(mysql.info)Adding UDF

For the UDF mechanism to work, functions must be written in C or C++
and your operating system must support dynamic loading.  The MySQL
source distribution includes a file `sql/udf_example.cc' that defines 5
new functions.  Consult this file to see how UDF calling conventions
work.

For `mysqld' to be able to use UDF functions, you should configure MySQL
with `--with-mysqld-ldflags=-rdynamic' The reason is that to on many
platforms (including Linux) you can load a dynamic library (with
`dlopen()') from a static linked program, which you would get if you
are using `--with-mysqld-ldflags=-all-static' If you want to use an UDF
that needs to access symbols from `mysqld' (like the `methaphone'
example in `sql/udf_example.cc' that uses `default_charset_info'), you
must link the program with `-rdynamic'.  (see `man dlopen').

For each function that you want to use in SQL statements, you should
define corresponding C (or C++) functions.  In the discussion below,
the name "xxx" is used for an example function name.  To distinquish
between SQL and C/C++ usage, `XXX()' (uppercase) indicates a SQL
function call, and `xxx()' (lowercase) indicates a C/C++ function call.

The C/C++ functions that you write to implement the interface for
`XXX()' are:

`xxx()' (required)
     The main function.  This is where the function result is computed.
     The correspondence between the SQL type and return type of your
     C/C++ function is shown below:

     *SQL type*    *C/C++ type*
     `STRING'      `char *'
     `INTEGER'     `long long'
     `REAL'        `double'

`xxx_init()' (optional)
     The initialization function for `xxx()'.  It can be used to:

        * Check the number of arguments to `XXX()'.

        * Check that the arguments are of a required type or,
          alternatively, tell MySQL to coerce arguments to the types
          you want when the main function is called.

        * Allocate any memory required by the main function.

        * Specify the maximum length of the result.

        * Specify (for `REAL' functions) the maximum number of decimals.

        * Specify whether or not the result can be `NULL'.

`xxx_deinit()' (optional)
     The deinitialization function for `xxx()'.  It should deallocate
     any memory allocated by the initialization function.

When a SQL statement invokes `XXX()', MySQL calls the initialization
function `xxx_init()' to let it perform any required setup, such as
argument checking or memory allocation.  If `xxx_init()' returns an
error, the SQL statement is aborted with an error message and the main
and deinitialization functions are not called.  Otherwise, the main
function `xxx()' is called once for each row.  After all rows have been
processed, the deinitialization function `xxx_deinit()' is called so it
can perform any required cleanup.

All functions must be thread safe (not just the main function, but the
initialization and deinitialization functions as well). This means that
you are not allowed to allocate any global or static variables that
change!  If you need memory, you should allocate it in `xxx_init()' and
free it in `xxx_deinit()'.