies.
- The `with'ed` unit has pragma Preelaborate or pragma Pure.
- The `with'ing` unit (that is the client) has an explicit pragma `Elaborate_All` for the `with'ed` unit.

If this rule is violated, that is if a unit with dynamic elaboration checks `with's` a unit that does not meet one of the above four criteria, then the binder (`gnatbind`) will issue a warning similar to that in the following example:

    warning: "x.ads" has dynamic elaboration checks and with's
    warning:   "y.ads" which has static elaboration checks

These warnings indicate that the rule has been violated, and that as a result elaboration checks may be missed in the resulting executable file. This warning may be suppressed using the `-ws` binder switch in the usual manner.

One useful application of this mixing rule is in the case of a subsystem which does not itself `with` units from the remainder of the application. In this case, the entire subsystem can be compiled with dynamic checks to resolve a circularity in the subsystem, without allowing the main application that uses this subsystem to be compiled using the more reliable default static model.

## What to Do If the Default Elaboration Behavior Fails

If the binder cannot find an acceptable order, it outputs detailed diagnostics. For example:

    @leftskip=0cm
    error: elaboration circularity detected
    info:   "proc (body)" must be elaborated before "pack (body)"
    info:     reason: Elaborate_All probably needed in unit "pack (body)"
    info:     recompile "pack (body)" with -gnatwl
    info:                             for full details
    info:       "proc (body)"
    info:         is needed by its spec:
    info:       "proc (spec)"
    info:         which is withed by:
    info:       "pack (body)"
    info:  "pack (body)" must be elaborated before "proc (body)"
    info:     reason: pragma Elaborate in unit "proc (body)"

In this case we have a cycle that the binder cannot break. On the one hand, there is an explicit pragma Elaborate in `proc` for `pack`. This means that the body of `pack` must be elaborated before the body of `proc`. On the other hand, there is elaboration code in `pack` that calls a subprogram in `proc`. This means that for maximum safety, there should really be a pragma Elaborate_All in `pack` for `proc` which would require that the body of `proc` be elaborated before the body of `pack`. Clearly both requirements cannot be satisfied. Faced with a circularity of this kind, you have three different options.

Fix the program
  The most desirable option from the point of view of long-term maintenance is to rearrange the program so that the elaboration problems are avoided. One useful technique is to place the elaboration code into separate child packages. Another is to move some of the initialization code to explicitly called subprograms, where the program controls the order of initialization explicitly. Although this is the most desirable option, it may be impractical and involve too much modification, especially in the case of complex legacy code.
Perform dynamic checks
  If the compilations are done using the `-gnatE` (dynamic elaboration check) switch, then GNAT behaves in a quite different manner. Dynamic checks are generated for all calls that could possibly result in raising an exception. With this switch, the compiler does not generate implicit `Elaborate_All` pragmas. The behavior then is exactly as specified in the Ada 95 Reference Manual. The binder will generate an executable program that may or may not raise `Program_Error`, and then it is the programmer's job to ensure that it does not raise an exception. Note that it is important to compile all units with the switch, it cannot be used selectively.
Suppress checks
  The drawback of dynamic checks is that they generate a significant overhead at run time, both in space and time. If you are absolutely sure that your program cannot raise any elaboration exceptions, then you can use the `-f` switch for the `gnatbind` step, or `-bargs -f` if you are using `gnatmake`. This switch tells the binder to ignore any implicit `Elaborate_All` pragmas that were generated by the compiler, and suppresses any circularity messages that they cause. The resulting executable will work properly if there are no elaboration problems, but if there are some order of elaboration problems they will not be detected, and unexpected results may occur.

It is hard to generalize on which of these three approaches should be taken. Obviously if it is possible to fix the program so that the default treatment works, this is preferable, but this may not always be practical. It is certainly simple enough to use `-gnatE` or `-f` but the danger in either case is that, even if the GNAT binder finds a correct elaboration order, it may not always do so, and certainly a binder from another Ada compiler might not. A combination of testing and analysis (for which the warnings generated with the `-gnatwl` switch can be useful) must be used to ensure that the program is free of errors. One switch that is useful in this testing is the `-h (horrible elaboration order)` switch for `gnatbind`. Normally the binder tries to find an order that has the best chance of of avoiding elaboration problems. With this switch, the binder plays a devil's advocate role, and tries to choose the order that has the best chance of failing. If your program works even with this switch, then it has a better chance of being error free, but this is still not a guarantee.

For an example of this approach in action, consider the C-tests (executable tests) from the ACVC suite. If these are compiled and run with the default treatment, then all but one of them succeed without generating any error diagnostics from the binder. However, there is one test that fails, and this is not surprising, because the whole point of this test is to ensure that the compiler can handle cases where it is impossible to determine a correct order statically, and it checks that an exception is indeed raised at run time.

This one test must be compiled and run using the `-gnatE` switch, and then it passes. Alternatively, the entire suite can be run using this switch. It is never wrong to run with the dynamic elaboration switch if your code is correct, and we assume that the C-tests are indeed correct (it is less efficient, but efficiency is not a factor in running the ACVC tests.)

## Elaboration for Access-to-Subprogram Values

The introduction of access-to-subprogram types in Ada 95 complicates the handling of elaboration. The trouble is that it becomes impossible to tell at compile time which procedure is being called. This means that it is not possible for the binder to analyze the elaboration requirements in this case.

If at the point at which the access value is created, the body of the subprogram is known to have been elaborated, then the access value is safe, and its use does not require a check. This may be achieved by appropriate arrangement of the order of declarations if the subprogram is in the current unit, or, if the subprogram is in another unit, by using pragma `Pure`, `Preelaborate`, or `Elaborate_Body` on the referenced unit.

If the referenced body is not known to have been elaborated at the point the access value is created, then any use of the access value must do a dynamic check, and this dynamic check will fail and raise a `Program_Error` exception if the body has not been elaborated yet. GNAT will generate the necessary checks, and in addition, if the `-gnatwl` switch is set, will generate warnings that such checks are required.

The use of dynamic dispatching for tagged types similarly generates a requirement for dynamic checks, and premature calls to any primitive operation of a tagged type before the body of the operation has been elaborated, will result in the raising of `Program_Error`.

## Summary of Procedures for Elaboration Control

First, compile your program with the default options, using none of the special elaboration control switches. If the binder successfully binds your program, then you can be confident that, apart from issues raised by the use of access-to-subprogram types and dynamic dispatching, the program is free of elaboration errors. If it is important that the program be portable, then use the `-gnatwl` switch to generate warnings about missing `Elaborate_All` pragmas, and supply the missing pragmas.

If the program fails to bind using the default static elaboration handling, then you can fix the program to eliminate the binder message, or recompile the entire program with the `-gnatE` switch to generate dynamic elaboration checks, or, if you are sure there really are no elaboration problems, use the `-f` switch for the binder to cause it to ignore implicit `Elaborate_All` pragmas generated by the compiler.

## Other Elaboration Order Considerations

This section has been entirely concerned with the issue of finding a valid elaboration order, as defined by the Ada Reference Manual. In a case where several elaboration orders are valid, the task is to find one of the possible valid elaboration orders (and the static model in GNAT will ensure that this is achieved).

The purpose of the elaboration rules in the Ada Reference Manual is to make sure that no entity is accessed before it has been elaborated. For a subprogram, this means that the spec and body must have been elaborated before the subprogram is called. For an object, this means that the object must have been elaborated before its value is read or written. A violation of either of these two requirements is an access before elaboration order, and this section has been all about avoiding such errors.

In the case where more than one order of elaboration is possible, in the sense that access before elaboration errors are avoided, then any one of the orders is "correct" in the sense that it meets the requirements of the Ada Reference Manual, and no such error occurs.

However, it may be the case for a given program, that there are constraints on the order of elaboration that come not from consideration of avoiding elaboration errors, but rather from extra-lingual logic requirements. Consider this example:

    with Init_Constants;
    package Constants is
       X : Integer := 0;
       Y : Integer := 0;
    end Constants;

    package Init_Constants is
       procedure Calc;
    end Init_Constants;

    with Constants;
    package body Init_Constants is
       procedure Calc is begin null; end;
    begin
       Constants.X := 3;
       Constants.Y := 4;
    end Init_Constants;

    with Constants;
    package Calc is
       Z : Integer := Constants.X + Constants.Y;
    end Calc;

    with Calc;
    with Text_IO; use Text_IO;
    procedure Main is
    begin
       Put_Line (Calc.Z'Img);
    end Main;

In this example, there is more than one valid order of elaboration. For example both the following are correct orders:

    Init_Constants spec
    Constants spec
    Calc spec
    Main body
    Init_Constants body

      and

    Init_Constants spec
    Init_Constants body
    Constants spec
    Calc spec
    Main body

There is no language rule to prefer one or the other, both are correct from an order of elaboration point of view. But the programmatic effects of the two orders are very different. In the first, the elaboration routine of `Calc` initializes `Z` to zero, and then the main program runs with this value of zero. But in the second order, the elaboration routine of `Calc` runs after the body of Init_Constants has set `X` and `Y` and thus `Z` is set to 7 before `Main` runs.

One could perhaps by applying pretty clever non-artificial intelligence to the situation guess that it is more likely that the second order of elaboration is the one desired, but there is no formal linguistic reason to prefer one over the other. In fact in this particular case, GNAT will prefer the second order, because of the rule that bodies are elaborated as soon as possible, but it's just luck that this is what was wanted (if indeed the second order was preferred).

If the program cares about the order of elaboration routines in a case like this, it is important to specify the order required. In this particular case, that could have been achieved by adding to the spec of Calc:

    pragma Elaborate_All (Constants);

which requires that the body (if any) and spec of `Constants`, as well as the body and spec of any unit `with`'ed by `Constants` be elaborated before `Calc` is elaborated.

Clearly no automatic method can always guess which alternative you require, and if you are working with legacy code that had constraints of this kind which were not properly specified by adding `Elaborate` or `Elaborate_All` pragmas, then indeed it is possible that two different compilers can choose different orders.

The `gnatbind` `-p` switch may be useful in smoking out problems. This switch causes bodies to be elaborated as late as possible instead of as early as possible. In the example above, it would have forced the choice of the first elaboration order. If you get different results when using this switch, and particularly if one set of results is right, and one is wrong as far as you are concerned, it shows that you have some missing `Elaborate` pragmas. For the example above, we have the following output:

    gnatmake -f -q main
    main
     7
    gnatmake -f -q main -bargs -p
    main
     0

It is of course quite unlikely that both these results are correct, so it is up to you in a case like this to investigate the source of the difference, by looking at the two elaboration orders that are chosen, and figuring out which is correct, and then adding the necessary `Elaborate_All` pragmas to ensure the desired order.

# The Cross-Referencing Tools `gnatxref` and `gnatfind`

The compiler generates cross-referencing information (unless you set the ``-gnatx'` switch), which are saved in the ``.ali'` files. This information indicates where in the source each entity is declared and referenced.

Before using any of these two tools, you need to compile successfully your application, so that GNAT gets a chance to generate the cross-referencing information.

The two tools `gnatxref` and `gnatfind` take advantage of this information to provide the user with the capability to easily locate the declaration and references to an entity. These tools are quite similar, the difference being that `gnatfind` is intended for locating definitions and/or references to a specified entity or entities, whereas `gnatxref` is oriented to generating a full report of all cross-references.

To use these tools, you must not compile your application using the ``-gnatx'` switch on the ``gnatmake'` command line (See Info file `gnat_ug', node `The GNAT Make Program gnatmake'). Otherwise, cross-referencing information will not be generated.

- gnatxref Switches
- gnatfind Switches
- Project Files
- Regular Expressions in gnatfind and gnatxref
- Examples of gnatxref Usage
- Examples of gnatfind Usage

## `gnatxref` Switches

The command lines for `gnatxref` is:

    $ gnatxref [switches] sourcefile1 [sourcefile2 ...]

where

`sourcefile1, sourcefile2`
  identifies the source files for which a report is to be generated. The 'with'ed units will be processed too. You must provide at least one file. These file names are considered to be regular expressions, so for instance specifying 'source*.adb' is the same as giving every file in the current directory whose name starts with 'source' and whose extension is 'adb'.

The switches can be :

`-a`
  If this switch is present, `gnatfind` and `gnatxref` will parse the read-only files found in the library search path. Otherwise, these files will be ignored. This option can be used to protect Gnat sources or your own libraries from being parsed, thus making `gnatfind` and `gnatxref` much faster, and their output much smaller.
`-aIDIR`
  When looking for source files also look in directory DIR. The order in which source file search is undertaken is the same as for ``gnatmake'`.
`-aODIR`
  When searching for library and object files, look in directory DIR. The order in which library files are searched is the same as for ``gnatmake'`.
`-d`
  If this switch is set `gnatxref` will output the parent type reference for each matching derived types.
`-f`
  If this switch is set, the output file names will be preceded by their directory (if the file was found in the search path). If this switch is not set, the directory will not be printed.
`-g`
  If this switch is set, information is output only for library-level entities, ignoring local entities. The use of this switch may accelerate `gnatfind` and `gnatxref`.
`-IDIR`
  Equivalent to ``-aODIR -aIDIR'`.
`-pFILE`
  Specify a project file to use See section Project Files. By default, `gnatxref` and `gnatfind` will try to locate a project file in the current directory. If a project file is either specified or found by the tools, then the content of the source directory and object directory lines are added as if they had been specified respectively by ``-aI'` and ``-aO'`.
`-u`
  Output only unused symbols. This may be really useful if you give your main compilation unit on the command line, as `gnatxref` will then display every unused entity and 'with'ed package.
`-v`
  Instead of producing the default output, `gnatxref` will generate a ``tags'` file that can be used by vi. For examples how to use this feature, see See section Examples of `gnatxref` Usage. The tags file is output to the standard output, thus you will have to redirect it to a file.

All these switches may be in any order on the command line, and may even appear after the file names. They need not be separated by spaces, thus you can say ``gnatxref -ag'` instead of ``gnatxref -a -g'`.

## `gnatfind` Switches

The command line for `gnatfind` is:

    $ gnatfind [switches] pattern[:sourcefile[:line[:column]]]
          [file1 file2 ...]

where

`pattern`
  An entity will be output only if it matches the regular expression found in ``pattern'`, see See section Regular Expressions in `gnatfind` and `gnatxref`. Omitting the pattern is equivalent to specifying ``*'`, which will match any entity. Note that if you do not provide a pattern, you have to provide both a sourcefile and a line. Entity names are given in Latin-1, with uppercase/lowercase equivalence for matching purposes. At the current time there is no support for 8-bit codes other than Latin-1, or for wide characters in identifiers.
`sourcefile`
  `gnatfind` will look for references, bodies or declarations of symbols referenced in ``sourcefile'`, at line ``line'` and column ``column'`. See see section Examples of `gnatfind` Usage for syntax examples.
`line`
  is a decimal integer identifying the line number containing the reference to the entity (or entities) to be located.
`column`
  is a decimal integer identifying the exact location on the line of the first character of the identifier for the entity reference. Columns are numbered from 1.
`file1 file2 ...`
  The search will be restricted to these files. If none are given, then the search will be done for every library file in the search path. These file must appear only after the pattern or sourcefile. These file names are considered to be regular expressions, so for instance specifying 'source*.adb' is the same as giving every file in the current directory whose name starts with 'source' and whose extension is 'adb'. Not that if you specify at least one file in this part, `gnatfind` may sometimes not be able to find the body of the subprograms...

