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<a href="https://gitlab.rtems.org/joel">Joel Sherrill</a>
commented on a
<a href="https://gitlab.rtems.org/rtems/rtos/rtems/-/issues/3791#note_143088">discussion</a>:
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<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Your survey looks good. I would say Linux is not compliant since it is using LIFO. And unfortunately, POSIX gives us the "if you support the _<em style="margin-top: 0;">POSIX_PRIORITY_SCHEDULING</em> <strong style="font-weight: 600;">option</strong>" then the blocking discipline for all message queues changes. This is a system implementation decision so there really is not portability. You cannot control what behavior your application is going to get.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">I think as a minimum, RTEMS should support a configuration option to toggle between FIFO and priority. If we don't do that much, then existing applications will have a change in behavior that will break backwards compatibility.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">Adding a field to <em style="margin-top: 0;">mq_attr</em> gives the user fine gained control on each message queue instance.</p>
<p dir="auto" style="color: #3a383f; margin: 0 0 1rem;" align="initial">I think adding an <em style="margin-top: 0;">mq_is_priority</em> to out <em>mq_attr</em> structure is a good move. And giving the user a way to define their own default <em>mq_attr</em> instance allows them change each entry in the default structure. It would let them also override maximum message size and maximum pending messages. It would take adding to confdefs.h a macro default for each field in the structure and another to turn on using them. The current default attributes could be a weak symbol that is overridden.</p>
<p dir="auto" style="color: #3a383f; margin: 0;" align="initial">I think this solution may also be needed for POSIX semaphores. They have the same language for the _<em style="margin-top: 0;">POSIX_PRIORITY_SCHEDULING</em> option</p>
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