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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 e85e65aec22f
children a1b03ba2fcd0
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/*
    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2006 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
*/
#include "SDL_config.h"

#ifndef _SDL_thread_c_h
#define _SDL_thread_c_h

/* Need the definitions of SYS_ThreadHandle */
#if SDL_THREADS_DISABLED
#include "generic/SDL_systhread_c.h"
#elif SDL_THREAD_BEOS
#include "beos/SDL_systhread_c.h"
#elif SDL_THREAD_DC
#include "dc/SDL_systhread_c.h"
#elif SDL_THREAD_OS2
#include "os2/SDL_systhread_c.h"
#elif SDL_THREAD_PTH
#include "pth/SDL_systhread_c.h"
#elif SDL_THREAD_PTHREAD
#include "pthread/SDL_systhread_c.h"
#elif SDL_THREAD_SPROC
#include "irix/SDL_systhread_c.h"
#elif SDL_THREAD_WIN32
#include "win32/SDL_systhread_c.h"
#elif SDL_THREAD_SYMBIAN
#include "symbian/SDL_systhread_c.h"
#else
#error Need thread implementation for this platform
#include "generic/SDL_systhread_c.h"
#endif
#include "../SDL_error_c.h"

/* This is the system-independent thread info structure */
struct SDL_Thread {
	Uint32 threadid;
	SYS_ThreadHandle handle;
	int status;
	SDL_error errbuf;
	void *data;
};

/* This is the function called to run a thread */
extern void SDL_RunThread(void *data);

#endif /* _SDL_thread_c_h */