At least one of 'sourcefile' or 'pattern' has to be present on the command line.

The following switches are available:

`-a`
  If this switch is present, `gnatfind` and `gnatxref` will parse the read-only files found in the library search path. Otherwise, these files will be ignored. This option can be used to protect Gnat sources or your own libraries from being parsed, thus making `gnatfind` and `gnatxref` much faster, and their output much smaller.
`-aIDIR`
  When looking for source files also look in directory DIR. The order in which source file search is undertaken is the same as for ``gnatmake'`.
`-aODIR`
  When searching for library and object files, look in directory DIR. The order in which library files are searched is the same as for ``gnatmake'`.
`-d`
  If this switch is set, then `gnatfind` will output the parent type reference for each matching derived types.
`-e`
  By default, `gnatfind` accept the simple regular expression set for ``pattern'`. If this switch is set, then the pattern will be considered as full Unix-style regular expression.
`-f`
  If this switch is set, the output file names will be preceded by their directory (if the file was found in the search path). If this switch is not set, the directory will not be printed.
`-g`
  If this switch is set, information is output only for library-level entities, ignoring local entities. The use of this switch may accelerate `gnatfind` and `gnatxref`.
`-IDIR`
  Equivalent to ``-aODIR -aIDIR'`.
`-pFILE`
  Specify a project file (see section Project Files) to use. By default, `gnatxref` and `gnatfind` will try to locate a project file in the current directory. If a project file is either specified or found by the tools, then the content of the source directory and object directory lines are added as if they had been specified respectively by ``-aI'` and ``-aO'`.
`-r`
  By default, `gnatfind` will output only the information about the declaration, body or type completion of the entities. If this switch is set, the `gnatfind` will locate every reference to the entities in the files specified on the command line (or in every file in the search path if no file is given on the command line).
`-s`
  If this switch is set, then `gnatfind` will output the content of the Ada source file lines were the entity was found.
`-t`
  If this switch is set, then `gnatfind` will output the type hierarchy for the specified type. It act like -d option but recursively from parent type to parent type. When this switch is set it is not possible to specify more than one file.

All these switches may be in any order on the command line, and may even appear after the file names. They need not be separated by spaces, thus you can say ``gnatxref -ag'` instead of ``gnatxref -a -g'`.

As stated previously, gnatfind will search in every directory in the search path. You can force it to look only in the current directory if you specify `*` at the end of the command line.

## Project Files

The project files allows a programmer to specify how to compile its application, where to find sources,... These files are used primarily by the Glide Ada mode, but they can also be used by the two tools `gnatxref` and `gnatfind`.

A project file name must end with ``.adp'`. If a single one is present in the current directory, then `gnatxref` and `gnatfind` will extract the information from it. If multiple project files are found, none of them is read, and you have to use the ``-p'` switch to specify the one you want to use.

The following lines can be included, even though most of them have default values which can be used in most cases. The lines can be entered in any order in the file. Except for ``src_dir'` and ``obj_dir'`, you can only have one instance of each line. If you have multiple instances, only the last one is taken into account.

`src_dir=DIR [default: "./"]`
  specifies a directory where to look for source files. Multiple src_dir lines can be specified and they will be searched in the order they are specified.
`obj_dir=DIR [default: "./"]`
  specifies a directory where to look for object and library files. Multiple obj_dir lines can be specified and they will be searched in the order they are specified
`comp_opt=SWITCHES [default: ""]`
  creates a variable which can be referred to subsequently by using the ``${comp_opt}'` notation. This is intended to store the default switches given to ``gnatmake'` and ``gcc'`.
`bind_opt=SWITCHES [default: ""]`
  creates a variable which can be referred to subsequently by using the ``${bind_opt}'` notation. This is intended to store the default switches given to ``gnatbind'`.
`link_opt=SWITCHES [default: ""]`
  creates a variable which can be referred to subsequently by using the ``${link_opt}'` notation. This is intended to store the default switches given to ``gnatlink'`.
`main=EXECUTABLE [default: ""]`
  specifies the name of the executable for the application. This variable can be referred to in the following lines by using the ``${main}'` notation.
`comp_cmd=COMMAND [default: "gcc -c -I${src_dir} -g -gnatq"]`
  specifies the command used to compile a single file in the application.
`make_cmd=COMMAND [default: "gnatmake ${main} -aI${src_dir} -aO${obj_dir} -g -gnatq -cargs ${comp_opt} -bargs ${bind_opt} -largs ${link_opt}"]`
  specifies the command used to recompile the whole application.
`run_cmd=COMMAND [default: "${main}"]`
  specifies the command used to run the application.
`debug_cmd=COMMAND [default: "gdb ${main}"]`
  specifies the command used to debug the application

`gnatxref` and `gnatfind` only take into account the ``src_dir'` and ``obj_dir'` lines, and ignore the others.

## Regular Expressions in `gnatfind` and `gnatxref`

As specified in the section about `gnatfind`, the pattern can be a regular expression. Actually, there are to set of regular expressions which are recognized by the program :

`globbing patterns`
  These are the most usual regular expression. They are the same that you generally used in a Unix shell command line, or in a DOS session. Here is a more formal grammar :

      @leftskip=.5cm
      regexp ::= term
      term   ::= elmt            -- matches elmt
      term   ::= elmt elmt       -- concatenation (elmt then elmt)
      term   ::= *               -- any string of 0 or more characters
      term   ::= ?               -- matches any character
      term   ::= [char {char}] -- matches any character listed
      term   ::= [char - char]   -- matches any character in range

`full regular expression`
  The second set of regular expressions is much more powerful. This is the type of regular expressions recognized by utilities such a ``grep'`. The following is the form of a regular expression, expressed in Ada reference manual style BNF is as follows

      @leftskip=.5cm
      regexp ::= term {| term} -- alternation (term or term ...)

      term ::= item {item}     -- concatenation (item then item)

      item ::= elmt              -- match elmt
      item ::= elmt *            -- zero or more elmt's
      item ::= elmt +            -- one or more elmt's
      item ::= elmt ?            -- matches elmt or nothing
      elmt ::= nschar            -- matches given character
      elmt ::= [nschar {nschar}]   -- matches any character listed
      elmt ::= [^ nschar {nschar}] -- matches any character not listed
      elmt ::= [char - char]     -- matches chars in given range
      elmt ::= \ char            -- matches given character
      elmt ::= .                 -- matches any single character
      elmt ::= ( regexp )        -- parens used for grouping

      char ::= any character, including special characters
      nschar ::= any character except ()[].*+?^

  Following are a few examples :
  `abcde|fghi`
    will match any of the two strings 'abcde' and 'fghi'.
  `abc*d`
    will match any string like 'abd', 'abcd', 'abccd', 'abcccd', and so on
  `[a-z]+`
    will match any string which has only lowercase characters in it (and at least one character

## Examples of `gnatxref` Usage

### General Usage

For the following examples, we will consider the following units :

    main.ads:
    1: with Bar;
    2: package Main is
    3:     procedure Foo (B : in Integer);
    4:     C : Integer;
    5: private
    6:     D : Integer;
    7: end Main;

    main.adb:
    1: package body Main is
    2:     procedure Foo (B : in Integer) is
    3:     begin
    4:        C := B;
    5:        D := B;
    6:        Bar.Print (B);
    7:        Bar.Print (C);
    8:     end Foo;
    9: end Main;

    bar.ads:
    1: package Bar is
    2:     procedure Print (B : Integer);
    3: end bar;

The first thing to do is to recompile your application (for instance, in that case just by doing a ``gnatmake main'`, so that GNAT generates the cross-referencing information. You can then issue any of the following commands:
`gnatxref main.adb`
  `gnatxref` generates cross-reference information for main.adb and every unit 'with'ed by main.adb. The output would be:

      @leftskip=0cm
      B                                                      Type: Integer
        Decl: bar.ads           2:22
      B                                                      Type: Integer
        Decl: main.ads          3:20
        Body: main.adb          2:20
        Ref:  main.adb          4:13     5:13     6:19
      Bar                                                    Type: Unit
        Decl: bar.ads           1:9
        Ref:  main.adb          6:8      7:8
             main.ads           1:6
      C                                                      Type: Integer
        Decl: main.ads          4:5
        Modi: main.adb          4:8
        Ref:  main.adb          7:19
      D                                                      Type: Integer
        Decl: main.ads          6:5
        Modi: main.adb          5:8
      Foo                                                    Type: Unit
        Decl: main.ads          3:15
        Body: main.adb          2:15
      Main                                                    Type: Unit
        Decl: main.ads          2:9
        Body: main.adb          1:14
      Print                                                   Type: Unit
        Decl: bar.ads           2:15
        Ref:  main.adb          6:12     7:12

  that is the entity `Main` is declared in main.ads, line 2, column 9, its body is in main.adb, line 1, column 14 and is not referenced any where. The entity `Print` is declared in bar.ads, line 2, column 15 and it it referenced in main.adb, line 6 column 12 and line 7 column 12.
`gnatxref package1.adb package2.ads`
  `gnatxref` will generates cross-reference information for package1.adb, package2.ads and any other package 'with'ed by any of these.

### Using gnatxref with vi

`gnatxref` can generate a tags file output, which can be used directly from ``vi'`. Note that the standard version of ``vi'` will not work properly with overloaded symbols. Consider using another free implementation of ``vi'`, such as ``vim'`.

    $ gnatxref -v gnatfind.adb > tags

will generate the tags file for `gnatfind` itself (if the sources are in the search path!).

From ``vi'`, you can then use the command ``:tag entity'` (replacing entity by whatever you are looking for), and vi will display a new file with the corresponding declaration of entity.

## Examples of `gnatfind` Usage

`gnatfind -f xyz:main.adb`
  Find declarations for all entities xyz referenced at least once in main.adb. The references are search in every library file in the search path. The directories will be printed as well (as the ``-f'` switch is set) The output will look like:

      directory/main.ads:106:14: xyz <= declaration
      directory/main.adb:24:10: xyz <= body
      directory/foo.ads:45:23: xyz <= declaration

  that is to say, one of the entities xyz found in main.adb is declared at line 12 of main.ads (and its body is in main.adb), and another one is declared at line 45 of foo.ads
`gnatfind -fs xyz:main.adb`
  This is the same command as the previous one, instead `gnatfind` will display the content of the Ada source file lines. The output will look like:

      directory/main.ads:106:14: xyz <= declaration
         procedure xyz;
      directory/main.adb:24:10: xyz <= body
         procedure xyz is
      directory/foo.ads:45:23: xyz <= declaration
         xyz : Integer;

  This can make it easier to find exactly the location your are looking for.
`gnatfind -r "*x*":main.ads:123 foo.adb`
  Find references to all entities containing an x that are referenced on line 123 of main.ads. The references will be searched only in main.adb and foo.adb.
`gnatfind main.ads:123`
  Find declarations and bodies for all entities that are referenced on line 123 of main.ads. This is the same as `gnatfind "*":main.adb:123`.
`gnatfind mydir/main.adb:123:45`
  Find the declaration for the entity referenced at column 45 in line 123 of file main.adb in directory mydir. Note that it is usual to omit the identifier name when the column is given, since the column position identifies a unique reference. The column has to be the beginning of the identifier, and should not point to any character in the middle of the identifier.

# File Name Krunching Using `gnatkr`

This chapter discusses the method used by the compiler to shorten the default file names chosen for Ada units so that they do not exceed the maximum length permitted. It also describes the `gnatkr` utility that can be used to determine the result of applying this shortening.

- About gnatkr
- Using gnatkr
- Krunching Method
- Examples of gnatkr Usage

## About `gnatkr`

The default file naming rule in GNAT is that the file name must be derived from the unit name. The exact default rule is as follows:

- Take the unit name and replace all dots by hyphens.
- If such a replacement occurs in the second character position of a name, and the first character is a, g, s, or i then replace the dot by the character ~ (tilde) instead of a minus.

The reason for this exception is to avoid clashes with the standard names for children of System, Ada, Interfaces, and GNAT, which use the prefixes s- a- i- and g- respectively.

The `-gnatknn` switch of the compiler activates a "krunching" circuit that limits file names to nn characters (where nn is a decimal integer). For example, using OpenVMS, where the maximum file name length is 39, the value of nn is usually set to 39, but if you want to generate a set of files that would be usable if ported to a system with some different maximum file length, then a different value can be specified. The default value of 39 for OpenVMS need not be specified.

The `gnatkr` utility can be used to determine the krunched name for a given file, when krunched to a specified maximum length.

## Using `gnatkr`

The `gnatkr` command has the form

    $ gnatkr name [length]

name can be an Ada name with dots or the GNAT name of the unit, where the dots representing child units or subunit are replaced by hyphens. The only confusion arises if a name ends in `.ads` or `.adb`. `gnatkr` takes this to be an extension if there are no other dots in the name and the whole name is in lowercase.

length represents the length of the krunched name. The default when no argument is given is 8 characters. A length of zero stands for unlimited, in other words do not chop except for system files which are always 8.

The output is the krunched name. The output has an extension only if the original argument was a file name with an extension.

## Krunching Method

The initial file name is determined by the name of the unit that the file contains. The name is formed by taking the full expanded name of the unit and replacing the separating dots with hyphens and using lowercase for all letters, except that a hyphen in the second character position is replaced by a tilde if the first character is a, i, g, or s. The extension is `.ads` for a specification and `.adb` for a body. Krunching does not affect the extension, but the file name is shortened to the specified length by following these rules:

- The name is divided into segments separated by hyphens, tildes or underscores and all hyphens, tildes, and underscores are eliminated. If this leaves the name short enough, we are done.
- If the name is too long, the longest segment is located (left-most if there are two of equal length), and shortened by dropping its last character. This is repeated until the name is short enough. As an example, consider the krunching of
  ``our-strings-wide_fixed.adb'` to fit the name into 8 characters as required by some operating systems.

      our-strings-wide_fixed 22
      our strings wide fixed 19
      our string  wide fixed 18
      our strin   wide fixed 17
      our stri    wide fixed 16
      our stri    wide fixe  15
      our str     wide fixe  14
      our str     wid  fixe  13
      our str     wid  fix   12
      ou  str     wid  fix   11
      ou  st      wid  fix   10
      ou  st      wi   fix   9
      ou  st      wi   fi    8
      Final file name: oustwifi.adb

- The file names for all predefined units are always krunched to eight characters. The krunching of these predefined units uses the following special prefix replacements:
  ``ada-'`
    replaced by ``a-'`
  ``gnat-'`
    replaced by ``g-'`
  ``interfaces-'`
    replaced by ``i-'`
  ``system-'`
    replaced by ``s-'`
  These system files have a hyphen in the second character position. That is why normal user files replace such a character with a tilde, to avoid confusion with system file names. As an example of this special rule, consider
  ``ada-strings-wide_fixed.adb'`, which gets krunched as follows:

      ada-strings-wide_fixed 22
      a-  strings wide fixed 18
      a-  string  wide fixed 17
      a-  strin   wide fixed 16
      a-  stri    wide fixed 15
      a-  stri    wide fixe  14
      a-  str     wide fixe  13
      a-  str     wid  fixe  12
      a-  str     wid  fix   11
      a-  st      wid  fix   10
      a-  st      wi   fix   9
      a-  st      wi   fi    8
      Final file name: a-stwifi.adb

Of course no file shortening algorithm can guarantee uniqueness over all possible unit names, and if file name krunching is used then it is your responsibility to ensure that no name clashes occur. The utility program `gnatkr` is supplied for conveniently determining the krunched name of a file.

