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POIf MessageBox()/related are synchronous, why doesn't my message loop freeze?
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copied!<p>Why is it that if I call a seemingly synchronous Windows function like <code>MessageBox()</code> inside of my message loop, the loop itself doesn't freeze as if I called <code>Sleep()</code> (or a similar function) instead? To illustrate my point, take the following skeletal <code>WndProc</code>:</p> <pre><code>int counter = 0; LRESULT CALLBACK WndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) { switch (msg) { case WM_CREATE: SetTimer(hwnd, 1, 1000, NULL); //start a 1 second timer break; case WM_PAINT: // paint/display counter variable onto window break; case WM_TIMER: //occurs every second counter++; InvalidateRect(hwnd, NULL, TRUE); //force window to repaint itself break; case WM_LBUTTONDOWN: //someone clicks the window MessageBox(hwnd, "", "", 0); MessageBeep(MB_OK); //play a sound after MessageBox returns break; //default .... } return 0; } </code></pre> <p>In the above example, the program's main function is to run a timer and display the counter's value every second. However, if the user clicks on our window, the program displays a message box and then beeps after the box is closed. </p> <p>Here's where it gets interesting: we can tell <code>MessageBox()</code> is a synchronous function because <code>MessageBeep()</code> doesn't execute until the message box is closed. However, the timer keeps running, and the window is repainted every second even while the message box is displayed. So while <code>MessageBox()</code> is apparently a blocking function call, other messages (<code>WM_TIMER</code>/<code>WM_PAINT</code>) can still be processed. That's fine, except if I substitute MessageBox for another blocking call like <code>Sleep()</code></p> <pre><code> case WM_LBUTTONDOWN: Sleep(10000); //wait 10 seconds MessageBeep(MB_OK); break; </code></pre> <p>This blocks my application entirely, and no message processing occurs for the 10 seconds (<code>WM_TIMER</code>/<code>WM_PAINT</code> aren't processed, the counter doesn't update, program 'freezes', etc). So why is it that <code>MessageBox()</code> allows message processing to continue while <code>Sleep()</code> doesn't? Given that my application is single-threaded, what is it that <code>MessageBox()</code> does to allow this functionality? Does the system 'replicate' my application thread, so that way it can finish the <code>WM_LBUTTONDOWN</code> code once <code>MessageBox()</code> is done, while still allowing the original thread to process other messages in the interim? (that was my uneducated guess)</p> <p>Thanks in advance</p>

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