14.1.3 Valeurs retournées, et gestion des erreurs

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 will be 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. For string functions, the string is returned in the result and length arguments. result is a buffer at least 255 bytes long. Set these to the contents and length of the return value. For example:

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

The string function return value normally also points to the result.

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 the error parameter 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() will not even be called for subsequent rows.) Note: In MySQL versions prior to 3.22.10, you should set both *error and *is_null:

*error = 1;
*is_null = 1;