4.4.2.3 Client Plugin Descriptors

Each client plugin must have a descriptor that provides information to the client plugin API. The descriptor structure begins with a fixed set of members common to all client plugins, followed by any members specific to the plugin type.

The st_mysql_client_plugin structure in the client_plugin.h file defines a “generic” descriptor that contains the common members:

struct st_mysql_client_plugin
{
  int type;
  unsigned int interface_version;
  const char *name;
  const char *author;
  const char *desc;
  unsigned int version[3];
  const char *license;
  void *mysql_api;
  int (*init)(char *, size_t, int, va_list);
  int (*deinit)();
  int (*options)(const char *option, const void *);
};

The common st_mysql_client_plugin descriptor structure members are used as follows. char * members should be specified as null-terminated strings.

• type: The plugin type. This must be one of the plugin-type values from client_plugin.h, such as MYSQL_CLIENT_AUTHENTICATION_PLUGIN.

• interface_version: The plugin interface version. For example, this is MYSQL_CLIENT_AUTHENTICATION_PLUGIN_INTERFACE_VERSION for an authentication plugin.

• name: A string that gives the plugin name. This is the name by which you refer to the plugin when you call mysql_options() with the MYSQL_DEFAULT_AUTH option or specify the --default-auth option to a MySQL client program.

• author: A string naming the plugin author. This can be whatever you like.

• desc: A string that provides a general description of the plugin. This can be whatever you like.

• version: The plugin version as an array of three integers indicating the major, minor, and teeny versions. For example, {1,2,3} indicates version 1.2.3.

• license: A string that specifies the license type.

• mysql_api: For internal use. Specify it as NULL in the plugin descriptor.

• init: A once-only initialization function, or NULL if there is no such function. The client library executes this function when it loads the plugin. The function returns zero for success and nonzero for failure.

  The init function uses its first two arguments to return an error message if an error occurs. The first argument is a pointer to a char buffer, and the second argument indicates the buffer length. Any message returned by the init function must be null-terminated, so the maximum message length is the buffer length minus one. The next arguments are passed to mysql_load_plugin(). The first indicates how many more arguments there are (0 if none), followed by any remaining arguments.

• deinit: A once-only deinitialization function, or NULL if there is no such function. The client library executes this function when it unloads the plugin. The function takes no arguments. It returns zero for success and nonzero for failure.

• options: A function for handling options passed to the plugin, or NULL if there is no such function. The function takes two arguments representing the option name and a pointer to its value. The function returns zero for success and nonzero for failure.

For a given client plugin type, the common descriptor members may be followed by additional members necessary to implement plugins of that type. For example, the st_mysql_client_plugin_AUTHENTICATION structure for authentication plugins has a function at the end that the client library calls to perform authentication.

To declare a plugin, use the mysql_declare_client_plugin() and mysql_end_client_plugin macros:

mysql_declare_client_plugin(plugin_type)
   ... members common to all client plugins ...
   ... type-specific extra members ...
mysql_end_client_plugin;

Do not specify the type or interface_version member explicitly. The mysql_declare_client_plugin() macro uses the plugin_type argument to generate their values automatically. For example, declare an authentication client plugin like this:

mysql_declare_client_plugin(AUTHENTICATION)
  "my_auth_plugin",
  "Author Name",
  "My Client Authentication Plugin",
  {1,0,0},
  "GPL",
  NULL,
  my_auth_init,
  my_auth_deinit,
  my_auth_options,
  my_auth_main
mysql_end_client_plugin;

This declaration uses the AUTHENTICATION argument to set the type and interface_version members to MYSQL_CLIENT_AUTHENTICATION_PLUGIN and MYSQL_CLIENT_AUTHENTICATION_PLUGIN_INTERFACE_VERSION.

Depending on the plugin type, the descriptor may have other members following the common members. For example, for an authentication plugin, there is a function (my_auth_main() in the descriptor just shown) that handles communication with the server. See Section 4.4.9, “Writing Authentication Plugins”.

Normally, a client program that supports the use of authentication plugins causes a plugin to be loaded by calling mysql_options() to set the MYSQL_DEFAULT_AUTH and MYSQL_PLUGIN_DIR options:

char *plugin_dir = "path_to_plugin_dir";
char *default_auth = "plugin_name";

/* ... process command-line options ... */

mysql_options(&mysql, MYSQL_PLUGIN_DIR, plugin_dir);
mysql_options(&mysql, MYSQL_DEFAULT_AUTH, default_auth);

Typically, the program will also accept --plugin-dir and --default-auth options that enable users to override the default values.

Should a client program require lower-level plugin management, the client library contains functions that take an st_mysql_client_plugin argument. See C API Client Plugin Interface.