This function must be called early within each created thread to initialize thread-specific variables. However, it may be unnecessarily to invoke it explicitly. Calling mysql_thread_init() is automatically handled my_init(), which itself is called by mysql_init(), mysql_library_init(), mysql_server_init(), and mysql_connect(). If you invoke any of those functions, mysql_thread_init() is called for you.

Zero for success. Nonzero if an error occurred.