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(mysql.info)UDF calling sequences

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UDF Calling Sequences
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The main function should be declared as shown below.  Note that the
return type and parameters differ, depending on whether you will
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);

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 are listed below.  The
initialization function should fill in any members that it wishes to
change.  (To use the default for a member, leave it unchanged.):

`my_bool maybe_null'
     `xxx_init()' should set `maybe_null' to `1' if `xxx()' can return
     `NULL'.  The default value is `1' if any of the arguments are
     declared `maybe_null'.

`unsigned int decimals'
     Number of decimals.  The default value is the maximum number of
     decimals in the arguments passed to the main function.  (For
     example, if the function is passed `1.34', `1.345', and `1.3', the
     default would be 3, because `1.345' has 3 decimals.

`unsigned int max_length'
     The maximum length of the string result.  The default value
     differs depending on the result type of the function.  For string
     functions, the default is the length of the longest argument.  For
     integer functions, the default is 21 digits.  For real functions,
     the default is 13 plus the number of decimals indicated by
     `initid->decimals'.  (For numeric functions, the length includes
     any sign or decimal point characters.)

`char *ptr'
     A pointer that the function can use for its own purposes.  For
     example, functions can use `initid->ptr' to communicate allocated
     memory between functions.  In `xxx_init()', allocate the memory
     and assign it to this pointer:

          initid->ptr = allocated_memory;

     In `xxx()' and `xxx_deinit()', refer to `initid->ptr' to use or
     deallocate the memory.