RTEMS POSIX API User's Guide
The mq_send() function adds the message in priority order to the message queue. Each message has an assigned a priority. The highest priority message is be at the front of the queue.
The maximum number of messages that a message queue may accept is specified at creation by the mq_maxmsg field of the attribute structure. If this amount is exceeded, the behavior of the process is determined according to what oflag was used when the message queue was opened. If the queue was opened with O_NONBLOCK flag set, the process does not block, and an error is returned. If the O_NONBLOCK flag was not set, the process does block and wait for space on the queue.
RTEMS POSIX API User's Guide
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