RTEMS POSIX API User's Guide
The mq_receive() function is used to receive the oldest of the highest priority message(s) from the message queue specified by mqdes. The messages are received in FIFO order within the priorities. The received message's priority is stored in the location referenced by the msg_prio. If the msg_prio is a NULL, the priority is discarded. The message is removed and stored in an area pointed to by msg_ptr whose length is of msg_len. The msg_len must be at least equal to the mq_msgsize attribute of the message queue.
The blocking behavior of the message queue is set by O_NONBLOCK at mq_open or by setting O_NONBLOCK in mq_flags in a call to mq_setattr. If this is a blocking queue, the process does block and wait on an empty queue. If this a non-blocking queue, the process does not block. Upon successful completion, mq_receive returns the length of the selected message in bytes and the message is removed from the queue.
RTEMS POSIX API User's Guide
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