Mercurial > sdl-ios-xcode
view README.Qtopia @ 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 | 2c5d4c22a2ac |
| children |
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============================================================================== Using the Simple DirectMedia Layer with Qtopia/OPIE ============================================================================== ============================================================================== I. Setting up the Qtopia development environment. This document will not explain how to setup the Qtopia development environment. That is outside the scope of the document. You can read more on this subject in this excellent howto: http://www.zauruszone.com/howtos/linux_compiler_setup_howto.html ============================================================================== II. Building the Simple DirectMedia Layer libraries using the arm cross-compiler This is somewhat tricky since the name of the compiler binaries differ from the standard. Also you should disable features not needed. The command below works for me. Note that it's all one line. You can also set the NM, LD etc environment variables separately. NM=arm-linux-nm LD=arm-linux-ld CC=arm-linux-gcc CXX=arm-linux-g++ RANLIB=arm-linux-ranlib AR=arm-linux-ar ./configure --enable-video-qtopia --disable-video-dummy --disable-video-fbcon --disable-video-dga --disable-arts --disable-esd --disable-alsa --disable-cdrom --disable-video-x11 --disable-nasm --prefix=/opt/Qtopia/sharp/ arm-unknown-linux-gnu One thing to note is that the above configure will include joystick support, even though you can't have joysticks on the Zaurus. The reason for this is to avoid link / compile / runtime errors with applications that have joystick support. ============================================================================== III. Building the Simple DirectMedia Layer test programs: After installing, making sure the correct sdl-config is in your path, run configure like this: NM=arm-linux-nm LD=arm-linux-ld CC=arm-linux-gcc CXX=arm-linux-g++ AR=arm-linux-ar ./configure arm-unknown-linux-gnu ============================================================================== IV. Application porting notes One thing I have noticed is that applications sometimes don't exit correctly. Their icon remains in the taskbar and they tend to relaunch themselves automatically. I believe this problem doesn't occur if you exit your application using the exit() method. However, if you end main() with 'return 0;' or so, this seems to happen. Also note that when running in landscape mode - i.e requesting a window that is HEIGHT pixels wide and WIDTH pixels high, where WIDTH and HEIGHT normally is 240 and 320 - the image is blitted so that the hardware buttons are on the left side of the display. This might not always be desirable but such is the code today. ============================================================================== V. Enjoy! :) If you have a project you'd like me to know about, or want to ask questions, go ahead and join the SDL developer's mailing list by sending e-mail to: sdl-request@libsdl.org and put "subscribe" into the subject of the message. Or alternatively you can use the web interface: http://www.libsdl.org/mailman/listinfo/sdl ============================================================================== VI. What is supported: Keyboard (Sharp Zaurus) Hardware buttons Stylus input (mouse) Video. Allows fullscreen both in portrait mode (up to WIDTHxHEIGHT size window) and in landscape mode (up to HEIGHTxWIDTH). All other SDL functionality works like a normal Linux system (threads, audio etc). -- David Hedbor <david@hedbor.org> http://david.hedbor.org/ http://eongames.com/