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.

Project files for embedded Visual C++ 3.0, 4.0 and 
Visual Studio 2005 can be found in VisualCE.zip

SDL supports GAPI and WinDib output for Windows CE.

GAPI driver supports:

- all possible WinCE devices (Pocket PC, Smartphones, HPC)
  with different orientations of video memory and resolutions.
- 4, 8 and 16 bpp devices
- special handling of 8bpp on 8bpp devices
- VGA mode, you can even switch between VGA and GAPI in runtime
  (between 240x320 and 480x640 for example). On VGA devices you can
  use either GAPI or VGA.
- Landscape mode and automatic rotation of buttons and stylus coordinates.
  To enable landscape mode make width of video screen bigger than height.
  For example: 
    SDL_SetVideoMode(320,240,16,SDL_FULLSCREEN)
- WM2005
- SDL_ListModes

NOTE:
There are several SDL features not available in the WinCE port of SDL.

- DirectX is not yet available
- Semaphores are not available
- Joystick support is not available
- CD-ROM control is not available

In addition, there are several features that run in "degraded" mode:

Preprocessor Symbol		Effect
===================		=================================

SDL_systimer.c:
USE_GETTICKCOUNT		Less accurate values for SDL time functions
USE_SETTIMER			Use only a single marginally accurate timer

SDL_syswm.c:
DISABLE_ICON_SUPPORT		Can't set the runtime window icon

SDL_sysmouse.c:
USE_STATIC_CURSOR		Only the arrow cursor is available

SDL_sysevents.c:
NO_GETKEYBOARDSTATE		Can't get modifier state on keyboard focus

SDL_dibevents.c:
NO_GETKEYBOARDSTATE		Very limited keycode translation

SDL_dibvideo.c:
NO_GETDIBITS			Can't distinguish between 15 bpp and 16 bpp
NO_CHANGEDISPLAYSETTINGS	No fullscreen support
NO_GAMMA_SUPPORT		Gamma correction not available