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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 8582c6a5ca16
children a1b03ba2fcd0
line wrap: on
line source

/*
    SDL - Simple DirectMedia Layer
    Copyright (C) 1997-2006 Sam Lantinga

    This library is SDL_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_sysaudio_h
#define _SDL_sysaudio_h

#include "SDL_mutex.h"
#include "SDL_thread.h"

/* The SDL audio driver */
typedef struct SDL_AudioDevice SDL_AudioDevice;

/* Define the SDL audio driver structure */
#define _THIS	SDL_AudioDevice *_this
#ifndef _STATUS
#define _STATUS	SDL_status *status
#endif
struct SDL_AudioDevice {
	/* * * */
	/* The name of this audio driver */
	const char *name;

	/* * * */
	/* The description of this audio driver */
	const char *desc;

	/* * * */
	/* Public driver functions */
	int  (*OpenAudio)(_THIS, SDL_AudioSpec *spec);
	void (*ThreadInit)(_THIS);	/* Called by audio thread at start */
	void (*WaitAudio)(_THIS);
	void (*PlayAudio)(_THIS);
	Uint8 *(*GetAudioBuf)(_THIS);
	void (*WaitDone)(_THIS);
	void (*CloseAudio)(_THIS);

	/* * * */
	/* Lock / Unlock functions added for the Mac port */
	void (*LockAudio)(_THIS);
	void (*UnlockAudio)(_THIS);

	/* * * */
	/* Data common to all devices */

	/* The current audio specification (shared with audio thread) */
	SDL_AudioSpec spec;

	/* An audio conversion block for audio format emulation */
	SDL_AudioCVT convert;

	/* Current state flags */
	int enabled;
	int paused;
	int opened;

	/* Fake audio buffer for when the audio hardware is busy */
	Uint8 *fake_stream;

	/* A semaphore for locking the mixing buffers */
	SDL_mutex *mixer_lock;

	/* A thread to feed the audio device */
	SDL_Thread *thread;
	Uint32 threadid;

	/* * * */
	/* Data private to this driver */
	struct SDL_PrivateAudioData *hidden;

	/* * * */
	/* The function used to dispose of this structure */
	void (*free)(_THIS);
};
#undef _THIS

typedef struct AudioBootStrap {
	const char *name;
	const char *desc;
	int (*available)(void);
	SDL_AudioDevice *(*create)(int devindex);
} AudioBootStrap;

#if SDL_AUDIO_DRIVER_BSD
extern AudioBootStrap BSD_AUDIO_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_PULSE
extern AudioBootStrap PULSE_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_OSS
extern AudioBootStrap DSP_bootstrap;
extern AudioBootStrap DMA_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_ALSA
extern AudioBootStrap ALSA_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_QNXNTO
extern AudioBootStrap QNXNTOAUDIO_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_SUNAUDIO
extern AudioBootStrap SUNAUDIO_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_DMEDIA
extern AudioBootStrap DMEDIA_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_ARTS
extern AudioBootStrap ARTS_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_ESD
extern AudioBootStrap ESD_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_NAS
extern AudioBootStrap NAS_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_DSOUND
extern AudioBootStrap DSOUND_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_WAVEOUT
extern AudioBootStrap WAVEOUT_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_PAUD
extern AudioBootStrap Paud_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_BAUDIO
extern AudioBootStrap BAUDIO_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_COREAUDIO
extern AudioBootStrap COREAUDIO_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_SNDMGR
extern AudioBootStrap SNDMGR_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_MINT
extern AudioBootStrap MINTAUDIO_GSXB_bootstrap;
extern AudioBootStrap MINTAUDIO_MCSN_bootstrap;
extern AudioBootStrap MINTAUDIO_STFA_bootstrap;
extern AudioBootStrap MINTAUDIO_XBIOS_bootstrap;
extern AudioBootStrap MINTAUDIO_DMA8_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_DISK
extern AudioBootStrap DISKAUD_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_DUMMY
extern AudioBootStrap DUMMYAUD_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_DC
extern AudioBootStrap DCAUD_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_NDS
extern AudioBootStrap NDSAUD_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_MMEAUDIO
extern AudioBootStrap MMEAUDIO_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_DART
extern AudioBootStrap DART_bootstrap;
#endif
#if SDL_AUDIO_DRIVER_EPOCAUDIO
extern AudioBootStrap EPOCAudio_bootstrap; 
#endif

/* This is the current audio device */
extern SDL_AudioDevice *current_audio;

#endif /* _SDL_sysaudio_h */