## Examples of `gnatkr` Usage

    @leftskip=0cm
    $ gnatkr very_long_unit_name.ads      --> velounna.ads
    $ gnatkr grandparent-parent-child.ads --> grparchi.ads
    $ gnatkr Grandparent.Parent.Child     --> grparchi
    $ gnatkr very_long_unit_name.ads/count=6 --> vlunna.ads
    $ gnatkr very_long_unit_name.ads/count=0 --> very_long_unit_name.ads

# Preprocessing Using `gnatprep`

The `gnatprep` utility provides a simple preprocessing capability for Ada programs. It is designed for use with GNAT, but is not dependent on any special features of GNAT.

- Using gnatprep
- Switches for gnatprep
- Form of Definitions File
- Form of Input Text for gnatprep

## Using `gnatprep`

To call `gnatprep` use

    $ gnatprep [-bcrsu] [-Dsymbol=value] infile outfile [deffile]

where

`infile`
  is the full name of the input file, which is an Ada source file containing preprocessor directives.
`outfile`
  is the full name of the output file, which is an Ada source in standard Ada form. When used with GNAT, this file name will normally have an ads or adb suffix.
`deffile`
  is the full name of a text file containing definitions of symbols to be referenced by the preprocessor. This argument is optional, and can be replaced by the use of the `-D` switch.
`switches`
  is an optional sequence of switches as described in the next section.

## Switches for `gnatprep`

`-b`
  Causes both preprocessor lines and the lines deleted by preprocessing to be replaced by blank lines in the output source file, preserving line numbers in the output file.
`-c`
  Causes both preprocessor lines and the lines deleted by preprocessing to be retained in the output source as comments marked with the special string "--! ". This option will result in line numbers being preserved in the output file.
`-Dsymbol=value`
  Defines a new symbol, associated with value. If no value is given on the command line, then symbol is considered to be `True`. This switch can be used in place of a definition file.
`-r`
  Causes a `Source_Reference` pragma to be generated that references the original input file, so that error messages will use the file name of this original file. The use of this switch implies that preprocessor lines are not to be removed from the file, so its use will force `-b` mode if `-c` has not been specified explicitly. Note that if the file to be preprocessed contains multiple units, then it will be necessary to `gnatchop` the output file from `gnatprep`. If a `Source_Reference` pragma is present in the preprocessed file, it will be respected by `gnatchop -r` so that the final chopped files will correctly refer to the original input source file for `gnatprep`.
`-s`
  Causes a sorted list of symbol names and values to be listed on the standard output file.
`-u`
  Causes undefined symbols to be treated as having the value FALSE in the context of a preprocessor test. In the absence of this option, an undefined symbol in a `#if` or `#elsif` test will be treated as an error.

Note: if neither `-b` nor `-c` is present, then preprocessor lines and deleted lines are completely removed from the output, unless -r is specified, in which case -b is assumed.

## Form of Definitions File

The definitions file contains lines of the form

    symbol := value

where symbol is an identifier, following normal Ada (case-insensitive) rules for its syntax, and value is one of the following:

- Empty, corresponding to a null substitution
- A string literal using normal Ada syntax
- Any sequence of characters from the set (letters, digits, period, underline).

Comment lines may also appear in the definitions file, starting with the usual `--`, and comments may be added to the definitions lines.

## Form of Input Text for `gnatprep`

The input text may contain preprocessor conditional inclusion lines, as well as general symbol substitution sequences. The preprocessor conditional inclusion commands have the form

    #if expression [then]
       lines
    #elsif expression [then]
       lines
    #elsif expression [then]
       lines
    ...
    #else
       lines
    #end if;

In this example, expression is defined by the following grammar:

    expression ::=  <symbol>
    expression ::=  <symbol> = "<value>"
    expression ::=  <symbol> = <symbol>
    expression ::=  <symbol> 'Defined
    expression ::=  not expression
    expression ::=  expression and expression
    expression ::=  expression or expression
    expression ::=  expression and then expression
    expression ::=  expression or else expression
    expression ::=  ( expression )

For the first test (expression ::= <symbol>) the symbol must have either the value true or false, that is to say the right-hand of the symbol definition must be one of the (case-insensitive) literals `True` or `False`. If the value is true, then the corresponding lines are included, and if the value is false, they are excluded.

The test (expression ::= <symbol> `'Defined`) is true only if the symbol has been defined in the definition file or by a `-D` switch on the command line. Otherwise, the test is false.

The equality tests are case insensitive, as are all the preprocessor lines.

If the symbol referenced is not defined in the symbol definitions file, then the effect depends on whether or not switch `-u` is specified. If so, then the symbol is treated as if it had the value false and the test fails. If this switch is not specified, then it is an error to reference an undefined symbol. It is also an error to reference a symbol that is defined with a value other than `True` or `False`.

The use of the `not` operator inverts the sense of this logical test, so that the lines are included only if the symbol is not defined. The `then` keyword is optional as shown

The `#` must be the first non-blank character on a line, but otherwise the format is free form. Spaces or tabs may appear between the `#` and the keyword. The keywords and the symbols are case insensitive as in normal Ada code. Comments may be used on a preprocessor line, but other than that, no other tokens may appear on a preprocessor line. Any number of `elsif` clauses can be present, including none at all. The `else` is optional, as in Ada.

The `#` marking the start of a preprocessor line must be the first non-blank character on the line, i.e. it must be preceded only by spaces or horizontal tabs.

Symbol substitution outside of preprocessor lines is obtained by using the sequence

    $symbol

anywhere within a source line, except in a comment. The identifier following the `$` must match one of the symbols defined in the symbol definition file, and the result is to substitute the value of the symbol in place of `$symbol` in the output file.

# The GNAT Library Browser `gnatls`

`gnatls` is a tool that outputs information about compiled units. It gives the relationship between objects, unit names and source files. It can also be used to check the source dependencies of a unit as well as various characteristics.

- Running gnatls
- Switches for gnatls
- Examples of gnatls Usage

## Running `gnatls`

The `gnatls` command has the form

    $ gnatls switches object_or_ali_file

The main argument is the list of object or ``ali'` files (see section The Ada Library Information Files) for which information is requested.

In normal mode, without additional option, `gnatls` produces a four-column listing. Each line represents information for a specific object. The first column gives the full path of the object, the second column gives the name of the principal unit in this object, the third column gives the status of the source and the fourth column gives the full path of the source representing this unit. Here is a simple example of use:

    $ gnatls *.o
    ./demo1.o            demo1            DIF demo1.adb
    ./demo2.o            demo2             OK demo2.adb
    ./hello.o            h1                OK hello.adb
    ./instr-child.o      instr.child      MOK instr-child.adb
    ./instr.o            instr             OK instr.adb
    ./tef.o              tef              DIF tef.adb
    ./text_io_example.o  text_io_example   OK text_io_example.adb
    ./tgef.o             tgef             DIF tgef.adb

The first line can be interpreted as follows: the main unit which is contained in object file ``demo1.o'` is demo1, whose main source is in ``demo1.adb'`. Furthermore, the version of the source used for the compilation of demo1 has been modified (DIF). Each source file has a status qualifier which can be:

`OK (unchanged)`
  The version of the source file used for the compilation of the specified unit corresponds exactly to the actual source file.
`MOK (slightly modified)`
  The version of the source file used for the compilation of the specified unit differs from the actual source file but not enough to require recompilation. If you use gnatmake with the qualifier `-m (minimal recompilation)`, a file marked MOK will not be recompiled.
`DIF (modified)`
  No version of the source found on the path corresponds to the source used to build this object.
`??? (file not found)`
  No source file was found for this unit.
`HID (hidden, unchanged version not first on PATH)`
  The version of the source that corresponds exactly to the source used for compilation has been found on the path but it is hidden by another version of the same source that has been modified.

## Switches for `gnatls`

`gnatls` recognizes the following switches:

`-a`
  Consider all units, including those of the predefined Ada library. Especially useful with `-d`.
`-d`
  List sources from which specified units depend on.
`-h`
  Output the list of options.
`-o`
  Only output information about object files.
`-s`
  Only output information about source files.
`-u`
  Only output information about compilation units.
`-aOdir`
`-aIdir`
`-Idir`
`-I-`
`-nostdinc`
  Source and Object path manipulation. Same meaning as the equivalent `gnatmake` flags (see section Switches for `gnatmake`).
`-v`
  Verbose mode. Output the complete source and object paths. Do not use the default column layout but instead use long format giving as much as information possible on each requested units, including special characteristics such as:
  `Preelaborable`
    The unit is preelaborable in the Ada 95 sense.
  `No_Elab_Code`
    No elaboration code has been produced by the compiler for this unit.
  `Pure`
    The unit is pure in the Ada 95 sense.
  `Elaborate_Body`
    The unit contains a pragma Elaborate_Body.
  `Remote_Types`
    The unit contains a pragma Remote_Types.
  `Shared_Passive`
    The unit contains a pragma Shared_Passive.
  `Predefined`
    This unit is part of the predefined environment and cannot be modified by the user.
  `Remote_Call_Interface`
    The unit contains a pragma Remote_Call_Interface.

## Example of `gnatls` Usage

Example of using the verbose switch. Note how the source and object paths are affected by the -I switch.

    $ gnatls -v -I.. demo1.o

    GNATLS 3.10w (970212) Copyright 1999 Free Software Foundation, Inc.

    Source Search Path:
       <Current_Directory>
       ../
       /home/comar/local/adainclude/

    Object Search Path:
       <Current_Directory>
       ../
       /home/comar/local/lib/gcc-lib/mips-sni-sysv4/2.7.2/adalib/

    ./demo1.o
       Unit =>
         Name   => demo1
         Kind   => subprogram body
         Flags  => No_Elab_Code
         Source => demo1.adb    modified

The following is an example of use of the dependency list. Note the use of the -s switch which gives a straight list of source files. This can be useful for building specialized scripts.

    $ gnatls -d demo2.o
    ./demo2.o   demo2        OK demo2.adb
                             OK gen_list.ads
                             OK gen_list.adb
                             OK instr.ads
                             OK instr-child.ads

    $ gnatls -d -s -a demo1.o
    demo1.adb
    /home/comar/local/adainclude/ada.ads
    /home/comar/local/adainclude/a-finali.ads
    /home/comar/local/adainclude/a-filico.ads
    /home/comar/local/adainclude/a-stream.ads
    /home/comar/local/adainclude/a-tags.ads
    gen_list.ads
    gen_list.adb
    /home/comar/local/adainclude/gnat.ads
    /home/comar/local/adainclude/g-io.ads
    instr.ads
    /home/comar/local/adainclude/system.ads
    /home/comar/local/adainclude/s-exctab.ads
    /home/comar/local/adainclude/s-finimp.ads
    /home/comar/local/adainclude/s-finroo.ads
    /home/comar/local/adainclude/s-secsta.ads
    /home/comar/local/adainclude/s-stalib.ads
    /home/comar/local/adainclude/s-stoele.ads
    /home/comar/local/adainclude/s-stratt.ads
    /home/comar/local/adainclude/s-tasoli.ads
    /home/comar/local/adainclude/s-unstyp.ads
    /home/comar/local/adainclude/unchconv.ads

# GNAT and Libraries

This chapter addresses some of the issues related to building and using a library with GNAT. It also shows how the GNAT runtime library can be recompiled.

- Creating an Ada Library
- Installing an Ada Library
- Using an Ada Library
- Rebuilding the GNAT Run-Time Library

## Creating an Ada Library

In the GNAT environment, a library has two components:

- Source files.
- Compiled code and Ali files. See section The Ada Library Information Files.

In order to use other packages section The GNAT Compilation Model requires a certain number of sources to be available to the compiler. The minimal set of sources required includes the specs of all the packages that make up the visible part of the library as well as all the sources upon which they depend. The bodies of all visible generic units must also be provided. Although it is not strictly mandatory, it is recommended that all sources needed to recompile the library be provided, so that the user can make full use of interunit inlining and source-level debugging. This can also make the situation easier for users that need to upgrade their compilation toolchain and thus need to recompile the library from sources.

The compiled code can be provided in different ways. The simplest way is to provide directly the set of objects produced by the compiler during the compilation of the library. It is also possible to group the objects into an archive using whatever commands are provided by the operating system. Finally, it is also possible to create a shared library (see option -shared in the GCC manual).

There are various possibilities for compiling the units that make up the library: for example with a Makefile section Using the GNU `make` Utility, or with a conventional script. For simple libraries, it is also possible to create a dummy main program which depends upon all the packages that comprise the interface of the library. This dummy main program can then be given to gnatmake, in order to build all the necessary objects. Here is an example of such a dummy program and the generic commands used to build an archive or a shared library.

    @leftskip=.7cm
    with My_Lib.Service1;
    with My_Lib.Service2;
    with My_Lib.Service3;
    procedure My_Lib_Dummy is
    begin
       null;
    end;

    # compiling the library
    $ gnatmake -c my_lib_dummy.adb

    # we don't need the dummy object itself
    $ rm my_lib_dummy.o my_lib_dummy.ali

    # create an archive with the remaining objects
    $ ar rc libmy_lib.a *.o
    # some systems may require "ranlib" to be run as well

    # or create a shared library
    $ gcc -shared -o libmy_lib.so *.o
    # some systems may require the code to have been compiled with -fPIC

When the objects are grouped in an archive or a shared library, the user needs to specify the desired library at link time, unless a pragma linker_options has been used in one of the sources:

    pragma Linker_Options ("-lmy_lib");

## Installing an Ada Library

In the GNAT model, installing a library consists in copying into a specific location the files that make up this library. It is possible to install the sources in a different directory from the other files (ALI, objects, archives) since the source path and the object path can easily be specified separately.

For general purpose libraries, it is possible for the system administrator to put those libraries in the default compiler paths. To achieve this, he must specify their location in the configuration files "ada_source_path" and "ada_object_path" that must be located in the GNAT installation tree at the same place as the gcc spec file. The location of the gcc spec file can be determined as follows:

    $ gcc -v

The configuration files mentioned above have simple format: each line in them must contain one unique directory name. Those names are added to the corresponding path in their order of appearance in the file. The names can be either absolute or relative, in the latter case, they are relative to where theses files are located.

"ada_source_path" and "ada_object_path" might actually not be present in a GNAT installation, in which case, GNAT will look for its run-time library in the directories "adainclude" for the sources and "adalib" for the objects and ALI files. When the files exist, the compiler does not look in "adainclude" and "adalib" at all, and thus the "ada_source_path" file must contain the location for the GNAT run-time sources (which can simply be "adainclude"). In the same way, the "ada_object_path" file must contain the location for the GNAT run-time objects (which can simply be "adalib").

It is possible to install a library before or after the standard GNAT library, by reordering the lines in the configuration files. In general, a library must be installed before the GNAT library if it redefines any part of it.

