# Incompatibliities between Make and Makepp

makepp was designed to be as close as possible to GNU make. However, because of the difference in philosophy, some of GNU make's features cannot be supported; others have not been implemented because I haven't had time. Also, in order to emulate GNU make's behavior precisely, you may in some cases have to add additional command line options to the makepp command line, as noted below. Most of the differences from GNU make are quite technical and only rarely cause problems.

Makepp will give warning messages for many things which the traditional unix make accepts without flinching. This is because there are better ways to do them with makepp. If these warnings annoy you, you can turn them off with the `--nowarn` command line option.

## Incompatibilities

- Target-specific variables are not yet supported. For example, the following will not work with makepp:

      xyz.o: CFLAGS := -g

- The `VPATH` variable is currently ignored. `vpath` statements are unsupported and will cause errors. Use repositories instead.
- Pattern rules only match files in the directory (or if you include the `--percent-subdirs` option, in its subdirectories). This means that a rule like this:

      %.o: %.c
      	$(CC) $(CFLAGS) -c $(input) -o $(output)

  will not be applied to files like `../shared/xyz.c`.

- A pattern rule present later in a makefile overrides one that is present earlier. This is backwards from GNU make.
- Default rules (pattern rules with no pattern dependencies) are not supported.
- The set of builtin implicit rules is somewhat different from those for GNU make, though the variable names are largely compatible. The builtin rules should successfully compile C/C++/Fortran programs, and in fact may be able to guess the proper libraries in some cases too. Support for Modula-2 and RatFor and other rare languages is deliberately not present, because I kept running into problems with GNU make's rules when I accidently reused the extensions for those languages.
- Archive members are not supported, and neither are the associated automatic variables `$%`, `$(%D)`, and `$(%F)`.
- Leading and trailing whitespace in variable assignments is ignored (even if the whitespace is followed by a comment). For more details on whitespace handling incompatibilities, see the section on `--norc-substitution`.
- Double colon rules are not currently supported; they are treated as single colon rules. Because of the way makepp is implemented, they will probably never be supported.
- Makepp does not attempt to rebuild files included with the `include` statement unless the makefile contains a rule for building them before the include statement is seen. (It will attempt to rebuild the makefile itself, however.)
- The `SHELL` variable is currently ignored. makepp always uses `/bin/sh`.
- Dependencies of anything on the Makefile still work, but are usually unnecessary. This is usually used to force a rebuild when compilation options change. makepp knows when build commands have changed without anything special in the makefile; it stores this on a file-by-file basis. If you change the makefile, it knows exactly which files need recompilation.
- `$?` is equivalent to `$+`. See the section on automatic variables for the reasons.
- Intermediate files are not deleted. (Because makepp insists on having all of the file dates be the same as they were on the last build, intermediate files must all be present or else rebuilds will occur.) There is no special status accorded to intermediate files. This means that the special targets `.INTERMEDIATE`, `.SECONDARY`, and `.PRECIOUS` are ignored.

- The only special target that is supported is `.PHONY`. The remaining are simply ingored.

  Specifically, GNU make has the following special targets:

  `.SUFFIXES`
    Makepp ignores `.SUFFIXES` except for the special case of `.SUFFIXES` with no dependencies, like this:

        .SUFFIXES:

    which tells it not to load any of its default rules.

  `.INTERMEDIATE`, `.SECONDARY`, `.PRECIOUS`
    No special status is accorded to intermediate files and so these targets are not meaningful.
  `.IGNORE`
    Put the word `ignore_error` (or a minus sign) in front of the command whose exit status is to be ignored.
  `.SILENT` Put the word `noecho` (or the `@` character) in front of the command which is not supposed to be echoed.
  `.DELETE_ON_ERROR`
  `.EXPORT_ALL_VARIABLES`
  `.NOEXPORT` These targets are not supported and are simply ignored.
  `.POSIX`
  `.DEFAULT`
    These targets are simply ignored.
- Several GNU make functions are not currently supported. These are `origin`, `call`, `error`, and `warning`.
- The `$(wildcard )` function matches not only files which exist, but also files which do not yet exist, but which have a rule which makepp has seen at the time the `$(wildcard )` function is evaluated.
- The `override` statement is not supported.
- `-include` will not attempt to make the include file if it doesn't exist. Also, if the file exists but is out of date with respect to its dependencies, it will not be remade; it is not considered an implicit target.

  This is usually used for files containing dependency information, and since makepp is able to compute a lot of this without depending on additional tools, `-include` is not as important as it used to be.

  The makefile itself is ordinarily considered an implicit target. It will be rebuilt and reread if any of its dependencies have changed since the last time makepp rebuilt it.

- "Canned command sequences" (strings defined with the `define` statement) are not supported.
- Makepp does not support the following environment variables (it does not set them up, and it just ignores them):
  - `MAKEOVERRIDES`
  - `MFLAGS`
  - `MAKELEVEL`
  - `MAKEFILES`

## Incompatibilities without the --percent-subdirs option

By default, `%` in a pattern rule does not match directories. Thus `%.c` matches only `.c` files in the current directory. If you want it to match files in subdirectories too, then add the `--percent-subdirs` option to the command line. You can also enable this in your makefile by the assignment `percent_subdirs=1`.

