Date: Thu, 27 Dec 2007 07:38:25 +0000 From: John Bartholomew Subject: [SDL] SDL Semaphore implementation broken on Windows? Hi, Over the past couple of days, I've been battling with SDL, SDL_Mixer and SMPEG to try to find an audio hang bug. I believe I've found the problem, which I think is a race condition inside SDL's semaphore implementation (at least the Windows implementation). The semaphore code uses Windows' built in semaphore functions, but it also maintains a separate count value. This count value is updated with bare increment and decrement operations in SemPost and SemWaitTimeout - no locking primitives to protect them. In tracking down the apparent audio bug, I found that at some point a semaphore's count value was being decremented to -1, which is clearly not a valid value for it to take. I'm still not certain exactly what sequence of operations is occuring for this to happen, but I believe that overall it's a race condition between a thread calling SemPost (which increments the count) and the thread on the other end calling SemWait (which decrements it). I will try to make a test case to verify this, but I'm not sure if I'll be able to (threading errors being difficult to reproduce even in the best circumstances). However, assuming this is the cause of my problems, there is a very simple fix: Windows provides InterlockedIncrement() and InterlockedDecrement() functions to perform increments and decrements which are guaranteed to be atomic. So the fix is in thread/win32/SDL_syssem.c: replace occurrences of --sem->count with InterlockedDecrement(&sem->count); and replace occurrences of ++sem->count with InterlockedIncrement(&sem->count); This is using SDL v1.2.12, built with VC++ 2008 Express, running on a Core 2 duo processor.

 ========================
  Using SDL with PicoGUI
 ========================

- Originally contributed by Micah Dowty <micahjd@users.sourceforge.net>

PicoGUI is a scalable GUI system with a unique architecture, primarily focused
on scalability to various embedded systems. You can find more information
including a FAQ at http://picogui.org

To use the patch:

  1. When compiling, add the "--enable-video-picogui" switch to ./configure

  2. When running your program, ensure that the picogui driver for SDL
     is in use by setting the SDL_VIDEODRIVER environment variable 
     to "picogui".

  3. The program must also be linked to the C client library for PicoGUI
     (libpgui.so). If the program is being compiled with a patched SDL 
     installed this should be done automatically. If you want to use an
     existing binary with PicoGUI, you can set the LD_PRELOAD environment
     variable to the path of your libpgui.so file.

Capabilities:

  So far only basic functionality is provided on true color (linear16/24/32)
  devices. Accessing a memory mapped bitmap, updating the display, and handling
  mouse/keyboard input. This functionality has been tested with several
  applications, including mplayer, Xine, sldroids, and Abuse.

TODO list:

  - YUV overlays will be helpful for watching video on set top boxes or other
    embedded devices that have some graphics acceleration hardware

  - Account for rotated bitmap storage in pgserver

  - Support for hiding or changing the cursor

  - The display should be centered when the SDL application is smaller
    than the PicoGUI panel

  - Fullscreen or any other special modes

  - Support for indexed and grayscale modes

  - Probably much more...

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