## Using an Ada Library

In order to use a Ada library, you need to make sure that this library is on both your source and object path section Search Paths and the Run-Time Library (RTL) and section Search Paths for `gnatbind`. For instance, you can use the library "mylib" installed in "/dir/my_lib_src" and "/dir/my_lib_obj" with the following commands:

    $ gnatmake -aI/dir/my_lib_src -aO/dir/my_lib_obj my_appl \
      -largs -lmy_lib

This can be simplified down to the following:

    $ gnatmake my_appl

when the following conditions are met:

- "/dir/my_lib_src" has been added by the user to the environment variable "ADA_INCLUDE_PATH", or by the administrator to the file "ada_source_path"
- "/dir/my_lib_obj" has been added by the user to the environment variable "ADA_OBJECTS_PATH", or by the administrator to the file "ada_object_path"
- a pragma linker_options, as mentioned in section Creating an Ada Library as been added to the sources.

## Rebuilding the GNAT Run-Time Library

It may be useful to recompile the GNAT library in various contexts, the most important one being the use of partition-wide configuration pragmas such as Normalize_Scalar. A special Makefile called `Makefile.adalib` is provided to that effect and can be found in the directory containing the GNAT library. The location of this directory depends on the way the GNAT environment has been installed and can be determined by means of the command:

    $ gnatls -v

The last entry in the object search path usually contains the gnat library. This Makefile contains its own documentation and in particular the set of instructions needed to rebuild a new library and to use it.

# Using the GNU `make` Utility

This chapter offers some examples of makefiles that solve specific problems. It does not explain how to write a makefile (see the GNU make documentation), nor does it try to replace the `gnatmake` utility (see section The GNAT Make Program `gnatmake`).

All the examples in this section are specific to the GNU version of make. Although `make` is a standard utility, and the basic language is the same, these examples use some advanced features found only in `GNU make`.

- Using gnatmake in a Makefile
- Automatically Creating a List of Directories
- Generating the Command Line Switches
- Overcoming Command Line Length Limits

## Using gnatmake in a Makefile

Complex project organizations can be handled in a very powerful way by using GNU make combined with gnatmake. For instance, here is a Makefile which allows you to build each subsystem of a big project into a separate shared library. Such a makefile allows you to significantly reduce the link time of very big applications while maintaining full coherence at each step of the build process.

The list of dependencies are handled automatically by `gnatmake`. The Makefile is simply used to call gnatmake in each of the appropriate directories.

Note that you should also read the example on how to automatically create the list of directories (see section Automatically Creating a List of Directories) which might help you in case your project has a lot of subdirectories.

    @leftskip=0cm
    @font@heightrm=cmr8
    @heightrm
    ## This Makefile is intended to be used with the following directory
    ## configuration:
    ##  - The sources are split into a series of csc (computer software components)
    ##    Each of these csc is put in its own directory.
    ##    Their name are referenced by the directory names.
    ##    They will be compiled into shared library (although this would also work
    ##    with static libraries
    ##  - The main program (and possibly other packages that do not belong to any
    ##    csc is put in the top level directory (where the Makefile is).
    ##       toplevel_dir __ first_csc  (sources) __ lib (will contain the library)
    ##                    \_ second_csc (sources) __ lib (will contain the library)
    ##                    \_ ...
    ## Although this Makefile is build for shared library, it is easy to modify
    ## to build partial link objects instead (modify the lines with -shared and
    ## gnatlink below)
    ##
    ## With this makefile, you can change any file in the system or add any new
    ## file, and everything will be recompiled correctly (only the relevant shared
    ## objects will be recompiled, and the main program will be re-linked).

    # The list of computer software component for your project. This might be
    # generated automatically.
    CSC_LIST=aa bb cc

    # Name of the main program (no extension)
    MAIN=main

    # If we need to build objects with -fPIC, uncomment the following line
    #NEED_FPIC=-fPIC

    # The following variable should give the directory containing libgnat.so
    # You can get this directory through 'gnatls -v'. This is usually the last
    # directory in the Object_Path.
    GLIB=...

    # The directories for the libraries
    # (This macro expands the list of CSC to the list of shared libraries, you
    # could simply use the expanded form :
    # LIB_DIR=aa/lib/libaa.so bb/lib/libbb.so cc/lib/libcc.so
    LIB_DIR=${foreach dir,${CSC_LIST},${dir}/lib/lib${dir}.so}

    ${MAIN}: objects ${LIB_DIR}
        gnatbind ${MAIN} ${CSC_LIST:%=-aO%/lib} -shared
        gnatlink ${MAIN} ${CSC_LIST:%=-l%}

    objects::
        # recompile the sources
        gnatmake -c -i ${MAIN}.adb ${NEED_FPIC} ${CSC_LIST:%=-I%}

    # Note: In a future version of GNAT, the following commands will be simplified
    # by a new tool, gnatmlib
    ${LIB_DIR}:
        mkdir -p ${dir $@ }
        cd ${dir $@ }; gcc -shared -o ${notdir $@ } ../*.o -L${GLIB} -lgnat
        cd ${dir $@ }; cp -f ../*.ali .

    # The dependencies for the modules
    # Note that we have to force the expansion of *.o, since in some cases make won't
    # be able to do it itself.
    aa/lib/libaa.so: ${wildcard aa/*.o}
    bb/lib/libbb.so: ${wildcard bb/*.o}
    cc/lib/libcc.so: ${wildcard cc/*.o}

    # Make sure all of the shared libraries are in the path before starting the
    # program
    run::
        LD_LIBRARY_PATH=pwd/aa/lib:pwd/bb/lib:pwd/cc/lib ./${MAIN}

    clean::
        ${RM} -rf ${CSC_LIST:%=%/lib}
        ${RM} ${CSC_LIST:%=%/*.ali}
        ${RM} ${CSC_LIST:%=%/*.o}
        ${RM} *.o *.ali ${MAIN}

## Automatically Creating a List of Directories

In most makefiles, you will have to specify a list of directories, and store it in a variable. For small projects, it is often easier to specify each of them by hand, since you then have full control over what is the proper order for these directories, which ones should be included...

However, in larger projects, which might involve hundreds of subdirectories, it might be more convenient to generate this list automatically.

The example below presents two methods. The first one, altough less general, gives you more control over the list. It involves wildcard characters, that are automatically expanded by `make`. Its shortcoming is that you need to explicitly specify some of the organization of your project, such as for instance the directory tree depth, whether some directories are found in a separate tree,...

The second method is the most general one. It requires an external program, called `find`, which is standard on all Unix systems. All the directories found under a given root directory will be added to the list.

    @leftskip=0cm
    @font@heightrm=cmr8
    @heightrm
    # The examples below are based on the following directory hierarchy:
    # All the directories can contain any number of files
    # ROOT_DIRECTORY ->  a  ->  aa  ->  aaa
    #                       ->  ab
    #                       ->  ac
    #                ->  b  ->  ba  ->  baa
    #                       ->  bb
    #                       ->  bc
    # This Makefile creates a variable called DIRS, that can be reused any time
    # you need this list (see the other examples in this section)

    # The root of your project's directory hierarchy
    ROOT_DIRECTORY=.

    ####
    # First method: specify explicitly the list of directories
    # This allows you to specify any subset of all the directories you need.
    ####

    DIRS := a/aa/ a/ab/ b/ba/

    ####
    # Second method: use wildcards
    # Note that the argument(s) to wildcard below should end with a '/'.
    # Since wildcards also return file names, we have to filter them out
    # to avoid duplicate directory names.
    # We thus use make's dir and sort functions.
    # It sets DIRs to the following value (note that the directories aaa and baa
    # are not given, unless you change the arguments to wildcard).
    # DIRS= ./a/a/ ./b/ ./a/aa/ ./a/ab/ ./a/ac/ ./b/ba/ ./b/bb/ ./b/bc/
    ####

    DIRS := ${sort ${dir ${wildcard ${ROOT_DIRECTORY}/*/ ${ROOT_DIRECTORY}/*/*/}}}

    ####
    # Third method: use an external program
    # This command is much faster if run on local disks, avoiding NFS slowdowns.
    # This is the most complete command: it sets DIRs to the following value:
    # DIRS= ./a ./a/aa ./a/aa/aaa ./a/ab ./a/ac ./b ./b/ba ./b/ba/baa ./b/bb ./b/bc
    ####

    DIRS := ${shell find ${ROOT_DIRECTORY} -type d -print}

## Generating the Command Line Switches

Once you have created the list of directories as explained in the previous section (see section Automatically Creating a List of Directories), you can easily generate the command line arguments to pass to gnatmake.

For the sake of completness, this example assumes that the source path is not the same as the object path, and that you have two separate lists of directories.

    # see "Automatically creating a list of directories" to create
    # these variables
    SOURCE_DIRS=
    OBJECT_DIRS=

    GNATMAKE_SWITCHES := ${patsubst %,-aI%,${SOURCE_DIRS}}
    GNATMAKE_SWITCHES += ${patsubst %,-aO%,${OBJECT_DIRS}}

    all:
            gnatmake ${GNATMAKE_SWITCHES} main_unit

## Overcoming Command Line Length Limits

One problem that might be encountered on big projects is that many operating systems limit the length of the command line. It is thus hard to give gnatmake the list of source and object directories.

This example shows how you can set up environment variables, which will make `gnatmake` behave exactly as if the directories had been specified on the command line, but have a much higher length limit (or even none on most systems).

It assumes that you have created a list of directories in your Makefile, using one of the methods presented in section Automatically Creating a List of Directories. For the sake of completness, we assume that the object path (where the ALI files are found) is different from the sources patch.

Note a small trick in the Makefile below: for efficiency reasons, we create two temporary variables (SOURCE_LIST and OBJECT_LIST), that are expanded immediatly by `make`. This way we overcome the standard make behavior which is to expand the variables only when they are actually used.

    @leftskip=0cm
    @font@heightrm=cmr8
    @heightrm
    # In this example, we create both ADA_INCLUDE_PATH and ADA_OBJECT_PATH.
    # This is the same thing as putting the -I arguments on the command line.
    # (the equivalent of using -aI on the command line would be to define
    #  only ADA_INCLUDE_PATH, the equivalent of -aO is ADA_OBJECT_PATH).
    # You can of course have different values for these variables.
    #
    # Note also that we need to keep the previous values of these variables, since
    # they might have been set before running 'make' to specify where the GNAT
    # library is installed.

    # see "Automatically creating a list of directories" to create these
    # variables
    SOURCE_DIRS=
    OBJECT_DIRS=

    empty:=
    space:=${empty} ${empty}
    SOURCE_LIST := ${subst ${space},:,${SOURCE_DIRS}}
    OBJECT_LIST := ${subst ${space},:,${OBJECT_DIRS}}
    ADA_INCLUDE_PATH += ${SOURCE_LIST}
    ADA_OBJECT_PATH += ${OBJECT_LIST}
    export ADA_INCLUDE_PATH
    export ADA_OBJECT_PATH

    all:
            gnatmake main_unit

# Finding Memory Problems with `gnatmem`

`gnatmem`, is a tool that monitors dynamic allocation and deallocation activity in a program, and displays information about incorrect deallocations and possible sources of memory leaks. Gnatmem provides three type of information:

- General information concerning memory management, such as the total number of allocations and deallocations, the amount of allocated memory and the high water mark, i.e. the largest amount of allocated memory in the course of program execution.
- Backtraces for all incorrect deallocations, that is to say deallocations which do not correspond to a valid allocation.
- Information on each allocation that is potentially the origin of a memory leak.

The `gnatmem` command has two modes. It can be used with `gdb` or with instrumented allocation and deallocation routines. The later mode is called the `GMEM` mode. Both modes produce the very same output.

- Running gnatmem (GDB Mode)
- Running gnatmem (GMEM Mode)
- Switches for gnatmem
- Examples of gnatmem Usage
- GDB and GMEM Modes
- Implementation Note

## Running `gnatmem` (GDB Mode)

The `gnatmem` command has the form

       $ gnatmem [-q] [n] [-o file] user_program [program_arg]*
    or
       $ gnatmem [-q] [n] -i file

Gnatmem must be supplied with the executable to examine, followed by its run-time inputs. For example, if a program is executed with the command:

    $ my_program arg1 arg2

then it can be run under `gnatmem` control using the command:

    $ gnatmem my_program arg1 arg2

The program is transparently executed under the control of the debugger section The GNAT Debugger GDB. This does not affect the behavior of the program, except for sensitive real-time programs. When the program has completed execution, `gnatmem` outputs a report containing general allocation/deallocation information and potential memory leak. For better results, the user program should be compiled with debugging options section Switches for `gcc`.

Here is a simple example of use:

*************** debut cc

    $ gnatmem test_gm

    Global information
    ------------------
       Total number of allocations        :  45
       Total number of deallocations      :   6
       Final Water Mark (non freed mem)   :  11.29 Kilobytes
       High Water Mark                    :  11.40 Kilobytes

    .
    .
    .
    Allocation Root # 2
    -------------------
     Number of non freed allocations    :  11
     Final Water Mark (non freed mem)   :   1.16 Kilobytes
     High Water Mark                    :   1.27 Kilobytes
     Backtrace                          :
       test_gm.adb:23 test_gm.alloc
    .
    .
    .

The first block of output give general information. In this case, the Ada construct "new" was executed 45 times, and only 6 calls to an unchecked deallocation routine occurred.

Subsequent paragraphs display information on all allocation roots. An allocation root is a specific point in the execution of the program that generates some dynamic allocation, such as a "new" construct. This root is represented by an execution backtrace (or subprogram call stack). By default the backtrace depth for allocations roots is 1, so that a root corresponds exactly to a source location. The backtrace can be made deeper, to make the root more specific.

## Running `gnatmem` (GMEM Mode)

The `gnatmem` command has the form

       $ gnatmem [-q] [n] -i gmem.out user_program [program_arg]*

The program must have been linked with the instrumented version of the allocation and deallocation routines. This is done with linking with the ``libgmem.a'` library. For better results, the user program should be compiled with debugging options section Switches for `gcc`. For example to build ``my_program'`:

    $ gnatmake -g my_program -largs -lgmem

Under Solaris it is required to build the application with `-funwind-tables` options section Switches for `gcc`.

When running ``my_program'` the file ``gmem.out'` is produced. This file contains information about all allocations and deallocations done by the program. It is produced by the instrumented allocations and deallocations routines and will be used by `gnatmem`.

Gnatmem must be supplied with the ``gmem.out'` file and the executable to examine followed by its run-time inputs. For example, if a program is executed with the command:

    $ my_program arg1 arg2

then ``gmem.out'` can be analysed by `gnatmem` using the command:

    $ gnatmem -i gmem.out my_program arg1 arg2

## Switches for `gnatmem`

`gnatmem` recognizes the following switches:

``-q``
  Quiet. Gives the minimum output needed to identify the origin of the memory leaks. Omit statistical information.
``n``
  N is an integer literal (usually between 1 and 10) which controls the depth of the backtraces defining allocation root. The default value for N is 1. The deeper the backtrace, the more precise the localization of the root. Note that the total number of roots can depend on this parameter.
``-o file``
  Direct the gdb output to the specified file. The `gdb` script used to generate this output is also saved in the file ``gnatmem.tmp'`.
``-i file``
  Do the `gnatmem` processing starting from ``file'` which has been generated by a previous call to `gnatmem` with the -o switch or ``gmem.out'` produced by `GMEM` mode. This is useful for post mortem processing.

