So what's the trick?

There's no trick! QNX OS does this automatically for you. (You can turn off priority inheritance if you don't want it; see the _NTO_CHF_FIXED_PRIORITY flag in the ChannelCreate() function's documentation.)

There's a minor design issue here, however. How do you revert the priority to what it was before it got changed?

Your server is running along, servicing requests from clients, adjusting its priority automagically when it unblocks from the MsgReceive() call. But when should it adjust its priority back to what it was before the MsgReceive() call changed it?

There are two cases to consider:

• The server performs some additional processing after it properly services the client. This should be done at the server's priority, not the client's.
• The server immediately does another MsgReceive() to handle the next client request.

In the first case, it would be incorrect for the server to run at the client's priority when it's no longer doing work for that client! The solution is fairly simple. Use the pthread_setschedparam() or pthread_setschedprio() function (discussed in the Processes and Threads chapter) to revert the priority back to what it should be.

What about the other case? The answer is subtly simple: Who cares?

Think about it. What difference does it make if the server becomes RECEIVE-blocked when it was priority 29 versus when it was priority 2? The fact of the matter is it's RECEIVE-blocked! It isn't getting any CPU time, so its priority is irrelevant. As soon as the MsgReceive() function unblocks the server, the (new) client's priority is inherited by the server and everything works as expected.