Question

My application (the bootstrap application for an installer that I’m working on needs to launch some other applications (my installer and third party installers for my installer’s prerequisites) and wait for them to complete. In order to allow the GUI to do screen updates while waiting for an app to complete, I put a message pump in the wait loop using the ‘MFC-compatible’ example in the Visual Studio documentation on idle loop processing as a guideline. My code (which is in a member function of a CWinApp-derived class) is as follows:

if (::CreateProcess(lpAppName, szCmdLineBuffer, NULL, NULL, TRUE, 0, NULL, NULL,
                    &StartupInfo, &ProcessInfo))
{
  ::GetExitCodeProcess(ProcessInfo.hProcess, &dwExitCode);
  if (bWait)
    while (dwExitCode == STILL_ACTIVE)
    {
      // In order to allow updates of the GUI to happen while we're waiting for
      // the application to finish, we must run a mini message pump here to
      // allow messages to go through and get processed.  This message pump
      // performs much like MFC's main message pump found in CWinThread::Run().
      MSG msg;
      while (::PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE))
      {
        if (!PumpMessage())
        {
          // a termination message (e.g. WM_DESTROY)
          // was processed, so we need to stop waiting
          dwExitCode = ERROR_CANT_WAIT;
          ::PostQuitMessage(0);
          break;
        }
      }

      // let MFC do its idle processing
      LONG nIdle = 0;
      while (OnIdle(nIdle++))
        ;

      if (dwExitCode == STILL_ACTIVE) // was a termination message processed?
      {
        // no; wait for .1 second to see if the application is finished
        ::WaitForSingleObject(ProcessInfo.hProcess, 100);
        ::GetExitCodeProcess(ProcessInfo.hProcess, &dwExitCode);
      }
    }
  ::CloseHandle(ProcessInfo.hProcess);
  ::CloseHandle(ProcessInfo.hThread);
}
else
  dwExitCode = ::GetLastError();

The problem that I’m having is that, at some point, this message pump seems to free up window and menu handles on the window that I have open at the time this code is run. I did a walk through in the debugger, and at no time did it ever get into the body of the if (!PumpMessage()) statement, so I don’t know what’s going on here to cause the window and menu handles to go south. If I don’t have the message pump, everything works fine, except that the GUI can’t update itself while the wait loop is running.

Does anyone have any ideas as to how to make this work? Alternatively, I’d like to launch a worker thread to launch the second app if bWait is TRUE, but I’ve never done anything with threads before, so I’ll need some advice on how to do it without introducing synchronization issues, etc. (Code examples would be greatly appreciated in either case.)

Answer 1

I’ve also posted this question on the Microsoft forums, and thanks to the help of one Doug Harris at Microsoft, I found out my problem with my HWND and HMENU values was, indeed due to stale CWwnd* and CMenu* pointers (obtained using GetMenu() and GetDialogItem() calls. Getting the pointers again after launching the second app solved that problem. Also, he pointed me to a web site* that showed a better way of doing my loop using MsgWaitForMultipleObjects() to control it that doesn’t involve the busy work of waiting a set amount of time and polling the process for an exit code.

My loop now looks like this:

if (bWait)
{
  // In order to allow updates of the GUI to happen while we're
  // waiting for the application to finish, we must run a message
  // pump here to allow messages to go through and get processed.
  LONG  nIdleCount = 0;
  for (;;)
  {
    MSG msg;
    if (::PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE))
      PumpMessage();
    else //if (!OnIdle(nIdleCount++))
    {
      nIdleCount = 0;
      if (!PeekMessage(&msg, NULL, 0, 0, PM_NOREMOVE))
      {
        DWORD nRes = ::MsgWaitForMultipleObjects(1, &ProcessInfo.hProcess,
                                                 FALSE, INFINITE, QS_ALLEVENTS);
        if (nRes == WAIT_OBJECT_0)
          break;
      }
    }
  }
}
::GetExitCodeProcess(ProcessInfo.hProcess, &dwExitCode);

*That Web site, if you’re curious, is: http://members.cox.net/doug_web/threads.htm