MySQL 5.7 C API Developer Guide
The MySQL C API includes functions enabling threaded client applications to be written. These functions provide control over thread initialization and termination with the client. See also Section 3.4, “Writing C API Threaded Client Programs”.
    Another C API function,
    mysql_thread_id(), has
    “thread” in its name but is not used for client
    threading purposes. Instead, it returns the ID of the server thread
    associated with the client, much like the
    CONNECTION_ID() SQL function. See
    Section 5.4.78, “mysql_thread_id()”.