## Example of `gnatmem` Usage

This section is based on the `GDB` mode of `gnatmem`. The same results can be achieved using `GMEM` mode. See section section Running `gnatmem` (GMEM Mode).

The first example shows the use of `gnatmem` on a simple leaking program. Suppose that we have the following Ada program:

    with Unchecked_Deallocation;
    procedure Test_Gm is

       type T is array (1..1000) of Integer;
       type Ptr is access T;
       procedure Free is new Unchecked_Deallocation (T, Ptr);
       A : Ptr;

       procedure My_Alloc is
       begin
          A := new T;
       end My_Alloc;

       procedure My_DeAlloc is
          B : Ptr := A;
       begin
          Free (B);
       end My_DeAlloc;

    begin
       My_Alloc;
       for I in 1 .. 5 loop
          for J in I .. 5 loop
             My_Alloc;
          end loop;
          My_Dealloc;
       end loop;
    end;

The program needs to be compiled with debugging option:

    $ gnatmake -g test_gm

`gnatmem` is invoked simply with

    $ gnatmem test_gm

which produces the following output:

    Global information
    ------------------
       Total number of allocations        :  18
       Total number of deallocations      :   5
       Final Water Mark (non freed mem)   :  53.00 Kilobytes
       High Water Mark                    :  56.90 Kilobytes

    Allocation Root # 1
    -------------------
     Number of non freed allocations    :  11
     Final Water Mark (non freed mem)   :  42.97 Kilobytes
     High Water Mark                    :  46.88 Kilobytes
     Backtrace                          :
       test_gm.adb:11 test_gm.my_alloc

    Allocation Root # 2
    -------------------
     Number of non freed allocations    :   1
     Final Water Mark (non freed mem)   :  10.02 Kilobytes
     High Water Mark                    :  10.02 Kilobytes
     Backtrace                          :
       s-secsta.adb:81 system.secondary_stack.ss_init

    Allocation Root # 3
    -------------------
     Number of non freed allocations    :   1
     Final Water Mark (non freed mem)   :  12 Bytes
     High Water Mark                    :  12 Bytes
     Backtrace                          :
       s-secsta.adb:181 system.secondary_stack.ss_init

Note that the GNAT run time contains itself a certain number of allocations that have no corresponding deallocation, as shown here for root #2 and root #1. This is a normal behavior when the number of non freed allocations is one, it locates dynamic data structures that the run time needs for the complete lifetime of the program. Note also that there is only one allocation root in the user program with a single line back trace: test_gm.adb:11 test_gm.my_alloc, whereas a careful analysis of the program shows that 'My_Alloc' is called at 2 different points in the source (line 21 and line 24). If those two allocation roots need to be distinguished, the backtrace depth parameter can be used:

    $ gnatmem 3 test_gm

which will give the following output:

    Global information
    ------------------
       Total number of allocations        :  18
       Total number of deallocations      :   5
       Final Water Mark (non freed mem)   :  53.00 Kilobytes
       High Water Mark                    :  56.90 Kilobytes

    Allocation Root # 1
    -------------------
     Number of non freed allocations    :  10
     Final Water Mark (non freed mem)   :  39.06 Kilobytes
     High Water Mark                    :  42.97 Kilobytes
     Backtrace                          :
       test_gm.adb:11 test_gm.my_alloc
       test_gm.adb:24 test_gm
       b_test_gm.c:52 main

    Allocation Root # 2
    -------------------
     Number of non freed allocations    :   1
     Final Water Mark (non freed mem)   :  10.02 Kilobytes
     High Water Mark                    :  10.02 Kilobytes
     Backtrace                          :
       s-secsta.adb:81  system.secondary_stack.ss_init
       s-secsta.adb:283 <system__secondary_stack___elabb>
       b_test_gm.c:33   adainit

    Allocation Root # 3
    -------------------
     Number of non freed allocations    :   1
     Final Water Mark (non freed mem)   :   3.91 Kilobytes
     High Water Mark                    :   3.91 Kilobytes
     Backtrace                          :
       test_gm.adb:11 test_gm.my_alloc
       test_gm.adb:21 test_gm
       b_test_gm.c:52 main

    Allocation Root # 4
    -------------------
     Number of non freed allocations    :   1
     Final Water Mark (non freed mem)   :  12 Bytes
     High Water Mark                    :  12 Bytes
     Backtrace                          :
       s-secsta.adb:181 system.secondary_stack.ss_init
       s-secsta.adb:283 <system__secondary_stack___elabb>
       b_test_gm.c:33   adainit

The allocation root #1 of the first example has been split in 2 roots #1 and #3 thanks to the more precise associated backtrace.

## GDB and GMEM Modes

The main advantage of the `GMEM` mode is that it is a lot faster than the `GDB` mode where the application must be monitored by a `GDB` script. But the `GMEM` mode is available only for DEC Unix, SGI Irix, Linux x86, Solaris (sparc and x86) and Windows 95/98/NT/2000 (x86).

The main advantage of the `GDB` mode is that it is available on all supported platforms. But it can be very slow if the application does a lot of allocations and deallocations.

## Implementation Note

- gnatmem Using GDB Mode
- gnatmem Using GMEM Mode

### `gnatmem` Using `GDB` Mode

`gnatmem` executes the user program under the control of `GDB` using a script that sets breakpoints and gathers information on each dynamic allocation and deallocation. The output of the script is then analyzed by `gnatmem` in order to locate memory leaks and their origin in the program. Gnatmem works by recording each address returned by the allocation procedure (`__gnat_malloc`) along with the backtrace at the allocation point. On each deallocation, the deallocated address is matched with the corresponding allocation. At the end of the processing, the unmatched allocations are considered potential leaks. All the allocations associated with the same backtrace are grouped together and form an allocation root. The allocation roots are then sorted so that those with the biggest number of unmatched allocation are printed first. A delicate aspect of this technique is to distinguish between the data produced by the user program and the data produced by the gdb script. Currently, on systems that allow probing the terminal, the gdb command "tty" is used to force the program output to be redirected to the current terminal while the `gdb` output is directed to a file or to a pipe in order to be processed subsequently by `gnatmem`.

### `gnatmem` Using `GMEM` Mode

This mode use the same algorithm to detect memory leak as the `GDB` mode of `gnatmem`, the only difference is in the way data are gathered. In `GMEM` mode the program is linked with instrumented version of `__gnat_malloc` and `__gnat_free` routines. Information needed to find memory leak are recorded by these routines in file ``gmem.out'`. This mode also require that the stack traceback be available, this is only implemented on some platforms section GDB and GMEM Modes.

# Finding Memory Problems with GNAT Debug Pool

The use of unchecked deallocation and unchecked conversion can easily lead to incorrect memory references. The problems generated by such references are usually difficult to tackle because the symptoms can be very remote from the origin of the problem. In such cases, it is very helpful to detect the problem as early as possible. This is the purpose of the Storage Pool provided by `GNAT.Debug_Pools`.

In order to use the GNAT specifc debugging pool, the user must associate a debug pool object with each of the access types that may be related to suspected memory problems. See Ada Reference Manual 13.11.

    type Ptr is access Some_Type;
    Pool : GNAT.Debug_Pools.Debug_Pool;
    for Ptr'Storage_Pool use Pool;

`GNAT.Debug_Pools` is derived from of a GNAT-specific kind of pool: the Checked_Pool. Such pools, like standard Ada storage pools, allow the user to redefine allocation and deallocation strategies. They also provide a checkpoint for each dereference, through the use of the primitive operation `Dereference` which is implicitly called at each dereference of an access value.

Once an access type has been associated with a debug pool, operations on values of the type may raise four distinct exceptions, which correspond to four potential kinds of memory corruption:

- `GNAT.Debug_Pools.Accessing_Not_Allocated_Storage`
- `GNAT.Debug_Pools.Accessing_Deallocated_Storage`
- `GNAT.Debug_Pools.Freeing_Not_Allocated_Storage`
- `GNAT.Debug_Pools.Freeing_Deallocated_Storage`

For types associated with a Debug_Pool, dynamic allocation is performed using the standard GNAT allocation routine. References to all allocated chunks of memory are kept in an internal dictionary. The deallocation strategy consists in not releasing the memory to the underlying system but rather to fill it with a memory pattern easily recognizable during debugging sessions: The memory pattern is the old IBM hexadecimal convention: 16#DEADBEEF#. Upon each dereference, a check is made that the access value denotes a properly allocated memory location. Here is a complete example of use of `Debug_Pools`, that includes typical instances of memory corruption:

    @leftskip=0cm
    with Gnat.Io; use Gnat.Io;
    with Unchecked_Deallocation;
    with Unchecked_Conversion;
    with GNAT.Debug_Pools;
    with System.Storage_Elements;
    with Ada.Exceptions; use Ada.Exceptions;
    procedure Debug_Pool_Test is

       type T is access Integer;
       type U is access all T;

       P : GNAT.Debug_Pools.Debug_Pool;
       for T'Storage_Pool use P;

       procedure Free is new Unchecked_Deallocation (Integer, T);
       function UC is new Unchecked_Conversion (U, T);
       A, B : aliased T;

       procedure Info is new GNAT.Debug_Pools.Print_Info(Put_Line);

    begin
       Info (P);
       A := new Integer;
       B := new Integer;
       B := A;
       Info (P);
       Free (A);
       begin
          Put_Line (Integer'Image(B.all));
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       begin
          Free (B);
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       B := UC(A'Access);
       begin
          Put_Line (Integer'Image(B.all));
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       begin
          Free (B);
       exception
          when E : others => Put_Line ("raised: " & Exception_Name (E));
       end;
       Info (P);
    end Debug_Pool_Test;

The debug pool mechanism provides the following precise diagnostics on the execution of this erroneous program:

    Debug Pool info:
      Total allocated bytes :  0
      Total deallocated bytes :  0
      Current Water Mark:  0
      High Water Mark:  0

    Debug Pool info:
      Total allocated bytes :  8
      Total deallocated bytes :  0
      Current Water Mark:  8
      High Water Mark:  8

    raised: GNAT.DEBUG_POOLS.ACCESSING_DEALLOCATED_STORAGE
    raised: GNAT.DEBUG_POOLS.FREEING_DEALLOCATED_STORAGE
    raised: GNAT.DEBUG_POOLS.ACCESSING_NOT_ALLOCATED_STORAGE
    raised: GNAT.DEBUG_POOLS.FREEING_NOT_ALLOCATED_STORAGE
    Debug Pool info:
      Total allocated bytes :  8
      Total deallocated bytes :  4
      Current Water Mark:  4
      High Water Mark:  8

# Creating Sample Bodies Using `gnatstub`

`gnatstub` creates body stubs, that is, empty but compilable bodies for library unit declarations.

To create a body stub, `gnatstub` has to compile the library unit declaration. Therefore, bodies can be created only for legal library units. Moreover, if a library unit depends semantically upon units located outside the current directory, you have to provide the source search path when calling `gnatstub`, see the description of `gnatstub` switches below.

- Running gnatstub
- Switches for gnatstub

## Running `gnatstub`

`gnatstub` has the command-line interface of the form

    $ gnatstub [switches] filename [directory]

where

`filename`
  is the name of the source file that contains a library unit declaration for which a body must be created. This name should follow the GNAT file name conventions. No crunching is allowed for this file name. The file name may contain the path information.
`directory`
  indicates the directory to place a body stub (default is the current directory)
`switches`
  is an optional sequence of switches as described in the next section

## Switches for `gnatstub`

`-f`
  If the destination directory already contains a file with a name of the body file for the argument spec file, replace it with the generated body stub.
`-hs`
  Put the comment header (i.e. all the comments preceding the compilation unit) from the source of the library unit declaration into the body stub.
`-hg`
  Put a sample comment header into the body stub.
`-IDIR`
`-I-`
  These switches have the same meaning as in calls to gcc. They define the source search path in the call to gcc issued by `gnatstub` to compile an argument source file.
`-in`
  (n is a decimal natural number). Set the indentation level in the generated body sample to n, '-i0' means "no indentation", the default indentation is 3.
`-k`
  Do not remove the tree file (i.e. the snapshot of the compiler internal structures used by `gnatstub`) after creating the body stub.
`-ln`
  (n is a decimal positive number) Set the maximum line length in the body stub to n, the default is 78.
`-q`
  Quiet mode: do not generate a confirmation when a body is successfully created or a message when a body is not required for an argument unit.
`-r`
  Reuse the tree file (if it exists) instead of creating it: instead of creating the tree file for the library unit declaration, gnatstub tries to find it in the current directory and use it for creating a body. If the tree file is not found, no body is created. `-r` also implies `-k`, whether or not `-k` is set explicitly.
`-t`
  Overwrite the existing tree file: if the current directory already contains the file which, according to the GNAT file name rules should be considered as a tree file for the argument source file, gnatstub will refuse to create the tree file needed to create a body sampler, unless `-t` option is set
`-v`
  Verbose mode: generate version information.

# Reducing the Size of Ada Executables with `gnatelim`

- About gnatelim
- Eliminate Pragma
- Tree Files
- Preparing Tree and Bind Files for gnatelim
- Running gnatelim
- Correcting the List of Eliminate Pragmas
- Making Your Executables Smaller
- Summary of the gnatelim Usage Cycle

## About `gnatelim`

When a program shares a set of Ada packages with other programs, it may happen that this program uses only a fraction of the subprograms defined in these packages. The code created for these unused subprograms increases the size of the executable.

`gnatelim` tracks unused subprograms in an Ada program and outputs a list of GNAT-specific `Eliminate` pragmas (see next section) marking all the subprograms that are declared but never called. By placing the list of `Eliminate` pragmas in the GNAT configuration file ``gnat.adc'` and recompiling your program, you may decrease the size of its executable, because the compiler will not generate the code for 'eliminated' subprograms.

`gnatelim` needs as its input data a set of tree files (see section Tree Files) representing all the components of a program to process and a bind file for a main subprogram (see section Preparing Tree and Bind Files for `gnatelim`).

## `Eliminate` Pragma

The simplified syntax of the Eliminate pragma used by `gnatelim` is:

    pragma Eliminate (Library_Unit_Name, Subprogram_Name);

where

`Library_Unit_Name`
  full expanded Ada name of a library unit
`Subprogram_Name`
  a simple or expanded name of a subprogram declared within this compilation unit

The effect of an `Eliminate` pragma placed in the GNAT configuration file ``gnat.adc'` is:

- If the subprogram `Subprogram_Name` is declared within the library unit `Library_Unit_Name`, the compiler will not generate code for this subprogram. This applies to all overloaded subprograms denoted by `Subprogram_Name`.
- If a subprogram marked by the pragma `Eliminate` is used (called) in a program, the compiler will produce an error message in the place where it is called.

## Tree Files

A tree file stores a snapshot of the compiler internal data structures at the very end of a successful compilation. It contains all the syntactic and semantic information for the copmiled unit and all the units upon which it depends semantically. To use tools that make use of tree files, you need to first produce the right set of tree files.

