Windows message loop

Question

Theres some reason for this code not reach the first else? I got it exactly the same from vairous sources. Than I did my own encapsulation. Everything goes fine. Window is created, messages are treated, events are generated to keyborad input in the client area, the gl canvas works fine (when I force it to draw).

The only problem is that message loop never leaves the first if. :/ I'm really stuck.

while (!done)                                       
{
    if (::PeekMessage (&msg, NULL, 0, 0, PM_REMOVE))
    {
        if (msg.message == WM_QUIT)                 
        {
            done = TRUE;                            
        }
        else                                        
        {
            ::TranslateMessage (&msg);              
            ::DispatchMessage (&msg);               
        }
    }
    else                                        
    {
        // Code is never reaching this!
        draw ();
        ::SwapBuffers(hDC);
        idle ();
    }
}
return msg.wParam;

Obviously something it posting new messages into the queue while Translate/Dispatch is done. You should just list all messages retrieved and deduce what message it is and why it appears. — sharptooth
– sharptooth, Commented Jul 16, 2009 at 7:18
Using spy i got a hard flow of WM_PAINT with hdc 0. No idea how this is being generated. — Ruben Trancoso
– Ruben Trancoso, Commented Jul 16, 2009 at 7:32

morechilli · Accepted Answer · 2009-07-16 08:46:25Z

7

In your case the message queue must never be empty - why? Well it depends on what the rest of your program is doing. Some possibilities:

Your code is posting new messages to the queue in a manner such that the queue doesn't get empty. I'd suggest logging out the message ids as they are handled.
You aren't handling paint messages - from msdn: "The PeekMessage function normally does not remove WM_PAINT messages from the queue. WM_PAINT messages remain in the queue until they are processed. However, if a WM_PAINT message has a NULL update region, PeekMessage does remove it from the queue."

Hope this helps.

[Edit] To handle WM_PAINT either call BeginPaint and EndPaint or forward to DefWindowProc

edited Jul 16, 2009 at 8:46

answered Jul 16, 2009 at 7:32

morechilli

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6 Comments

Ruben Trancoso Over a year ago

looks like thats the problem. But, the my MainWndProc is returning 0 from WM_PAINT. What I know is that every message you treat, you return 0. So, in theory, no message should be left.

Daniel Earwicker Over a year ago

He's handling WM_PAINT because he's seeing things appearing.

Ruben Trancoso Over a year ago

Added this code and it start to work as expected. But would like to have some explanation if possible please. case WM_PAINT: BeginPaint(hwnd, &ps); EndPaint(hwnd, &ps); return 0;

Michael Over a year ago

WM_PAINT isn't a real message - it's just a flag on the window that says it has invalidated regions (WM_PAINT never handled.) As long as that flag is present, WM_PAINT will be generated.

morechilli Over a year ago

looks like you can also do it by forwarding wm_paint to DefWindowProc but it probably does the same thing

|

jussij · Accepted Answer · 2009-07-16 07:51:06Z

4

Make sure you are processing the WM_PAINT correctly.

By this I mean make sure you are calling BeginPaint and EndPaint from inside the WM_PAINT message, otherwise you will be confusing Windows into thinking your application still needs to be painted.

answered Jul 16, 2009 at 7:51

jussij

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Comments

Emmanuel Caradec · Accepted Answer · 2009-07-16 07:20:02Z

0

May be there is always a message waiting ?

answered Jul 16, 2009 at 7:20

Emmanuel Caradec

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Comments

Naveen · Accepted Answer · 2009-07-16 07:20:26Z

0

PeekMessage will return 0 only if there are no messages in the message queue. Since there are messages to be dispatched in the message queue, it is returning a non-zero value and your else condition is never executed.

answered Jul 16, 2009 at 7:20

Naveen

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1 Comment

sharptooth Over a year ago

I guess he is asking about this - why doesn't it ever happen that there're no messages left.

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