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 項25.3.4.1. 「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.)