Mercurial > sdl-ios-xcode
view README.Watcom @ 4105:84882a89ca50 SDL-1.2
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.
author | Sam Lantinga <slouken@libsdl.org> |
---|---|
date | Fri, 28 Dec 2007 22:05:17 +0000 |
parents | 488eba319a25 |
children | 8c72321542f6 |
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Using SDL under Windows with the OpenWatcom compiler ==================================================== Prerequisites ------------- I have done the port under Windows XP Home with SP2 installed. Windows 2000 should also be working. I'm not so sure about ancient Windows NT, since only DirectX 3 is available there. Building should be possible, but running the compiled applications will probalbly fail with SDL_VIDEODRIVER=directx. The windib driver should work, though. To compile and use the SDL with Open Watcom you will need the following: - Open Watcom compiler. I used version 1.5. The environment variables PATH, WATCOM and INCLUDE need to be set appropriately - please consult the OpenWatcom documentation and instructions given during the installation of the compiler. My setup looks like this in owvars.bat: set WATCOM=C:\watcom set INCLUDE=%WATCOM%\h;%WATCOM%\h\nt set PATH=%PATH%;%WATCOM%\binnt;%WATCOM%\binw - A fairly recent DirectX SDK. The original unmodified DX8 SDK works, as well as the minimal DirectX 7 SDK from the Allegro download site (<http://alleg.sourceforge.net/files/dx70_min.zip>). - The SDL sources from Subversion - The file Watcom-Win32.zip (now available in Subversion) Building the Library -------------------- 1) In the SDL base directory extract the archive Watcom-Win32.zip. This creates a subdirectory named 'watcom'. 2) The makefile expects the environment variable DXDIR to be set to the base directory of a DirectX SDK. I have tried a stock DX8 SDK from Microsoft as well as the minimal DirectX 7 SDK from the Allegro download site. You can also edit the makefile directly and hard code your path to the SDK on your system. I have this in my setup: set DXDIR=D:\devel\DX8_SDK 3) Enter the watcom directory and run wmake sdl 4) All tests from the test directory are working and can be built by running wmake tests Notes: The makefile offers some options to tweak the way the library is built. You have at your disposal the option to build a static (default) library, or a DLL (with tgt=dll). You can also choose whether to build a Release (default) or a Debug version (with build=debug) of the tests and library. Please consult the usage comment at the top of the makefile for usage instructions. If you specify a test target (i.e. 'wmake tests' for all tests, or selected targets like 'wmake testgl testvidinfo testoverlay2'), the tests are always freshly compiled and linked. This is done to minimise hassle when switching between library versions (static vs. DLL), because they require subtly different options. Also, the test executables are put directly into the test directory, so they can find their data files. The clean target of the makefile removes the test executables and the SDL.dll file from the test directory. To use the library in your own projects with Open Watcom, you can use the way the tests are built as base of your own build environment. The library can also be built with the stack calling convention of the compiler (-6s instead of -6r). Test applications ----------------- I've tried to make all tests work. The following table gives an overview of the current status. Testname Status ~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ checkkeys + graywin + loopwave + testalpha + testbitmap + testdyngl + testerror + testfile + testgamma + testgl + testhread + testiconv - (all failed) testkeys + testlock + testoverlay + (needs 'set SDL_VIDEODRIVER=directx') testoverlay2 + (needs 'set SDL_VIDEODRIVER=directx') testpalette + testplatform + testsem + testsprite + testtimer + testver + testvidinfo + testwin ? (fading doesn't seem right) testwm + torturethread + testcdrom + testjoystick not tested threadwin + testcursor + TODO ---- There is room for further improvement: - Test joystick functionality. - Investigate fading issue in 'testwin' test. - Fix the UTF-8 support. - Adapt the makefile/object file list to support more target systems - Use "#pragma aux" syntax for the CPU info functions. Questions and Comments ---------------------- Please direct any questions or comments to me: <mailto:macpete@gmx.de> Happy Coding! Marc Peter