POSIX中正确的消息队列使用

Question

I'm quite bewildered by the use of message queues in realtime OS. The code that was given seems to have message queues used down to the bone: even passing variables to another class object is done through MQ. I always have a concept of MQ used in IPC. Question is: what is a proper use of a message queue?

Answer 1

In realtime OS environments you often face the problem that you have to guarantee execution of code at a fixed schedule. Eg you may have a function that gets called exactly each 10 milliseconds. Not earlier, not later.

To guarantee such hard timing constraints you have to write code that must not block the time critical code under any circumstances.

The posix thread synchronization primitives from cannot be used here. You must never lock a mutex or aqurie a semaphore from time critical code because a different process/thread may already have it locked. However, often you are allowed to unblock some other thread from time critical code (eg releasing a semaphore is okay).

In such environments message queues are a nice choice to exchange data because they offer a clean way to pass data from one thread to another without ever blocking.

Using queues to just set variables may sound like overkill, but it is very good software design. If you do it that way you have a well-defined interface to your time critical code.

Also it helps to write deterministic code because you'll never run into the problem of race-conditions. If you set variables via message-queues you can be sure that the time critical code sees the messages in the same order as they have been sent. When mixing direct memory access and messages you can't guarantee this.

Answer 2

Message Queues are predominantly used as an IPC Mechanism , whenever there needs to be exchange of data between two different processes. However, sometimes Message Queues are also used for thread context switching. For eg:
You register some callback with a software layer which sits on top of driver. The callback is returned to you in the context of the driver. It is a thread spawned by the driver. Now you cannot hog this thread of driver by doing a lot of processing in it. So one may add the data returned in callback in a message Queue, which has application threads blocked on it for performing the processing on the data.

I dont see why one should use Message Queues for replacing just normal function calls.