GNAT produces correct tree files when -gnatt -gnatc options are set in a gcc call. The tree files have an .adt extension. Therefore, to produce a tree file for the compilation unit contained in a file named ``foo.adb'`, you must use the command

    $ gcc -c -gnatc -gnatt foo.adb

and you will get the tree file ``foo.adt'`. compilation.

## Preparing Tree and Bind Files for `gnatelim`

A set of tree files covering the program to be analyzed with `gnatelim` and the bind file for the main subprogram does not have to be in the current directory. '-T' gnatelim option may be used to provide the search path for tree files, and '-b' option may be used to point to the bind file to process (see section Running `gnatelim`)

If you do not have the appropriate set of tree files and the right bind file, you may create them in the current directory using the following procedure.

Let `Main_Prog` be the name of a main subprogram, and suppose this subprogram is in a file named ``main_prog.adb'`.

To create a bind file for `gnatelim`, run `gnatbind` for the main subprogram. `gnatelim` can work with both Ada and C bind files; when both are present, it uses the Ada bind file. The following commands will build the program and create the bind file:

    $ gnatmake -c Main_Prog
    $ gnatbind main_prog

To create a minimal set of tree files covering the whole program, call `gnatmake` for this program as follows:

    $ gnatmake -f -c -gnatc -gnatt Main_Prog

The `-c` gnatmake option turns off the bind and link steps, that are useless anyway because the sources are compiled with `-gnatc` option which turns off code generation.

The `-f` gnatmake option forces recompilation of all the needed sources.

This sequence of actions will create all the data needed by `gnatelim` from scratch and therefore guarantee its consistency. If you would like to use some existing set of files as `gnatelim` output, you must make sure that the set of files is complete and consistent. You can use the `-m` switch to check if there are missed tree files

Note, that `gnatelim` needs neither object nor ALI files.

## Running `gnatelim`

`gnatelim` has the following command-line interface:

    $ gnatelim [options] name

`name` should be a full expanded Ada name of a main subprogram of a program (partition).

`gnatelim` options:

`-q`
  Quiet mode: by default `gnatelim` generates to the standard error stream a trace of the source file names of the compilation units being processed. This option turns this trace off.
`-v`
  Verbose mode: `gnatelim` version information is printed as Ada comments to the standard output stream.
`-a`
  Also look for subprograms from the GNAT run time that can be eliminated.
`-m`
  Check if any tree files are missing for an accurate result.
`\-T\dir`
  When looking for tree files also look in directory dir
`\-b\bind_file`
  Specifices bind_file as the bind file to process. If not set, the name of the bind file is computed from the full expanded Ada name of a main subprogram.
`-dx`
  Activate internal debugging switches. x is a letter or digit, or string of letters or digits, which specifies the type of debugging mode desired. Normally these are used only for internal development or system debugging purposes. You can find full documentation for these switches in the body of the `Gnatelim.Options` unit in the compiler source file ``gnatelim-options.adb'`.

`gnatelim` sends its output to the standard output stream, and all the tracing and debug information is sent to the standard error stream. In order to produce a proper GNAT configuration file ``gnat.adc'`, redirection must be used:

    $ gnatelim Main_Prog > gnat.adc

or

    $ gnatelim Main_Prog >> gnat.adc

In order to append the `gnatelim` output to the existing contents of ``gnat.adc'`.

## Correcting the List of Eliminate Pragmas

In some rare cases it may happen that `gnatelim` will try to eliminate subprograms which are actually called in the program. In this case, the compiler will generate an error message of the form:

    file.adb:106:07: cannot call eliminated subprogram "My_Prog"

You will need to manually remove the wrong `Eliminate` pragmas from the ``gnat.adc'` file. It is advised that you recompile your program from scratch after that because you need a consistent ``gnat.adc'` file during the entire compilation.

## Making Your Executables Smaller

In order to get a smaller executable for your program you now have to recompile the program completely with the new ``gnat.adc'` file created by `gnatelim` in your current directory:

    $ gnatmake -f Main_Prog

(you will need `-f` option for gnatmake to recompile everything with the set of pragmas `Eliminate` you have obtained with `gnatelim`).

Be aware that the set of `Eliminate` pragmas is specific to each program. It is not recommended to merge sets of `Eliminate` pragmas created for different programs in one ``gnat.adc'` file.

## Summary of the gnatelim Usage Cycle

Here is a quick summary of the steps to be taken in order to reduce the size of your executables with `gnatelim`. You may use other GNAT options to control the optimization level, to produce the debugging information, to set search path, etc.

1. Produce a bind file and a set of tree files

       $ gnatmake -c Main_Prog
       $ gnatbind main_prog
       $ gnatmake -f -c -gnatc -gnatt Main_Prog

2. Generate a list of `Eliminate` pragmas

       $ gnatelim Main_Prog >[>] gnat.adc

3. Recompile the application

       $ gnatmake -f Main_Prog

# Other Utility Programs

This chapter discusses some other utility programs available in the Ada environment.

- Using Other Utility Programs with GNAT
- The gnatpsys Utility Program
- The gnatpsta Utility Program
- The External Symbol Naming Scheme of GNAT
- Ada Mode for Glide
- Converting Ada Files to html with gnathtml
- Installing gnathtml

## Using Other Utility Programs with GNAT

The object files generated by GNAT are in standard system format and in particular the debugging information uses this format. This means programs generated by GNAT can be used with existing utilities that depend on these formats.

In general, any utility program that works with C will also often work with Ada programs generated by GNAT. This includes software utilities such as gprof (a profiling program), `gdb` (the FSF debugger), and utilities such as Purify.

## The `gnatpsys` Utility Program

Many of the definitions in package System are implementation-dependent. Furthermore, although the source of the package System is available for inspection, it uses special attributes for parameterizing many of the critical values, so the source is not informative for the casual user.

The `gnatpsys` utility is designed to deal with this situation. It is an Ada program that dynamically determines the values of all the relevant parameters in System, and prints them out in the form of an Ada source listing for System, displaying all the values of interest. This output is generated to ``stdout'`.

To determine the value of any parameter in package System, simply run `gnatpsys` with no qualifiers or arguments, and examine the output. This is preferable to consulting documentation, because you know that the values you are getting are the actual ones provided by the executing system.

## The `gnatpsta` Utility Program

Many of the definitions in package Standard are implementation-dependent. However, the source of this package does not exist as an Ada source file, so these values cannot be determined by inspecting the source. They can be determined by examining in detail the coding of ``cstand.adb'` which creates the image of Standard in the compiler, but this is awkward and requires a great deal of internal knowledge about the system.

The `gnatpsta` utility is designed to deal with this situation. It is an Ada program that dynamically determines the values of all the relevant parameters in Standard, and prints them out in the form of an Ada source listing for Standard, displaying all the values of interest. This output is generated to ``stdout'`.

To determine the value of any parameter in package Standard, simply run `gnatpsta` with no qualifiers or arguments, and examine the output. This is preferable to consulting documentation, because you know that the values you are getting are the actual ones provided by the executing system.

## The External Symbol Naming Scheme of GNAT

In order to interpret the output from GNAT, when using tools that are originally intended for use with other languages, it is useful to understand the conventions used to generate link names from the Ada entity names.

All link names are in all lowercase letters. With the exception of library procedure names, the mechanism used is simply to use the full expanded Ada name with dots replaced by double underscores. For example, suppose we have the following package spec:

    package QRS is
       MN : Integer;
    end QRS;

The variable `MN` has a full expanded Ada name of `QRS.MN`, so the corresponding link name is `qrs__mn`. Of course if a `pragma Export` is used this may be overridden:

    package Exports is
       Var1 : Integer;
       pragma Export (Var1, C, External_Name => "var1_name");
       Var2 : Integer;
       pragma Export (Var2, C, Link_Name => "var2_link_name");
    end Exports;

In this case, the link name for Var1 is whatever link name the C compiler would assign for the C function var1_name. This typically would be either var1_name or _var1_name, depending on operating system conventions, but other possibilities exist. The link name for Var2 is var2_link_name, and this is not operating system dependent.

One exception occurs for library level procedures. A potential ambiguity arises between the required name `_main` for the C main program, and the name we would otherwise assign to an Ada library level procedure called `Main` (which might well not be the main program).

To avoid this ambiguity, we attach the prefix `_ada_` to such names. So if we have a library level procedure such as

    procedure Hello (S : String);

the external name of this procedure will be _ada_hello.

## Ada Mode for `Glide`

The Glide mode for programming in Ada (both, Ada83 and Ada95) helps the user in understanding existing code and facilitates writing new code. It furthermore provides some utility functions for easier integration of standard Emacs features when programming in Ada.

### General Features:

- Full Integrated Development Environment :
  - support of 'project files' for the configuration (directories, compilation options,...)
  - compiling and stepping through error messages.
  - running and debugging your applications within Glide.
- easy to use for beginners by pull-down menus,
- user configurable by many user-option variables.

### Ada Mode Features That Help Understanding Code:

- functions for easy and quick stepping through Ada code,
- getting cross reference information for identifiers (e.g. find the defining place by a keystroke),
- displaying an index menu of types and subprograms and move point to the chosen one,
- automatic color highlighting of the various entities in Ada code.

### Glide Support for Writing Ada Code:

- switching between spec and body files with possible autogeneration of body files,
- automatic formating of subprograms parameter lists.
- automatic smart indentation according to Ada syntax,
- automatic completion of identifiers,
- automatic casing of identifiers, keywords, and attributes,
- insertion of statement templates,
- filling comment paragraphs like filling normal text,

For more information, please see See section Ada Mode for `Glide`.

## Converting Ada Files to html with `gnathtml`

This `Perl` script allows Ada source files to be browsed using standard Web browsers. For installation procedure, see the section See section Installing `gnathtml`.

Ada reserved keywords are highlighted in a bold font and Ada comments in a blue font. Unless your program was compiled with the gcc `-gnatx` switch to suppress the generation of cross-referencing information, user defined variables and types will appear in a different color; you will be able to click on any identifier and go to its declaration.

The command line is as follow:

    $ perl gnathtml.pl [switches] ada-files

You can pass it as many Ada files as you want. `gnathtml` will generate an html file for every ada file, and a global file called ``index.htm'`. This file is an index of every identifier defined in the files.

The available switches are the following ones :

`-83`
  Only the subset on the Ada 83 keywords will be highlighted, not the full Ada 95 keywords set.
`-cc color`
  This option allows you to change the color used for comments. The default value is green. The color argument can be any name accepted by html.
`-d`
  If the ada files depend on some other files (using for instance the `with` command, the latter will also be converted to html. Only the files in the user project will be converted to html, not the files in the run-time library itself.
`-D`
  This command is the same as -d above, but `gnathtml` will also look for files in the run-time library, and generate html files for them.
`-f`
  By default, gnathtml will generate html links only for global entities ('with'ed units, global variables and types,...). If you specify the `-f` on the command line, then links will be generated for local entities too.
`-l number`
  If this switch is provided and number is not 0, then `gnathtml` will number the html files every number line.
`-I dir`
  Specify a directory to search for library files (``.ali'` files) and source files. You can provide several -I switches on the command line, and the directories will be parsed in the order of the command line.
`-o dir`
  Specify the output directory for html files. By default, gnathtml will saved the generated html files in a subdirectory named ``html/'`.
`-p file`
  If you are using Emacs and the most recent Emacs Ada mode, which provides a full Integrated Development Environment for compiling, checking, running and debugging applications, you may be using ``.adp'` files to give the directories where Emacs can find sources and object files. Using this switch, you can tell gnathtml to use these files. This allows you to get an html version of your application, even if it is spread over multiple directories.
`-sc color`
  This option allows you to change the color used for symbol definitions. The default value is red. The color argument can be any name accepted by html.
`-t file`
  This switch provides the name of a file. This file contains a list of file names to be converted, and the effect is exactly as though they had appeared explicitly on the command line. This is the recommended way to work around the command line length limit on some systems.

## Installing `gnathtml`

`Perl` needs to be installed on your machine to run this script. `Perl` is freely available for almost every architecture and Operating System via the Internet.

On Unix systems, you may want to modify the first line of the script `gnathtml`, to explicitly tell the Operating system where Perl is. The syntax of this line is :

    #!full_path_name_to_perl

Alternatively, you may run the script using the following command line:

    $ perl gnathtml.pl [switches] files

# Running and Debugging Ada Programs

This chapter discusses how to debug Ada programs. An incorrect Ada program may be handled in three ways by the GNAT compiler:

1. The illegality may be a violation of the static semantics of Ada. In that case GNAT diagnoses the constructs in the program that are illegal. It is then a straightforward matter for the user to modify those parts of the program.
2. The illegality may be a violation of the dynamic semantics of Ada. In that case the program compiles and executes, but may generate incorrect results, or may terminate abnormally with some exception.
3. When presented with a program that contains convoluted errors, GNAT itself may terminate abnormally without providing full diagnostics on the incorrect user program.

- The GNAT Debugger GDB
- Running GDB
- Introduction to GDB Commands
- Using Ada Expressions
- Calling User-Defined Subprograms
- Ada Exceptions
- Ada Tasks
- Debugging Generic Units
- GNAT Abnormal Termination or Failure to Terminate
- Naming Conventions for GNAT Source Files
- Getting Internal Debugging Information

## The GNAT Debugger GDB

`GDB` is a general purpose, platform-independent debugger that can be used to debug mixed-language programs compiled with `GCC`, and in particular is capable of debugging Ada programs compiled with GNAT. The latest versions of `GDB` are Ada-aware and can handle complex Ada data structures. The manual Debugging with GDB contains full details on the usage of `GDB`, including a section on its usage on programs. This manual should be consulted for full details. The section that follows is a brief introduction to the philosophy and use of `GDB`.

When GNAT programs are compiled, the compiler optionally writes debugging information into the generated object file, including information on line numbers, and on declared types and variables. This information is separate from the generated code. It makes the object files considerably larger, but it does not add to the size of the actual executable that will be loaded into memory, and has no impact on run-time performance. The generation of debug information is triggered by the use of the -g switch in the gcc or gnatmake command used to carry out the compilations. It is important to emphasize that the use of these options does not change the generated code.

The debugging information is written in standard system formats that are used by many tools, including debuggers and profilers. The format of the information is typically designed to describe C types and semantics, but GNAT implements a translation scheme which allows full details about Ada types and variables to be encoded into these standard C formats. Details of this encoding scheme may be found in the file exp_dbug.ads in the GNAT source distribution. However, the details of this encoding are, in general, of no interest to a user, since `GDB` automatically performs the necessary decoding.

When a program is bound and linked, the debugging information is collected from the object files, and stored in the executable image of the program. Again, this process significantly increases the size of the generated executable file, but it does not increase the size of the executable program itself. Furthermore, if this program is run in the normal manner, it runs exactly as if the debug information were not present, and takes no more actual memory.

However, if the program is run under control of `GDB`, the debugger is activated. The image of the program is loaded, at which point it is ready to run. If a run command is given, then the program will run exactly as it would have if `GDB` were not present. This is a crucial part of the `GDB` design philosophy. `GDB` is entirely non-intrusive until a breakpoint is encountered. If no breakpoint is ever hit, the program will run exactly as it would if no debugger were present. When a breakpoint is hit, `GDB` accesses the debugging information and can respond to user commands to inspect variables, and more generally to report on the state of execution.

