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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 fb74445b4d19
children 72a00fe65ffe
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Simple DirectMedia Layer CREDITS
Thanks to everyone who made this possible, including:

* Cliff Matthews, for giving me a reason to start this project. :)
 -- Executor rocks!  *grin*

* Scott Call, for making a home for SDL on the 'Net... Thanks! :)

* The Linux Fund, C Magazine, Educational Technology Resources Inc.,
  Gareth Noyce, Jesse Pavel, Keith Kitchin, Jeremy Horvath, Thomas Nicholson,
  Hans-Peter Gygax, the Eternal Lands Development Team, Lars Brubaker,
  and Phoenix Kokido for financial contributions

* Gaëtan de Menten for writing the PHP and SQL behind the SDL website

* Tim Jones for the new look of the SDL website

* Marco Kraus for setting up SDL merchandise

* Martin Donlon for his work on the SDL Documentation Project

* Ryan Gordon for helping everybody out and keeping the dream alive. :)

* Mattias Engdegård, for help with the Solaris port and lots of other help

* Max Watson, Matt Slot, and Kyle for help with the MacOS Classic port

* Stan Shebs, for the initial Mac OS X port

* Eric Wing, Max Horn, and Darrell Walisser for unflagging work on the Mac OS X port

* Patrick Trainor, Jim Boucher, and Mike Gorchak for the QNX Neutrino port

* Carsten Griwodz for the AIX port

* Gabriele Greco, for the Amiga port

* Patrice Mandin, for the Atari port

* Hannu Viitala for the EPOC port

* Marcus Mertama for the S60 port.

* Peter Valchev for nagging me about the OpenBSD port until I got it right. :)

* Kent B Mein, for a place to do the IRIX port

* Ash, for a place to do the OSF/1 Alpha port

* David Sowsy, for help with the BeOS port

* Eugenia Loli, for endless work on porting SDL games to BeOS

* Jon Taylor for the GGI front-end

* Paulus Esterhazy, for the Visual C++ testing and libraries

* Brenda Tantzen, for Metrowerks CodeWarrior on MacOS

* Chris Nentwich, for the Hermes assembly blitters

* Michael Vance and Jim Kutter for the X11 OpenGL support

* Stephane Peter, for the AAlib front-end and multi-threaded timer idea.

* Jon Atkins for SDL_image, SDL_mixer and SDL_net documentation

* Peter Wiklund, for the 1998 winning SDL logo,
  and Arto Hamara, Steven Wong, and Kent Mein for other logo entries.

* Arne Claus, for the 2004 winning SDL logo,
  and Shandy Brown, Jac, Alex Lyman, Mikkel Gjoel, #Guy, Jonas Hartmann,
  Daniel Liljeberg,  Ronald Sowa, DocD, Pekka Jaervinen, Patrick Avella,
  Erkki Kontilla, Levon Gavalian, Hal Emerich, David Wiktorsson,
  S. Schury and F. Hufsky, Ciska de Ruyver, Shredweat, Tyler Montbriand,
  Martin Andersson, Merlyn Wysard, Fernando Ibanez, David Miller,
  Andre Bommele, lovesby.com, Francisco Camenforte Torres, and David Igreja
  for other logo entries.

* Bob Pendleton and David Olofson for being long time contributors to
  the SDL mailing list.

* Everybody at Loki Software, Inc. for their great contributions!

 And a big hand to everyone else who gave me appreciation, advice,
 and suggestions, especially the good folks on the SDL mailing list.

THANKS! :)

  -- Sam Lantinga			<slouken@libsdl.org>