mq_close will return zero after successful completion. All notification requests that have been successfully attached to the message queue through the descriptor being closed will be removed, so another process will be able to set up notification. Calling mq_close with an invalid descriptor will result in a mq_close returning -1 and errno being set to EBADF. mq_close removes the association between the descriptor being closed and the message queue it represents. It is undefined what will happen if mqdes is used after mq_close() is successfully called and mq_open() has not been called again.