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 license;          /* the plugin license (PLUGIN_LICENSE_XXX)      */
  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;
  void * __reserved1;   /* placeholder for system variables             */
  void * __reserved2;   /* placeholder for config options               */
};

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

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:

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.

In the st_mysql_plugin structure that defines a plugin's general declaration, the info member points to a type-specific plugin descriptor. For a full-text parser plugin, the descriptor corresponds to the st_mysql_ftparser structure in the plugin.h file:

struct st_mysql_ftparser
{
  int interface_version;
  int (*parse)(MYSQL_FTPARSER_PARAM *param);
  int (*init)(MYSQL_FTPARSER_PARAM *param);
  int (*deinit)(MYSQL_FTPARSER_PARAM *param);
};

As shown by the structure definition, the descriptor has a version number (MYSQL_FTPARSER_INTERFACE_VERSION for full-text parser plugins) and contains pointers to three functions. The init and deinit members should point to a function or be set to 0 if the function is not needed. The parse member must point to the function that performs the parsing.

A full-text parser plugin is used in two different contexts, indexing and searching. In both contexts, the server calls the initialization and deinitialization functions at the beginning and end of processing each SQL statement that causes the plugin to be invoked. However, during statement processing, the server calls the main parsing function in context-specific fashion:

Note that the plugin declaration in the plugin library descriptor has initialization and deinitialization functions, and so does the plugin descriptor to which it points. These pairs of functions have different purposes and are invoked for different reasons:

Each interface function named in the plugin descriptor should return zero for success or non-zero for failure, and each of them receives an argument that points to a MYSQL_FTPARSER_PARAM structure containing the parsing context. The structure has this definition:

typedef struct st_mysql_ftparser_param
{
  int (*mysql_parse)(struct st_mysql_ftparser_param *,
                     char *doc, int doc_len);
  int (*mysql_add_word)(struct st_mysql_ftparser_param *,
                        char *word, int word_len,
                        MYSQL_FTPARSER_BOOLEAN_INFO *boolean_info);
  void *ftparser_state;
  void *mysql_ftparam;
  struct charset_info_st *cs;
  char *doc;
  int length;
  int flags;
  enum enum_ftparser_mode mode;
} MYSQL_FTPARSER_PARAM;

Note: The definition shown is current as of MySQL 5.1.12. It is incompatible with versions of MySQL 5.1 older than 5.1.12.

The structure members are used as follows:

If the parser is called in boolean mode, the param->mode value will be MYSQL_FTPARSER_FULL_BOOLEAN_INFO. The MYSQL_FTPARSER_BOOLEAN_INFO structure that the parser uses for passing token information to the server looks like this:

typedef struct st_mysql_ftparser_boolean_info
{
  enum enum_ft_token_type type;
  int yesno;
  int weight_adjust;
  bool wasign;
  bool trunc;
  /* These are parser state and must be removed. */
  byte prev;
  byte *quot;
} MYSQL_FTPARSER_BOOLEAN_INFO;

The parser should fill in the structure members as follows:

Plugins should not use the prev and quot members of the MYSQL_FTPARSER_BOOLEAN_INFO structure.

This section provides a step-by-step procedure for creating a plugin library. It shows how to develop a library that contains a full-text parsing plugin named simple_parser. This plugin performs parsing based on simpler rules than those used by the MySQL built-in full-text parser: Words are non-empty runs of whitespace characters.

Each plugin library has the following contents:

The easiest way to follow the instructions in this section is to use the source code in the plugin/fulltext directory of a MySQL source distribution. The instructions assume that you make a copy of that directory and use it to build the plugin library. To make a copy of the directory, use the following commands, which assume that the MySQL source tree is in a directory named mysql-5.1 under your current directory:

shell> mkdir fulltext_plugin
shell> cp mysql-5.1/plugin/fulltext/* fulltext_plugin

If you are copying files from a BitKeeper source tree, cp will display an error message about the SCCS directory, which you can ignore.

After copying the source files, use the following procedure to create a plugin library:

Note how neither “case” nor “insensitive” match “case-insensitive” the way that they would for the built-in parser.

