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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 Tue, 17 Feb 2009 05:39:18 +0000
parents cf09447f3d3c
children 245a7d79577c
line wrap: on
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/*
    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2009 Sam Lantinga

    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
    License as published by the Free Software Foundation; either
    version 2.1 of the License, or (at your option) any later version.

    This library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
    Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public
    License along with this library; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

    Sam Lantinga
    slouken@libsdl.org
*/

/* Try to get a standard set of platform defines */

#ifndef _SDL_platform_h
#define _SDL_platform_h

#if defined(_AIX)
#undef __AIX__
#define __AIX__		1
#endif
#if defined(__BEOS__)
#undef __BEOS__
#define __BEOS__	1
#endif
#if defined(bsdi) || defined(__bsdi) || defined(__bsdi__)
#undef __BSDI__
#define __BSDI__	1
#endif
#if defined(_arch_dreamcast)
#undef __DREAMCAST__
#define __DREAMCAST__	1
#endif
#if defined(__FreeBSD__) || defined(__DragonFly__)
#undef __FREEBSD__
#define __FREEBSD__	1
#endif
#if defined(hpux) || defined(__hpux) || defined(__hpux__)
#undef __HPUX__
#define __HPUX__	1
#endif
#if defined(sgi) || defined(__sgi) || defined(__sgi__) || defined(_SGI_SOURCE)
#undef __IRIX__
#define __IRIX__	1
#endif
#if defined(linux) || defined(__linux) || defined(__linux__)
#undef __LINUX__
#define __LINUX__	1
#endif

#if defined(__APPLE__)
/* lets us know what version of Mac OS X we're compiling on */
#include "AvailabilityMacros.h"
#ifdef MAC_OS_X_VERSION_10_3
#include "TargetConditionals.h" /* this header is in 10.3 or later */
#if TARGET_OS_IPHONE
/* if compiling for iPhone */
#undef __IPHONEOS__
#define __IPHONEOS__ 1
#undef __MACOSX__
#else
/* if not compiling for iPhone */
#undef __MACOSX__
#define __MACOSX__	1
#endif /* TARGET_OS_IPHONE */
#else
/* if earlier verion of Mac OS X than version 10.3 */
#undef __MACOSX__
#define __MACOSX__	1
#endif

#endif /* defined(__APPLE__) */

#if defined(__NetBSD__)
#undef __NETBSD__
#define __NETBSD__	1
#endif
#if defined(__OpenBSD__)
#undef __OPENBSD__
#define __OPENBSD__	1
#endif
#if defined(__OS2__)
#undef __OS2__
#define __OS2__		1
#endif
#if defined(osf) || defined(__osf) || defined(__osf__) || defined(_OSF_SOURCE)
#undef __OSF__
#define __OSF__		1
#endif
#if defined(__QNXNTO__)
#undef __QNXNTO__
#define __QNXNTO__	1
#endif
#if defined(riscos) || defined(__riscos) || defined(__riscos__)
#undef __RISCOS__
#define __RISCOS__	1
#endif
#if defined(__SVR4)
#undef __SOLARIS__
#define __SOLARIS__	1
#endif
#if defined(WIN32) || defined(_WIN32)
#undef __WIN32__
#define __WIN32__	1
#endif

#if defined(__NDS__)
#undef __NINTENDODS__
#define __NINTENDODS__	1
#endif

#endif /* _SDL_platform_h */