Re: Loop for AfxBeginThread

From:
"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>
Newsgroups:
microsoft.public.vc.mfc
Date:
Tue, 05 Jun 2007 21:02:04 -0400
Message-ID:
<bdidnfuQGoAMmvvbnZ2dnUVZ_hudnZ2d@comcast.com>
jp2code wrote:

I'm reading your essay on worker threads, but I won't be able to impliment
asynchronous I/O because we include WinCE / PocketPC devices, and they do
not support overlapped operations.

Currently, I have a main thread for writing, a worker thread for reading,
and a global BOOL value called (ironically) g_Overlapped.

Here is how I use it:

Main Thread - Writing:
  while (g_Overlapped)
    Sleep(1);
  g_Overlapped = TRUE;
  g_Overlapped = WriteFile( ... );
  if (g_Overlapped)
  {
    g_Overlapped = FALSE;
    // code to process
  }

Worker Thread - Reading:
  while (g_Overlapped)
    Sleep(1);
  g_Overlapped = TRUE;
  g_Overlapped = ReadFile( ... );
  if (g_Overlapped)
  {
    g_Overlapped = FALSE;
    // code to process
  }

This should work, wouldn't you think?


No. You don't understand the race conditions that are inherent in
multithreading. You cannot test the flag and then assume that it is
unchanged in the next instruction. Synchronization primitives are
necessary: You can't do it with if, while, etc.

You need to stop coding and work on the design. (Or find some
experienced help.)

--
Scott McPhillips [MVP VC++]

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