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OpenGL Multithreaded OpenGL problem (Win32)

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Zorbfish    214
#include <windows.h>

HWND hWnd = 0;
BOOL done = FALSE;

LRESULT WinProc(HWND hWnd, UINT Msg, WPARAM wParam, LPARAM lParam) {
   switch (Msg) {
      /* Ignore paint messages */
      case WM_PAINT:
      case WM_ERASEBKGND:
         return 0;
      case WM_DESTROY:
   return DefWindowProc(hWnd, Msg, wParam, lParam);

/* Creates a GL context attached to the window owned
   by the other thread and paints the client area blue */

DWORD GLThread(LPVOID lpParams) {
   HDC hDC = GetDC(hWnd);
   HGLRC hRC = 0;
   /* Set absolute minimum format attributes; i.e. select default mode */
   pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR);
   pfd.nVersion = 1;
   pfd.iPixelType = PFD_TYPE_RGBA;
   SetPixelFormat(hDC, ChoosePixelFormat(hDC, &pfd), &pfd);
   hRC = wglCreateContext(hDC);
   wglMakeCurrent(hDC, hRC);
   glClearColor(0.0f, 0.0f, 1.0f, 0.0f);
   while (!done) {
      /* Yield to other threads */
   return 0;

INT WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, INT nShowCmd) {
   WNDCLASSEX wcx = {0};
   HANDLE hThread = 0;
   DWORD iThread = 0;
   DWORD wait = 0;
   MSG msg = {0};
   wcx.cbSize = sizeof(WNDCLASSEX); = CS_OWNDC | CS_VREDRAW | CS_HREDRAW;
   wcx.lpfnWindowProc = WinProc;
   wcx.hInstance = hInstance;
   wcx.lpszClassName = "MTGL";
   /* No background brush */
   hWnd = CreateWindowEx(WS_EX_APPWINDOW, wcx.lpszClassName, "Untitled",
      240, 0, 0, hInstance, 0);
   ShowWindow(hWnd, nShowCmd);
   /* Start OpenGL */
   hThread = CreateThread(0, 0, GLThread, 0, &iThread);
   while (!done) {
      if (GetMessage(&msg, 0, 0) > 0) {
      } else {
         done = TRUE;
   return (INT)msg.wParam;

I'm trying to run my game logic in a different thread from the Win32 message pump so that my game doesn't stall when a message is being processed by Windows. I've been reading up on multithreading OpenGL but I am running into a problem. The above code has a race condition. I can easily cause the host window to become hung if I resize or move it quickly across the screen. The window will then stop responding and the mouse cursor will not hover over the taskbar. I have to open up the task manager and kill it manually. I can fix the problem if I introduce synchronization through a mutex or event object around the OpenGL and message pump code. I suspect there's something going on under the hood that's causing the problem (GDI, window handle, etc). I defeat the purpose behind using the two threads in the first place by synchornizing them since the mutex would not be released until a message is processed. I've been trying to troubleshoot this for a couple of days now but the information on the net is sparse at best. I've read the few posts in this forum about multithread OpenGL but none of them seem to address this issue. Any help would be appreciated.

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Mike2343    1202
Few things I noticed.

A) You're recreating the pixel format each time, this is just a MAJOR waste of processing cycles. I set it once and forget it until something changes with the window (like resizing, etc).

B) I call my rendering loop in my while (!done) message pump, like I see most people do. I multithread things like AI, audio, resource loading, scene graph, etc. But not my rendering loop.

Now, have you worked with multithreading a lot before? Just wondering because if you haven't you might want to start off a bit simpler. Remember, rendering the scene is the last thing to happen and normally is the least amount of work (AI, scene graph, physics, collision detection all take a lot more CPU cycles).

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udaykv    126
When it goes into race, try breaking into the application and check the stack trace. Also, try syncing SwapBuffers call with DispatchMessage.

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Zorbfish    214
Sorry to those who responded that I never got back to.

A) No. The pixelformat is created just once and then for the remainder of the thread it executes inside the while block.

B) I'm simply interested in finding out why it was racing.

And yes I do have multithreading experience under my belt.

I had tried that but when the problem occurs Windows stop working. About the only thing I can do is bring up the taskmanager and kill the application.

I thought GetMessage automatically blocks when there is no message queued (maybe I read the msdn docs wrong). I changed it to use MsgWaitForMultipleObjects instead.

Ultimately I did find a solution that works from what I have tested. I finally tracked the problem down to the window's HDC object. Apparently the main thread and the OpenGL thread began to race for control of the DC and every once and awhile they ran into each other. It was happening when the window tried to process a WM_NCPAINT message to redraw the frame of its window while the OpenGL thread was trying to draw in the client area using the same DC. So far by just synchronizing the DC I have not run into anymore problems.

If anyone is interested I'll post the code as soon as I clean it up.

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