## Incompatibilities without the -m option

By default, makepp will attempt to rebuild all targets if any of the dependencies have changed since the last build, or if the command has changed. This is normally what you want. Sometimes, however, you don't want the target to be rebuilt if it has been modified apart from the control of makepp (e.g., by editing it, or by running a program manually to make the file). You can force makepp to use the traditional make algorithm, which only rebuilds if any of the targets are newer than the dependencies, by adding the option `-m target_newer` to the command line.

As a special exception, any targets which are built while rebuilding the makefile are automatically checked using the `target_newer` method in order to avoid problems with configure procedures.

## Incompatibilities without the --traditional-recursion option

Recursive invocations of make are often considered to be an unsafe practice (see the section on recursive make in the discussion of deprecated idioms), but they are extremely common in existing makefiles. Makepp supports recursive make for backward compatibility; for new makefiles, it is much better to use the `load_makefile` statement, or makepp's implicit makefile loading mechanism.

Sometimes recursive make invocations set variables on the command line:

    target: dependencies
    	cd subdir && $(MAKE) VAR=val VAR2=val2 some-other-target

If you do this, you may have to add the `--noimplicit-load` option to the command line to avoid conflicts between what the parent process and the child process think the build rules are.

In order to be able to use repositories for variant builds, and to help make recursive invocations of make safter, makepp normally does not actually invoke itself recursively even if you tell it to. Instead, a subprocess communicates with the parent process, and the actual build is done by the parent process.

This works in most cases, but there are a few incompatibilities. (All of these incompatibilities are removed by adding the `--traditional-recursive-make` option to the command line.)

- You may not invoke several makefiles from the same directory, e.g., the following will not work:

      target: dependencies
      	$(MAKE) -f other_makefile targets

  However, this will work:

      target: dependencies
      	cd subdir && $(MAKE) -f other_makefile targets

- Changes in environment variables are not passed to the recursive make process; the environment of the parent is used instead.
- The `MAKEFLAGS` variable is not set up, and altering it has no effect.

This may seem like a long list of restrictions, but many makefiles obey them. For example, as far as I know, all makefiles produced by `automake` follow these restrictions.

All of these restrictions go away if you add the `--traditional-recursive-make` option to the command line, but that has the following undesirable side effects:

- Recursive makes do not execute in parallel, even if the parent does.
- Recursive make processes do not know anything about repositories.
- Each recursive make process produces its own log file, in the directory it is invoked in, instead of producing one log file for the entire build.

Even with the `--traditional-recursive-make` option, there are a few remaining incompatibilities:

- The environment variables `MAKEOVERRIDES` and `MFLAGS` not set up, and are ignored.

## Incompatibilities without the --norc-substitution option

Rc-style substitution is the default way makepp performs variable substitution into text strings because it very rarely breaks legacy makefiles and is often useful in new makefiles. However, it does introduce occasional incompatibilities in the substitution of variables not surrounded by spaces. For example,

    INCLUDE_PREFIX := -I/some/include/dir -I
    INCLUDES := $(INCLUDE_PREFIX)/other/include/dir

will set `INCLUDES` to

    -I/some/include/dir/other/include/dir -I/other/include/dir

if rc-style substitution is enabled, whereas GNU make would set it to

    -I/some/include/dir -I/other/include/dir

There is also an incompatibility in the handling of whitespace in a variable:

    null :=
    T := -o $(null)		# T contains -o followed by one space.
    OUTFILE = $(T)outfile

will set `OUTFILE` to `-ooutfile` if rc-style substitution is enabled, whereas GNU make would set it to `-o outfile`.

Both of these incompatibilities are removed by the `--norc-substitution` option. Note, however, that even with `--norc-substitution`, makepp still treats whitespace incompatibly in some situations:

    T := -o # Don't delete this comment.

GNU make sets `T` to contain `-o` followed by a space, whereas makepp strips out the trailing space anyway. If you want the trailing space, you must specify `--norc-substitution` and also set `T` using the technique involving a dummy variable such as `null`, as shown above.

## Command line incompatibilities

At present, you may not combine single character options into a single word. E.g., you may not say `makepp -ejk 2` instead of `makepp -e -j 2 -k`.

Makepp supports a few of make's more useful command line options. The following, however, are not supported, and are ignored after a warning message is printed:

- `-d` (`--debug`)
- `-i`
- `-I`
- `-l` (`--load-average` or `--max-load`)
- `-n` (`--just-print`, `--dry-run`, or `--recon`)
- `-o` (`--old-file` or `--assume-old`)
- `-p` (`--print-data-base`)
- `-q` (`--question`)
- -r (`--no-builtin-rules`)
  (Note that makepp never has any builtin rules.)
- `-R` (`--no-builtin-variables`)
  (Note that makepp never has any builtin variables.)
- `-S` (`--no-keep-going` or `--stop`)
- `-t` (`--touch`)
- `-w` (`--print-directory`)
- `--no-print-directory`
- `-W` (`--what-if` or `--new-file` or `--assume-new`)
- `--warn-undefined-variables`

Some of these can be easily supported if anyone cares.

Note that the `-q` option at present actually does something different--it suppresses some of makepp's chatty informational messages.

The `-m` option also has a different function.

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Last modified: Wed May 16 10:00:56 PDT 2001