MySQL plugins can be written in C or C++ (or another language that can use C calling conventions). One feature of C++ is that you can use non-constant variables to initialize global structures. However, if you write a C++ plugin, you should not use this feature. Members of structures such as the st_mysql_plugin structure should be initialized with constant variables. The simple_parser descriptor shown earlier is allowable in a C++ plugin because it satisfies that requirement:

mysql_declare_plugin
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  "simple_parser",            /* name                            */
  "MySQL AB",                 /* author                          */
  "Simple Full-Text Parser",  /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status               /* status variables                */
}
mysql_declare_plugin_end;

Here is another valid way to write the descriptor. It uses constant variables to indicate the plugin name, author, and description:

const char *simple_parser_name = "simple_parser";
const char *simple_parser_author = "MySQL AB";
const char *simple_parser_description = "Simple Full-Text Parser";

mysql_declare_plugin
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  simple_parser_name,         /* name                            */
  simple_parser_author,       /* author                          */
  simple_parser_description,  /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status               /* status variables                */
}
mysql_declare_plugin_end;

However, the following descriptor is invalid. It uses structure members to indicate the plugin name, author, and description, but structures are not considered constant initializers in C++:

typedef struct
{
  const char *name;
  const char *author;
  const char *description;
} plugin_info;

plugin_info parser_info = {
  "simple_parser",
  "MySQL AB",
  "Simple Full-Text Parser"
};

mysql_declare_plugin
{
  MYSQL_FTPARSER_PLUGIN,      /* type                            */
  &simple_parser_descriptor,  /* descriptor                      */
  parser_info.name,           /* name                            */
  parser_info.author,         /* author                          */
  parser_info.description,    /* description                     */
  PLUGIN_LICENSE_GPL,         /* plugin license                  */
  simple_parser_plugin_init,  /* init function (when loaded)     */
  simple_parser_plugin_deinit,/* deinit function (when unloaded) */
  0x0001,                     /* version                         */
  simple_status               /* status variables                */
}
mysql_declare_plugin_end;

This section describes the different functions that you need to define when you create a simple UDF. Section 26.3.4, “Adding a New User-Defined Function”, describes the order in which MySQL calls these functions.

The main xxx() function should be declared as shown in this section. Note that the return type and parameters differ, depending on whether you declare the SQL function XXX() to return STRING, INTEGER, or REAL in the CREATE FUNCTION statement:

For STRING functions:

char *xxx(UDF_INIT *initid, UDF_ARGS *args,
          char *result, unsigned long *length,
          char *is_null, char *error);

For INTEGER functions:

long long xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

For REAL functions:

double xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

DECIMAL functions return string values and should be declared the same way as STRING functions. ROW functions are not implemented.

The initialization and deinitialization functions are declared like this:

my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

void xxx_deinit(UDF_INIT *initid);

The initid parameter is passed to all three functions. It points to a UDF_INIT structure that is used to communicate information between functions. The UDF_INIT structure members follow. The initialization function should fill in any members that it wishes to change. (To use the default for a member, leave it unchanged.)

This section describes the different functions that you need to define when you create an aggregate UDF. Section 26.3.4, “Adding a New User-Defined Function”, describes the order in which MySQL calls these functions.

The xxx() function for an aggregate UDF should be declared the same way as for a non-aggregate UDF. See Section 26.3.4.1, “UDF Calling Sequences for Simple Functions”.

For an aggregate UDF, MySQL calls the xxx() function after all rows in the group have been processed. You should normally never access its UDF_ARGS argument here but instead return a value based on your internal summary variables.

Return value handling in xxx() should be done the same way as for a non-aggregate UDF. See Section 26.3.4.4, “UDF Return Values and Error Handling”.

The xxx_reset() and xxx_add() functions handle their UDF_ARGS argument the same way as functions for non-aggregate UDFs. See Section 26.3.4.3, “UDF Argument Processing”.

The pointer arguments to is_null and error are the same for all calls to xxx_reset(), xxx_clear(), xxx_add() and xxx(). You can use this to remember that you got an error or whether the xxx() function should return NULL. You should not store a string into *error! error points to a single-byte variable, not to a string buffer.

*is_null is reset for each group (before calling xxx_clear()). *error is never reset.

If *is_null or *error are set when xxx() returns, MySQL returns NULL as the result for the group function.

The args parameter points to a UDF_ARGS structure that has the members listed here:

The initialization function should return 0 if no error occurred and 1 otherwise. If an error occurs, xxx_init() should store a null-terminated error message in the message parameter. The message is returned to the client. The message buffer is MYSQL_ERRMSG_SIZE characters long, but you should try to keep the message to less than 80 characters so that it fits the width of a standard terminal screen.

The return value of the main function xxx() is the function value, for long long and double functions. A string function should return a pointer to the result and set *result and *length to the contents and length of the return value. For example:

memcpy(result, "result string", 13);
*length = 13;

The result buffer that is passed to the xxx() function is 255 bytes long. If your result fits in this, you don't have to worry about memory allocation for results.

