Message queue descriptor.
Buffer to write the message to
Size of the buffer provided as msg_ptr. Must be greater than the mq_msgsize attribute of the queue.
If not null, set to the priority of this message.
Specify a ceiling on the time to block if the queue is empty.
On success, mq_receive() returns the number of bytes in the received message; on error, -1 is returned, with errno set to indicate the error
Receive the oldest message with the highest priority the message queue, wait up to a certain timeout.