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Error Using PostMessage from within a Class Defined in a Worker Thread

I have a Windows program (developed using MSVS 2008 in C) composed of three threads:

1) Main (GUI) windows thread
2) Windows worker thread that acts as a TCP server
3) Windows worker thread that acts as a TCP client

Each worker thread has its own window procedure to handle messages.  In addition, the TCP client & server code are defined in separate classes.  Each class is used in its corresponding worker thread.  Finally, I save a copy of the thread window handles in globals, which I use with PostMessage when I need them.  

The TCP client thread gathers data from a remote source and places it in global cache.  It then posts a message to the server thread to read this data, so that it can distribute it to those connected to this server.  The TCP code works, but the PostMessage does not.

I did some further testing and this is what I found.
* Calling PostMessage directly from within any class function does not work (even though PostMessage does not return an error)

void CServerClass::cache_message (char message[])

PostMessage (hwnd, WM_SEND, 0, 0);


* Calling PostMessage directly inside the window procedure works but outside a class function

LRESULT CALLBACK ClientWndProc (HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
      if (message == WM_XXX) {
            PostMessage (hwnd, WM_SEND, 0, 0);
            return 0;

      return DefWindowProc (hwnd, message, wParam, lParam) ;

* Calling PostMessage inside a non class function works

void pmessage (void)

PostMessage (hwnd, WM_SEND, 0, 0);


* Calling PostMessage indirectly from within a class function works.  That is, if I define a nonclass function that calls PostMessage, and then call this function from within a class function, this nonclass function's PostMessage succeeds.

void CServerClass::cache_message2 (char message[])
      //PostMessage (hwnd, WM_SEND, 0, 0); //xxx does not work
      pmessage (); // this works


Thus, calling PostMessage only works if calling from within a nonclass function.  Does any know why this is?    Is it possible to use PostMessage directly from within a class?  What other methods may I use?
Editors IDEsMicrosoft Development.NET Programming

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Last Comment

8/22/2022 - Mon

Are the worker-thread windows getting *any* messages?
It is unusual for a worker thread to process GUI functions... unusual problems always arise and this might be one of them.
For diagnostic, try a SendMessage -- with a breakpoint in the target's Winproc.  Also, use Spy++ to watch the messages.
As to class vs. non-class... it is possible that a base-class implementation is overiding the normal Win32 API fn.  You can use:
   ::PostMessage( hwnd, WM_SEND, 0,0); // two leading colons
to be sure you use the Win API fn rather than some overridden method.  Also, attempting to single-step into the call might reveal what code is actually handling it.

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