## Running GDB

The debugger can be launched directly and simply from emacs which allows to browse and modify directly the source code during the debugging session, See section Ada Mode for `Glide`. Here is described the basic use of `GDB` is text mode.

The command to run `GDB` is

    $ gdb program

where `program` is the name of the executable file. This activates the debugger and results in a prompt for debugger commands. The simplest command is simply `run`, which causes the program to run exactly as if the debugger were not present. The following section describes some of the additional commands that can be given to `GDB`.

## Introduction to GDB Commands

`GDB` contains a large repertoire of commands. The manual Debugging with GDB includes extensive documentation on the use of these commands, together with examples of their use. Furthermore, the command help invoked from within `GDB` activates a simple help facility which summarizes the available commands and their options. In this section we summarize a few of the most commonly used commands to give an idea of what `GDB` is about. You should create a simple program with debugging information and experiment with the use of these `GDB` commands on the program as you read through the following section.

`set args arguments`
  The arguments list above is a list of arguments to be passed to the program on a subsequent run command, just as though the arguments had been entered on a normal invocation of the program. The `set args` command is not needed if the program does not require arguments.
`run`
  The `run` command causes execution of the program to start from the beginning. If the program is already running, that is to say if you are currently positioned at a breakpoint, then a prompt will ask for confirmation that you want to abandon the current execution and restart.
`breakpoint location`
  The breakpoint command sets a breakpoint, that is to say a point at which execution will halt and `GDB` will await further commands. location is either a line number within a file, given in the format `file:linenumber`, or it is the name of a subprogram. If you request that a breakpoint be set on a subprogram that is overloaded, a prompt will ask you to specify on which of those subprograms you want to breakpoint. You can also specify that all of them should be breakpointed. If the program is run and execution encounters the breakpoint, then the program stops and `GDB` signals that the breakpoint was encountered by printing the line of code before which the program is halted.
`breakpoint exception name`
  A special form of the breakpoint command which breakpoints whenever exception name is raised. If name is omitted, then a breakpoint will occur when any exception is raised.
`print expression`
  This will print the value of the given expression. Most simple Ada expression formats are properly handled by `GDB`, so the expression can contain function calls, variables, operators, and attribute references.
`continue`
  Continues execution following a breakpoint, until the next breakpoint or the termination of the program.
`step`
  Executes a single line after a breakpoint. If the next statement is a subprogram call, execution continues into (the first statement of) the called subprogram.
`next`
  Executes a single line. If this line is a subprogram call, executes and returns from the call.
`list`
  Lists a few lines around the current source location. In practice, it is usually more convenient to have a separate edit window open with the relevant source file displayed. Successive applications of this command print subsequent lines. The command can be given an argument which is a line number, in which case it displays a few lines around the specified one.
`backtrace`
  Displays a backtrace of the call chain. This command is typically used after a breakpoint has occurred, to examine the sequence of calls that leads to the current breakpoint. The display includes one line for each activation record (frame) corresponding to an active subprogram.
`up`
  At a breakpoint, `GDB` can display the values of variables local to the current frame. The command `up` can be used to examine the contents of other active frames, by moving the focus up the stack, that is to say from callee to caller, one frame at a time.
`down`
  Moves the focus of `GDB` down from the frame currently being examined to the frame of its callee (the reverse of the previous command),
`frame n`
  Inspect the frame with the given number. The value 0 denotes the frame of the current breakpoint, that is to say the top of the call stack.

The above list is a very short introduction to the commands that `GDB` provides. Important additional capabilities, including conditional breakpoints, the ability to execute command sequences on a breakpoint, the ability to debug at the machine instruction level and many other features are described in detail in Debugging with GDB. Note that most commands can be abbreviated (for example, c for continue, bt for backtrace).

## Using Ada Expressions

`GDB` supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is

- That `GDB` should provide basic literals and access to operations for arithmetic, dereferencing, field selection, indexing, and subprogram calls, leaving more sophisticated computations to subprograms written into the program (which therefore may be called from `GDB`).
- That type safety and strict adherence to Ada language restrictions are not particularly important to the `GDB` user.
- That brevity is important to the `GDB` user.

Thus, for brevity, the debugger acts as if there were implicit `with` and `use` clauses in effect for all user-written packages, thus making it unnecessary to fully qualify most names with their packages, regardless of context. Where this causes ambiguity, `GDB` asks the user's intent.

For details on the supported Ada syntax, see Debugging with GDB.

## Calling User-Defined Subprograms

An important capability of `GDB` is the ability to call user-defined subprograms while debugging. This is achieved simply by entering a subprogram call statement in the form:

    call subprogram-name (parameters)

The keyword `call` can be omitted in the normal case where the `subprogram-name` does not coincide with any of the predefined `GDB` commands.

The effect is to invoke the given subprogram, passing it the list of parameters that is supplied. The parameters can be expressions and can include variables from the program being debugged. The subprogram must be defined at the library level within your program, and `GDB` will call the subprogram within the environment of your program execution (which means that the subprogram is free to access or even modify variables within your program).

The most important use of this facility is in allowing the inclusion of debugging routines that are tailored to particular data structures in your program. Such debugging routines can be written to provide a suitably high-level description of an abstract type, rather than a low-level dump of its physical layout. After all, the standard `GDB print` command only knows the physical layout of your types, not their abstract meaning. Debugging routines can provide information at the desired semantic level and are thus enormously useful.

For example, when debugging GNAT itself, it is crucial to have access to the contents of the tree nodes used to represent the program internally. But tree nodes are represented simply by an integer value (which in turn is an index into a table of nodes). Using the `print` command on a tree node would simply print this integer value, which is not very useful. But the PN routine (defined in file treepr.adb in the GNAT sources) takes a tree node as input, and displays a useful high level representation of the tree node, which includes the syntactic category of the node, its position in the source, the integers that denote descendant nodes and parent node, as well as varied semantic information. To study this example in more detail, you might want to look at the body of the PN procedure in the stated file.

## Breaking on Ada Exceptions

You can set breakpoints that trip when your program raises selected exceptions.

`break exception`
  Set a breakpoint that trips whenever (any task in the) program raises any exception.
`break exception name`
  Set a breakpoint that trips whenever (any task in the) program raises the exception name.
`break exception unhandled`
  Set a breakpoint that trips whenever (any task in the) program raises an exception for which there is no handler.
`info exceptions`
`info exceptions regexp`
  The `info exceptions` command permits the user to examine all defined exceptions within Ada programs. With a regular expression, regexp, as argument, prints out only those exceptions whose name matches regexp.

## Ada Tasks

`GDB` allows the following task-related commands:

`info tasks`
  This command shows a list of current Ada tasks, as in the following example:

      @leftskip=0cm
      (gdb) info tasks
        ID       TID P-ID   Thread Pri State                 Name
         1   8088000   0   807e000  15 Child Activation Wait main_task
         2   80a4000   1   80ae000  15 Accept/Select Wait    b
         3   809a800   1   80a4800  15 Child Activation Wait a
      *  4   80ae800   3   80b8000  15 Running               c

  In this listing, the asterisk before the first task indicates it to be the currently running task. The first column lists the task ID that is used to refer to tasks in the following commands.
`break linespec task taskid`
`break linespec task taskid if ...`
  These commands are like the `break ... thread ...`. linespec specifies source lines. Use the qualifier ``task taskid'` with a breakpoint command to specify that you only want `GDB` to stop the program when a particular Ada task reaches this breakpoint. taskid is one of the numeric task identifiers assigned by `GDB`, shown in the first column of the ``info tasks'` display. If you do not specify ``task taskid'` when you set a breakpoint, the breakpoint applies to all tasks of your program. You can use the `task` qualifier on conditional breakpoints as well; in this case, place ``task taskid'` before the breakpoint condition (before the `if`).
`task taskno`
  This command allows to switch to the task referred by taskno. In particular, This allows to browse the backtrace of the specified task. It is advised to switch back to the original task before continuing execution otherwise the scheduling of the program may be perturbated.

For more detailed information on the tasking support, see Debugging with GDB.

## Debugging Generic Units

GNAT always uses code expansion for generic instantiation. This means that each time an instantiation occurs, a complete copy of the original code is made, with appropriate substitutions of formals by actuals.

It is not possible to refer to the original generic entities in `GDB`, but it is always possible to debug a particular instance of a generic, by using the appropriate expanded names. For example, if we have

    procedure g is

       generic package k is
          procedure kp (v1 : in out integer);
       end k;

       package body k is
          procedure kp (v1 : in out integer) is
          begin
             v1 := v1 + 1;
          end kp;
       end k;

       package k1 is new k;
       package k2 is new k;

       var : integer := 1;

    begin
       k1.kp (var);
       k2.kp (var);
       k1.kp (var);
       k2.kp (var);
    end;

Then to break on a call to procedure kp in the k2 instance, simply use the command:

    (gdb) break g.k2.kp

When the breakpoint occurs, you can step through the code of the instance in the normal manner and examine the values of local variables, as for other units.

## GNAT Abnormal Termination or Failure to Terminate

When presented with programs that contain serious errors in syntax or semantics, GNAT may on rare occasions experience problems in operation, such as aborting with a segmentation fault or illegal memory access, raising an internal exception, terminating abnormally, or failing to terminate at all. In such cases, you can activate various features of GNAT that can help you pinpoint the construct in your program that is the likely source of the problem.

The following strategies are presented in increasing order of difficulty, corresponding to your experience in using GNAT and your familiarity with compiler internals.

1. Run `gcc` with the `-gnatf`. This first switch causes all errors on a given line to be reported. In its absence, only the first error on a line is displayed. The `-gnatdO` switch causes errors to be displayed as soon as they are encountered, rather than after compilation is terminated. If GNAT terminates prematurely or goes into an infinite loop, the last error message displayed may help to pinpoint the culprit.
2. Run `gcc` with the `-v (verbose)` switch. In this mode, `gcc` produces ongoing information about the progress of the compilation and provides the name of each procedure as code is generated. This switch allows you to find which Ada procedure was being compiled when it encountered a code generation problem.
3. Run `gcc` with the `-gnatdc` switch. This is a GNAT specific switch that does for the front-end what `-v` does for the back end. The system prints the name of each unit, either a compilation unit or nested unit, as it is being analyzed.
4. Finally, you can start `gdb` directly on the `gnat1` executable. `gnat1` is the front-end of GNAT, and can be run independently (normally it is just called from `gcc`). You can use `gdb` on `gnat1` as you would on a C program (but see section The GNAT Debugger GDB for caveats). The `where` command is the first line of attack; the variable `lineno` (seen by `print lineno`), used by the second phase of `gnat1` and by the `gcc` backend, indicates the source line at which the execution stopped, and `input_file name` indicates the name of the source file.

## Naming Conventions for GNAT Source Files

In order to examine the workings of the GNAT system, the following brief description of its organization may be helpful:

- Files with prefix ``sc'` contain the lexical scanner.
- All files prefixed with ``par'` are components of the parser. The numbers correspond to chapters of the Ada 95 Reference Manual. For example, parsing of select statements can be found in ``par-ch9.adb'`.
- All files prefixed with ``sem'` perform semantic analysis. The numbers correspond to chapters of the Ada standard. For example, all issues involving context clauses can be found in ``sem_ch10.adb'`. In addition, some features of the language require sufficient special processing to justify their own semantic files: sem_aggr for aggregates, sem_disp for dynamic dispatching, etc.
- All files prefixed with ``exp'` perform normalization and expansion of the intermediate representation (abstract syntax tree, or AST). these files use the same numbering scheme as the parser and semantics files. For example, the construction of record initialization procedures is done in ``exp_ch3.adb'`.
- The files prefixed with ``bind'` implement the binder, which verifies the consistency of the compilation, determines an order of elaboration, and generates the bind file.
- The files ``atree.ads'` and ``atree.adb'` detail the low-level data structures used by the front-end.
- The files ``sinfo.ads'` and ``sinfo.adb'` detail the structure of the abstract syntax tree as produced by the parser.
- The files ``einfo.ads'` and ``einfo.adb'` detail the attributes of all entities, computed during semantic analysis.
- Library management issues are dealt with in files with prefix ``lib'`.
- Ada files with the prefix ``a-'` are children of `Ada`, as defined in Annex A.
- Files with prefix ``i-'` are children of `Interfaces`, as defined in Annex B.
- Files with prefix ``s-'` are children of `System`. This includes both language-defined children and GNAT run-time routines.
- Files with prefix ``g-'` are children of `GNAT`. These are useful general-purpose packages, fully documented in their specifications. All the other ``.c'` files are modifications of common `gcc` files.

## Getting Internal Debugging Information

Most compilers have internal debugging switches and modes. GNAT does also, except GNAT internal debugging switches and modes are not secret. A summary and full description of all the compiler and binder debug flags are in the file ``debug.adb'`. You must obtain the sources of the compiler to see the full detailed effects of these flags.

The switches that print the source of the program (reconstructed from the internal tree) are of general interest for user programs, as are the options to print the full internal tree, and the entity table (the symbol table information). The reconstructed source provides a readable version of the program after the front-end has completed analysis and expansion, and is useful when studying the performance of specific constructs. For example, constraint checks are indicated, complex aggregates are replaced with loops and assignments, and tasking primitives are replaced with run-time calls.

# Performance Considerations

The GNAT system provides a number of options that allow a trade-off between

- performance of the generated code
- speed of compilation
- minimization of dependences and recompilation
- the degree of run-time checking.

The defaults (if no options are selected) aim at improving the speed of compilation and minimizing dependences, at the expense of performance of the generated code:

- no optimization
- no inlining of subprogram calls
- all run-time checks enabled except overflow and elaboration checks

These options are suitable for most program development purposes. This chapter describes how you can modify these choices, and also provides some guidelines on debugging optimized code.

- Controlling Run-Time Checks
- Optimization Levels
- Debugging Optimized Code
- Inlining of Subprograms

## Controlling Run-Time Checks

By default, GNAT produces all run-time checks, except arithmetic overflow checking for integer operations (that includes division by zero) and checks for access before elaboration on subprogram calls. Two gnat switches, `-gnatp` and `-gnato` allow this default to be modified. See section Run-Time Checks.

Our experience is that the default is suitable for most development purposes.

We treat integer overflow specially because these are quite expensive and in our experience are not as important as other run-time checks in the development process.

Elaboration checks are off by default, and also not needed by default, since GNAT uses a static elaboration analysis approach that avoids the need for run-time checking. This manual contains a full chapter discussing the issue of elaboration checks, and if the default is not satisfactory for your use, you should read this chapter.

Note that the setting of the switches controls the default setting of the checks. They may be modified using either `pragma Suppress` (to remove checks) or `pragma Unsuppress` (to add back suppressed checks) in the program source.

## Optimization Levels

The default is optimization off. This results in the fastest compile times, but GNAT makes absolutely no attempt to optimize, and the generated programs are considerably larger and slower than when optimization is enabled. You can use the `-On` switch, where n is an integer from 0 to 3, on the `gcc` command line to control the optimization level:

`-O0`
  no optimization (the default)
`-O1`
  medium level optimization
`-O2`
  full optimization
`-O3`
  full optimization, and also attempt automatic inlining of small subprograms within a unit (see section Inlining of Subprograms).

