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25.2.5.2 Type-Specific Plugin Structures and Functions »
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25.2.5.1. General Plugin Structures and Functions

Every plugin must have a general plugin declaration. The declaration corresponds to the st_mysql_plugin structure in the plugin.h file: 

struct st_mysql_plugin
{
  int type;             /* the plugin type (a MYSQL_XXX_PLUGIN value)   */
  void *info;           /* pointer to type-specific plugin descriptor   */
  const char *name;     /* plugin name                                  */
  const char *author;   /* plugin author (for SHOW PLUGINS)             */
  const char *descr;    /* general descriptive text (for SHOW PLUGINS ) */
  int (*init)(void);    /* the function to invoke when plugin is loaded */
  int (*deinit)(void);  /* the function to invoke when plugin is unloaded */
  unsigned int version; /* plugin version (for SHOW PLUGINS)            */
  struct st_mysql_show_var *status_vars;
};

The st_mysql_plugin structure is common to every type of plugin. Its members should be filled in as follows:

  • type

    The plugin type. This must be one of the plugin-type values from plugin.h. For a full-text parser plugin, the type value is MYSQL_FTPARSER_PLUGIN.

  • info

    A pointer to the descriptor for the plugin. Unlike the general plugin declaration structure, this descriptor's structure depends on the particular type of plugin. Each descriptor has a version number that indicates the API version for that type of plugin, plus any other members needed. The descriptor for full-text plugins is described in 項25.2.5.2. 「Type-Specific Plugin Structures and Functions」.

  • name

    The plugin name. This is the name that will be listed in the plugin table and by which you refer to the plugin in SQL statements such as INSTALL PLUGIN and UNINSTALL PLUGIN.

  • author

    The plugin author. This can be whatever you like.

  • desc

    A general description of the plugin. This can be whatever you like.

  • init

    A once-only initialization function. This is executed when the plugin is loaded, which happens for INSTALL PLUGIN or, for plugins listed in the plugin table, at server startup. The function takes no arguments. It returns zero for success and non-zero for failure. If an init function is unneeded for a plugin, it can be specified as 0.

  • deinit

    A once-only deinitialization function. This is executed when the plugin is unloaded, which happens for UNINSTALL PLUGIN or, for plugins listed in the plugin table, at server shutdown. The function takes no arguments. It returns zero for success and non-zero for failure. If a deinit function is unneeded for a plugin, it can be specified as 0.

  • version

    The plugin version number. When the plugin is installed, this value can be retrieved from the INFORMATION_SCHEMA.PLUGINS table. The value includes major and minor numbers. If you write the value as a hex constant, the format is 0xMMNN, where MM and NN are the major and minor numbers, respectively. For example, 0x0302 represents version 3.2.

  • status_vars

    A pointer to a structure for status variables associated with the plugin, or 0 if there are no such variables. When the plugin is installed, these variables are displayed in the output of the SHOW STATUS statement.

The init and deinit functions in the general plugin declaration are invoked only when loading and unloading the plugin. They have nothing to do with use of the plugin such as happens when an SQL statement causes the plugin to be invoked.

The status_vars member, if not 0, points to an array of st_mysql_show_var structures, each of which describes one status variable, followed by a structure with all members set to 0. The st_mysql_show_var structure has this definition: 

struct st_mysql_show_var {
  const char *name;
  char *value;
  enum enum_mysql_show_type type;
};

When the plugin is installed, the plugin name and the name value are joined with an underscore to form the name displayed by SHOW STATUS.

The following table shows the allowable status variable type values and what the corresponding variable should be:

Type Meaning
SHOW_BOOL Pointer to a boolean variable
SHOW_INT Pointer to an integer variable
SHOW_LONG Pointer to a long integer variable
SHOW_LONGLONG Pointer to a longlong integer variable
SHOW_CHAR A string
SHOW_CHAR_PTR Pointer to a string
SHOW_ARRAY Pointer to another st_mysql_show_var array
SHOW_FUNC Pointer to a function

For the SHOW_FUNC type, the function is called and fills in its out parameter, which then provides information about the variable to be displayed. The function has this calling sequence: 

#define SHOW_VAR_FUNC_BUFF_SIZE 1024

typedef int (*mysql_show_var_func) (void *thd,
                                    struct st_mysql_show_var *out,
                                    char *buf);

Plugins should consider the thd parameter to be read-only.