RTEMS POSIX API User's Guide
The mq_setattr() function is used to set attributes associated with the open message queue description referenced by the message queue descriptor specified by mqdes. The *omqstat represents the old or previous attributes. If omqstat is non-NULL, the function mq_setattr() stores, in the location referenced by omqstat, the previous message queue attributes and the current queue status. These values are the same as would be returned by a call to mq_getattr() at that point.
There is only one mq_attr.mq_flag that can be altered by this call. This is the flag that deals with the blocking and non-blocking behavior of the message queue. If the flag is set then the message queue is non-blocking, and requests to send or receive do not block while waiting for resources. If the flag is not set, then message send and receive may involve waiting for an empty queue or waiting for a message to arrive.
RTEMS POSIX API User's Guide
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