Higher optimization levels perform more global transformations on the program and apply more expensive analysis algorithms in order to generate faster and more compact code. The price in compilation time, and the resulting improvement in execution time, both depend on the particular application and the hardware environment. You should experiment to find the best level for your application.

Note: Unlike some other compilation systems, `gcc` has been tested extensively at all optimization levels. There are some bugs which appear only with optimization turned on, but there have also been bugs which show up only in unoptimized code. Selecting a lower level of optimization does not improve the reliability of the code generator, which in practice is highly reliable at all optimization levels.

## Debugging Optimized Code

Since the compiler generates debugging tables for a compilation unit before it performs optimizations, the optimizing transformations may invalidate some of the debugging data. You therefore need to anticipate certain anomalous situations that may arise while debugging optimized code. This section describes the most common cases.

1. The "hopping Program Counter": Repeated 'step' or 'next' commands show the PC bouncing back and forth in the code. This may result from any of the following optimizations:
   - Common subexpression elimination: using a single instance of code for a quantity that the source computes several times. As a result you may not be able to stop on what looks like a statement.
   - Invariant code motion: moving an expression that does not change within a loop, to the beginning of the loop.
   - Instruction scheduling: moving instructions so as to overlap loads and stores (typically) with other code, or in general to move computations of values closer to their uses. Often this causes you to pass an assignment statement without the assignment happening and then later bounce back to the statement when the value is actually needed. Placing a breakpoint on a line of code and then stepping over it may, therefore, not always cause all the expected side-effects.
2. The "big leap": More commonly known as cross-jumping, in which two identical pieces of code are merged and the program counter suddenly jumps to a statement that is not supposed to be executed, simply because it (and the code following) translates to the same thing as the code that was supposed to be executed. This effect is typically seen in sequences that end in a jump, such as a `goto`, a `return`, or a `break` in a C `switch` statement.
3. The "roving variable": The symptom is an unexpected value in a variable. There are various reasons for this effect:
   - In a subprogram prologue, a parameter may not yet have been moved to its "home".
   - A variable may be dead, and its register re-used. This is probably the most common cause.
   - As mentioned above, the assignment of a value to a variable may have been moved.
   - A variable may be eliminated entirely by value propagation or other means. In this case, GCC may incorrectly generate debugging information for the variable
   In general, when an unexpected value appears for a local variable or parameter you should first ascertain if that value was actually computed by your program, as opposed to being incorrectly reported by the debugger. Record fields or array elements in an object designated by an access value are generally less of a problem, once you have ascertained that the access value is sensible. Typically, this means checking variables in the preceding code and in the calling subprogram to verify that the value observed is explainable from other values (one must apply the procedure recursively to those other values); or re-running the code and stopping a little earlier (perhaps before the call) and stepping to better see how the variable obtained the value in question; or continuing to step from the point of the strange value to see if code motion had simply moved the variable's assignments later.

## Inlining of Subprograms

A call to a subprogram in the current unit is inlined if all the following conditions are met:

- The optimization level is at least `-O1`.
- The called subprogram is suitable for inlining: It must be small enough and not contain nested subprograms or anything else that `gcc` cannot support in inlined subprograms.
- The call occurs after the definition of the body of the subprogram.
- Either `pragma Inline` applies to the subprogram or it is small and automatic inlining (optimization level `-O3`) is specified.

Calls to subprograms in `with`'ed units are normally not inlined. To achieve this level of inlining, the following conditions must all be true:

- The optimization level is at least `-O1`.
- The called subprogram is suitable for inlining: It must be small enough and not contain nested subprograms or anything else `gcc` cannot support in inlined subprograms.
- The call appears in a body (not in a package spec).
- There is a `pragma Inline` for the subprogram.
- The `-gnatn` switch is used in the `gcc` command line

Note that specifying the `-gnatn` switch causes additional compilation dependencies. Consider the following:

    package R is
       procedure Q;
       pragma Inline (Q);
    end R;
    package body R is
       ...
    end R;

    with R;
    procedure Main is
    begin
       ...
       R.Q;
    end Main;

With the default behavior (no `-gnatn` switch specified), the compilation of the `Main` procedure depends only on its own source, ``main.adb'`, and the spec of the package in file ``r.ads'`. This means that editing the body of `R` does not require recompiling `Main`.

On the other hand, the call `R.Q` is not inlined under these circumstances. If the `-gnatn` switch is present when `Main` is compiled, the call will be inlined if the body of `Q` is small enough, but now `Main` depends on the body of `R` in ``r.adb'` as well as on the spec. This means that if this body is edited, the main program must be recompiled. Note that this extra dependency occurs whether or not the call is in fact inlined by `gcc`.

The use of front end inlining with `-gnatN` generates similar additional dependencies.

Note: The `-fno-inline` switch can be used to prevent all inlining. This switch overrides all other conditions and ensures that no inlining occurs. The extra dependences resulting from `-gnatn` will still be active, even if this switch is used to suppress the resulting inlining actions.

# Index

## -

- `--GCC=compiler_name` (`gnatlink`)
- `--GCC=compiler_name` (`gnatmake`)
- `--GNATBIND=binder_name` (`gnatmake`)
- `--GNATLINK=linker_name` (`gnatmake`)
- `--LINK=` (`gnatlink`)
- `-83` (`gnathtml`)
- `-A` (`gnatbind`)
- `-A` (`gnatlink`)
- `-a` (`gnatls`)
- `-A` (`gnatmake`)
- `-a` (`gnatmake`)
- `-aI` (`gnatmake`)
- `-aL` (`gnatmake`)
- `-aO` (`gnatmake`)
- `-B` (`gcc`)
- `-b` (`gcc`)
- `-b` (`gnatbind`)
- `-B` (`gnatlink`)
- `-b` (`gnatlink`)
- `-bargs` (`gnatmake`)
- `-c` (`gcc`)
- `-c` (`gnatbind`)
- `-C` (`gnatbind`)
- `-c` (`gnatchop`)
- `-C` (`gnatlink`)
- `-c` (`gnatmake`)
- `-cargs` (`gnatmake`)
- `-d` (`gnathtml`)
- `-d` (`gnatls`)
- `-e` (`gnatbind`)
- `-f` (`gnatbind`)
- `-f` (`gnathtml`)
- `-f` (`gnatlink`)
- `-f` (`gnatmake`)
- `-fno-inline` (`gcc`)
- -fstack-check
- `-funwind-tables` (`gcc`)
- `-g` (`gcc`)
- `-g` (`gnatlink`)
- `-gnat83` (`gcc`)
- `-gnata` (`gcc`)
- `-gnatb` (`gcc`)
- `-gnatc` (`gcc`)
- `-gnatD` (`gcc`)
- `-gnatdc` switch
- `-gnatE` (`gcc`), `-gnatE` (`gcc`)
- `-gnatf` (`gcc`)
- `-gnatG` (`gcc`)
- `-gnati` (`gcc`)
- `-gnatk` (`gcc`)
- `-gnatl` (`gcc`)
- `-gnatm` (`gcc`)
- `-gnatn` switch
- `-gnatN` switch
- `-gnatN` (`gcc`)
- `-gnatn` (`gcc`), `-gnatn` (`gcc`)
- `-gnato` (`gcc`), `-gnato` (`gcc`)
- `-gnatp` (`gcc`), `-gnatp` (`gcc`)
- `-gnatq` (`gcc`)
- `-gnatR` (`gcc`)
- `-gnats` (`gcc`)
- `-gnatT` (`gcc`)
- `-gnatt` (`gcc`)
- `-gnatu` (`gcc`)
- `-gnatU` (`gcc`)
- `-gnatv` (`gcc`)
- `-gnatW` (`gcc`)
- `-gnatwA` (`gcc`)
- `-gnatwa` (`gcc`)
- `-gnatwc` (`gcc`)
- `-gnatwC` (`gcc`)
- `-gnatwe` (`gcc`)
- `-gnatwh` (`gcc`)
- `-gnatwH` (`gcc`)
- `-gnatwi` (`gcc`)
- `-gnatwI` (`gcc`)
- `-gnatwl` (`gcc`)
- `-gnatwL` (`gcc`)
- `-gnatwo` (`gcc`)
- `-gnatwO` (`gcc`)
- `-gnatwp` (`gcc`)
- `-gnatwP` (`gcc`)
- `-gnatwR` (`gcc`)
- `-gnatwr` (`gcc`)
- `-gnatws` (`gcc`)
- `-gnatwU` (`gcc`)
- `-gnatwu` (`gcc`)
- `-gnatx` (`gcc`)
- `-h` (`gnatbind`), `-h` (`gnatbind`)
- `-h` (`gnatls`)
- `-I-` (`gcc`)
- `-I-` (`gnatmake`)
- `-I` (`gcc`)
- `-I` (`gnathtml`)
- `-i` (`gnatmake`)
- `-I` (`gnatmake`)
- `-i` (`gnatmem`)
- `-j` (`gnatmake`)
- `-k` (`gnatchop`)
- `-k` (`gnatmake`)
- `-l` (`gnatbind`)
- `-l` (`gnathtml`)
- `-L` (`gnatmake`)
- `-largs` (`gnatmake`)
- `-M` (`gnatbind`)
- `-m` (`gnatbind`)
- `-M` (`gnatmake`)
- `-m` (`gnatmake`)
- `-n` (`gnatbind`)
- `-n` (`gnatlink`)
- `-n` (`gnatmake`)
- `-nostdinc` (`gnatmake`)
- `-nostdlib` (`gnatmake`)
- `-o` (`gcc`)
- `-O` (`gcc`), `-O` (`gcc`)
- `-O` (`gnatbind`)
- `-o` (`gnatbind`)
- `-o` (`gnathtml`)
- `-o` (`gnatlink`)
- `-o` (`gnatls`)
- `-o` (`gnatmake`)
- `-o` (`gnatmem`)
- `-p` (`gnathtml`)
- `-pass-exit-codes` (`gcc`)
- `-q` (`gnatchop`)
- `-q` (`gnatmake`)
- `-q` (`gnatmem`)
- `-r` (`gnatchop`)
- `-S` (`gcc`)
- `-s` (`gnatbind`)
- `-s` (`gnatls`), `-s` (`gnatls`)
- `-s` (`gnatmake`)
- `-sc` (`gnathtml`)
- `-t` (`gnatbind`)
- `-t` (`gnathtml`)
- `-u` (`gnatls`)
- `-u` (`gnatmake`)
- `-v -v` (`gnatlink`)
- `-V` (`gcc`)
- `-v` (`gcc`)
- `-v` (`gnatbind`)
- `-v` (`gnatchop`)
- `-v` (`gnatlink`)
- `-v` (`gnatmake`)
- `-w`
- `-w` (`gnatchop`)
- `-we` (`gnatbind`)
- `-ws` (`gnatbind`)
- `-x` (`gnatbind`)
- `-z` (`gnatbind`)
- `-z` (`gnatmake`)

## _

- __gnat_finalize
- __gnat_initialize
- __gnat_set_globals, __gnat_set_globals
- _main

## a

- Access before elaboration
- Access-to-subprogram
- ACVC, Ada 83 tests
- Ada, Ada
- Ada 83 compatibility
- Ada 95 Language Reference Manual
- Ada expressions
- Ada Library Information files
- Ada.Characters.Latin_1
- ADA_INCLUDE_PATH
- ADA_OBJECTS_PATH
- adafinal, adafinal
- adainit, adainit
- ``ali'` files
- Annex A
- Annex B
- Asm
- Assert
- Assertions

## b

- Binder consistency checks
- Binder output file
- Binder, multiple input files
- Breakpoints and tasks

## c

- C
- C++
- Calling Conventions
- Check, elaboration
- Check, overflow
- Checks, access before elaboration
- Checks, division by zero
- Checks, elaboration
- Checks, overflow
- Checks, suppressing
- COBOL
- code page 437
- code page 850
- Combining GNAT switches
- Compilation model
- Configuration pragmas
- Consistency checks, in binder
- Convention Ada
- Convention Asm
- Convention Assembler
- Convention C
- Convention C++
- Convention COBOL
- Convention Fortran
- Convention Stdcall
- Convention Stubbed
- Conventions
- CR

## d

- Debug
- Debug Pool
- Debugger
- Debugging
- Debugging Generic Units
- Debugging information, including
- Debugging options
- Dependencies, producing list
- Dependency rules
- Division by zero

## e

- Elaborate
- Elaborate_All
- Elaborate_Body
- Elaboration checks, Elaboration checks
- Elaboration control, Elaboration control
- Elaboration of library tasks
- Elaboration order control
- Eliminate
- End of source file
- Error messages, suppressing
- EUC Coding
- Exceptions
- Export

## f

- FF
- File names, File names
- File naming schemes, alternative
- Foreign Languages
- Fortran

## g

- gdb
- Generic formal parameters
- Generics, Generics
- `GMEM` (`gnatmem`)
- GNAT, GNAT
- GNAT Abnormal Termination or Failure to Terminate
- GNAT compilation model
- GNAT library
- ``gnat.adc'`, ``gnat.adc'`
- gnat1
- gnat_argc
- gnat_argv
- GNAT_STACK_LIMIT
- gnatbind
- gnatchop
- gnatelim
- gnatfind
- gnatkr
- gnatlink
- gnatls
- gnatmake
- gnatmem
- gnatprep
- gnatstub
- gnatxref
- GNU make

## h

- HT

## i

- Inline, Inline
- Inlining
- Interfaces, Interfaces
- Interfacing to Ada
- Interfacing to Assembler
- Interfacing to C
- Interfacing to C++
- Interfacing to COBOL
- Interfacing to Fortran
- Internal trees, writing to file

## l

- Latin-1, Latin-1
- Latin-2
- Latin-3
- Latin-4
- LF
- Library browser
- Library tasks, elaboration issues
- Library, building, installing
- Linker libraries

## m

- Machine_Overflows
- Main Program
- make
- makefile
- Mixed Language Programming
- Multiple units, syntax checking

## n

- `n` (`gnatmem`)
- No code generated
- No_Entry_Calls_In_Elaboration_Code

## o

- Object file list
- Order of elaboration
- Other Ada compilers
- Overflow checks, Overflow checks

## p

- Parallel make
- Performance
- pragma Elaborate
- pragma Elaborate_All
- pragma Elaborate_Body
- pragma Inline
- pragma Preelaborate
- pragma Pure
- pragma Suppress
- pragma Unsuppress
- Pragmas, configuration
- Preelaborate
- Pure

## r

- Recompilation, by `gnatmake`
- RTL, RTL

## s

- SDP_Table_Build
- Search paths, for `gnatmake`
- Shift JIS Coding
- Source file, end
- Source files, suppressing search
- Source files, use by binder
- Source_File_Name pragma, Source_File_Name pragma
- Source_Reference
- Stack Overflow Checking
- Stdcall
- stderr, stderr
- stdout
- storage, pool, memory corruption
- Stubbed
- Style checking
- SUB
- Subunits
- Suppress, Suppress
- Suppressing checks
- System, System
- System.IO

## t

- Task switching
- Tasks
- Time Slicing
- Time stamp checks, in binder
- Tree file
- Typographical conventions

## u

- Unsuppress, Unsuppress
- Upper-Half Coding

## v

- VT

## w

- Warning messages
- Warnings
- Writing internal trees

## z

- Zero Cost Exceptions

This document was generated on 10 May 2001 using the texi2html translator version 1.51.