RTEMS POSIX API User's Guide
If the message queue already exists, mq_open() opens it, if the message queue does not exist, mq_open() creates it. When a message queue is created, the geometry of the message queue is contained in the attribute structure that is passed in as an argument. This includes mq_msgsize that dictates the maximum size of a single message, and the mq_maxmsg that dictates the maximum number of messages the queue can hold at one time. The blocking or non-blocking behavior of the queue can also specified.
RTEMS POSIX API User's Guide
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