If your string function needs to return a string longer than 255 bytes, you must allocate the space for it with malloc() in your xxx_init() function or your xxx() function and free it in your xxx_deinit() function. You can store the allocated memory in the ptr slot in the UDF_INIT structure for reuse by future xxx() calls. See Section 26.3.4.1, “UDF Calling Sequences for Simple Functions”.

To indicate a return value of NULL in the main function, set *is_null to 1:

*is_null = 1;

To indicate an error return in the main function, set *error to 1:

*error = 1;

If xxx() sets *error to 1 for any row, the function value is NULL for the current row and for any subsequent rows processed by the statement in which XXX() was invoked. (xxx() is not even called for subsequent rows.)

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file sql/udf_example.c that is included in the MySQL source distribution.

The immediately following instructions are for Unix. Instructions for Windows are given later in this section.

The udf_example.c file contains the following functions:

A dynamically loadable file should be compiled as a sharable object file, using a command something like this:

shell> gcc -shared -o udf_example.so udf_example.c

If you are using gcc with configure and libtool (which is how MySQL is configured), you should be able to create udf_example.so with a simpler command:

shell> make udf_example.la

After you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from udf_example.c using gcc directly produces a file named udf_example.so. Compiling the shared object using make produces a file named something like udf_example.so.0.0.0 in the .libs directory (the exact name may vary from platform to platform). Copy the shared object to the server's plugin directory and name it udf_example.so. This directory is given by the value of the plugin_dir system variable. (Note: This a change in MySQL 5.1. For earlier versions of MySQL, the shared object can be located in any directory that is searched by your system's dynamic linker.)

On some systems, the ldconfig program that configures the dynamic linker does not recognize a shared object unless its name begins with lib. In this case you should rename a file such as udf_example.so to libudf_example.so.

On Windows, you can compile user-defined functions by using the following procedure:

After the shared object file has been installed, notify mysqld about the new functions with these statements:

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME 'udf_example.so';
mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME 'udf_example.so';
mysql> CREATE FUNCTION lookup RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE FUNCTION reverse_lookup
    ->        RETURNS STRING SONAME 'udf_example.so';
mysql> CREATE AGGREGATE FUNCTION avgcost
    ->        RETURNS REAL SONAME 'udf_example.so';

Functions can be deleted using DROP FUNCTION:

mysql> DROP FUNCTION metaphon;
mysql> DROP FUNCTION myfunc_double;
mysql> DROP FUNCTION myfunc_int;
mysql> DROP FUNCTION lookup;
mysql> DROP FUNCTION reverse_lookup;
mysql> DROP FUNCTION avgcost;

The CREATE FUNCTION and DROP FUNCTION statements update the func system table in the mysql database. The function's name, type and shared library name are saved in the table. You must have the INSERT and DELETE privileges for the mysql database to create and drop functions.

You should not use CREATE FUNCTION to add a function that has previously been created. If you need to reinstall a function, you should remove it with DROP FUNCTION and then reinstall it with CREATE FUNCTION. You would need to do this, for example, if you recompile a new version of your function, so that mysqld gets the new version. Otherwise, the server continues to use the old version.

An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION. All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable.

If the new function will be referred to in statements that will be replicated to slave servers, you must ensure that every slave server also has the function available. Otherwise, replication will fail on the slaves when they attempt to invoke the function.

MySQL takes the following measures to prevent misuse of user-defined functions.

You must have the INSERT privilege to be able to use CREATE FUNCTION and the DELETE privilege to be able to use DROP FUNCTION. This is necessary because these statements add and delete rows from the mysql.func table.

UDFs should have at least one symbol defined in addition to the xxx symbol that corresponds to the main xxx() function. These auxiliary symbols correspond to the xxx_init(), xxx_deinit(), xxx_reset(), xxx_clear(), and xxx_add() functions. mysqld also supports an --allow-suspicious-udfs option that controls whether UDFs that have only an xxx symbol can be loaded. By default, the option is off, to prevent attempts at loading functions from shared object files other than those containing legitimate UDFs. If you have older UDFs that contain only the xxx symbol and that cannot be recompiled to include an auxiliary symbol, it may be necessary to specify the --allow-suspicious-udfs option. Otherwise, you should avoid enabling this capability.

UDF object files cannot be placed in arbitrary directories. They must be located in the server's plugin directory. This directory is given by the value of the plugin_dir system variable. (Note: This a change in MySQL 5.1. For earlier versions of MySQL, the shared object can be located in any directory that is searched by your system's dynamic linker.)