Building clients with ZOOM

YAZ User's Guide and Reference
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# Chapter 3. Building clients with ZOOM

ZOOM is an acronym for 'Z39.50 Object-Orientation Model' and is an initiative started by Mike Taylor (Mike is from the UK, which explains the peculiar name of the model). The goal of ZOOM is to provide a common Z39.50 client API not bound to a particular programming language or toolkit.

The lack of a simple Z39.50 client API for YAZ has become more and more apparent over time. So when the first ZOOM specification became available, an implementation for YAZ was quickly developed. For the first time, it is now as easy (or easier!) to develop clients than servers with YAZ. This chapter describes the ZOOM C binding. Before going futher, please reconsider whether C is the right programming language for the job. There are other language bindings available for YAZ, and still more are in active development. See the ZOOM website at zoom.z3950.org for more information.

In order to fully understand this chapter you should read and try the example programs `zoomtst1.c`, `zoomtst2.c`, .. in the `zoom` directory.

The C language misses many features found in object oriented languages such as C++, Java, etc. For example, you'll have to manually, destroy all objects you create, even though you may think of them as temporary. Most objects has a `_create` - and a `_destroy` variant. All objects are in fact pointers to internal stuff, but you don't see that because of typedefs. All destroy methods should gracefully ignore a `NULL` pointer.

# Connections

The Connection object is a session with a target.

       #include <yaz/zoom.h>

       Z3950_connection Z3950_connection_new (const char *host, int portnum);

       Z3950_connection Z3950_connection_create (Z3950_options options);

       void Z3950_connection_connect(Z3950_connection c, const char *host,
                                     int portnum);
       void Z3950_connection_destroy (Z3950_connection c);

Connection objects are created with either function `Z3950_connection_new` or `Z3950_connection_create`. The former creates and automatically attempts to establish a network connection with the target. The latter doesn't establish a connection immediately, thus allowing you to specify options before establishing network connection using the function `Z3950_connection_connect`. If the portnumber, `portnum`, is zero, the `host` is consulted for a port specification. If no port is given, 210 is used. A colon denotes the beginning of a port number in the host string. If the host string includes a slash, the following part specifies a database for the connection.

Connection objects should be destroyed using the function `Z3950_connection_destroy`.

        const char *Z3950_connection_option (Z3950_connection c,
                                             const char *key,
                                             const char *val);
        const char *Z3950_connection_host (Z3950_connection c);

The `Z3950_connection_option` allows you to inspect or set an option given by `key` for the connection. If `val` is non-`NULL` that holds the new value for option. Otherwise, if `val` is `NULL`, the option is unchanged. The function returns the previous value of the option.

Table 3-1. ZOOM Connection Options

Option | Description | Default
--- | --- | ---
implementationName | Name of Your client | none
user | Authentication user name | none
group | Authentication group name | none
pass | Authentication password | none
proxy | Proxy host | none
async | If true (1) the connection operates in asynchronous operation which means that all calls are non-blocking except `Z3950_event`. | 0
maximumRecordSize | Maximum size of single record. | 1 MB
preferredMessageSize | Maximum size of multiple records. | 1 MB

Function `Z3950_connection_host` returns the host for the connection as specified in a call to `Z3950_connection_new` or `Z3950_connection_connect`. This function returns `NULL` if host isn't set for the connection.

         int Z3950_connection_error (Z3950_connection c, const char **cp,
                                     const char **addinfo);

Use `Z3950_connection_error` to check for errors for the last operation(s) performed. The function returns zero if no errors occurred; non-zero otherwise indicating the error. Pointers `cp` and `addinfo` holds messages for the error and additional-info if passed as non-`NULL`.

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