Message Thread Using IoCompletionPort

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Introduction

Windows has a simplified built-in mechanism for communication among threads, using message loops. APIs such as GetMessage(), PeekMessage() can be used to achieve this. E.g.,

MSG msg;
PeekMessage(
    &msg,
    NULL,
    0,
    0,
    PM_NOREMOVE);
while (true)
{
    BOOL bRet = GetMessage(
            &msg,
            NULL,
            0,
            0);
}

Once the loop is setup, use PostThreadMessage() to send the message across. E.g.,

PostThreadMessage(
    nThreadId,
    nMsg,
    wParam,
    lParam);

In fact, message based communication is the building block of Windows. Of course, it has serious security implications as the source of the message is not known to the receiver, but that's a topic for another article.

In this article, I used this concept and the MFC macro style message handler declaration and wrapped them in a class that lets you send messages in two different modes:

• PostMessage
• SendMessage with timeout

The Code

In order to construct the message loop, I used CreateIoCompletionPort(), GetQueuedCompletionStatus() instead of the above mentioned method. The reasons for this are as follows:

• IoCompletionPorts are fast and less resource intensive.
• No restriction as in the other methods where the thread needs to belong to the same desktop, etc.

OK, let's see an example. By the way, the attached files have a Visual Studio based project that you can compile and run. Check the DemoThread class.

First, derive a class from MsgThread:

// DemoThread.h
class DemoThread : public MsgThread
{
    DECLARE_MSG_MAP();
private:
    void OnMsgGoGetPizza(
        WPARAM wParam,
        LPARAM lParam);
};
// DemoThread.cpp
BEGIN_MSG_MAP(DemoThread, MsgThread)
    ON_MSG(kMsgGoGetPizza, OnMsgGoGetPizza ) 
END_MSG_MAP()
void
DemoThread::OnMsgGoGetPizza
(
    WPARAM wParam,
    LPARAM lParam
)
{
}

Elsewhere in your code:

DemoThread rThread1;
rThread1.Start();
rThread1.PostMessage(
    kMsgGoGetPizza,
    (WPARAM) 5,
    0);
rThread1.Stop();

And that's it. Just be careful; if you use SendMessage, you might end up in a deadlock if you haven't designed things properly.

Just include MsgHandler.[h,cpp] and MsgThread.[h,cpp] in your project. I hope you find it useful. Any comments or suggestions are most welcome.