(mysql.info) udf-return-values

 
 24.2.4.4 UDF Return Values and Error Handling
 .............................................
 
 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 
 udf-calling.
 
 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.)
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