Mercurial > sdl-ios-xcode
view src/thread/win32/SDL_syssem.c @ 3069:caefe2344f65
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 | 99210400e8b9 |
children | 6f3308d4b6cd |
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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 */ #include "SDL_config.h" /* Semaphore functions using the Win32 API */ #define WIN32_LEAN_AND_MEAN #include <windows.h> #include "SDL_thread.h" #if defined(_WIN32_WCE) && (_WIN32_WCE < 300) #include "win_ce_semaphore.h" #endif struct SDL_semaphore { #if defined(_WIN32_WCE) && (_WIN32_WCE < 300) SYNCHHANDLE id; #else HANDLE id; #endif Uint32 volatile count; }; /* Create a semaphore */ SDL_sem * SDL_CreateSemaphore(Uint32 initial_value) { SDL_sem *sem; /* Allocate sem memory */ sem = (SDL_sem *) SDL_malloc(sizeof(*sem)); if (sem) { /* Create the semaphore, with max value 32K */ #if defined(_WIN32_WCE) && (_WIN32_WCE < 300) sem->id = CreateSemaphoreCE(NULL, initial_value, 32 * 1024, NULL); #else sem->id = CreateSemaphore(NULL, initial_value, 32 * 1024, NULL); #endif sem->count = initial_value; if (!sem->id) { SDL_SetError("Couldn't create semaphore"); SDL_free(sem); sem = NULL; } } else { SDL_OutOfMemory(); } return (sem); } /* Free the semaphore */ void SDL_DestroySemaphore(SDL_sem * sem) { if (sem) { if (sem->id) { #if defined(_WIN32_WCE) && (_WIN32_WCE < 300) CloseSynchHandle(sem->id); #else CloseHandle(sem->id); #endif sem->id = 0; } SDL_free(sem); } } int SDL_SemWaitTimeout(SDL_sem * sem, Uint32 timeout) { int retval; DWORD dwMilliseconds; if (!sem) { SDL_SetError("Passed a NULL sem"); return -1; } if (timeout == SDL_MUTEX_MAXWAIT) { dwMilliseconds = INFINITE; } else { dwMilliseconds = (DWORD) timeout; } #if defined(_WIN32_WCE) && (_WIN32_WCE < 300) switch (WaitForSemaphoreCE(sem->id, dwMilliseconds)) { #else switch (WaitForSingleObject(sem->id, dwMilliseconds)) { #endif case WAIT_OBJECT_0: InterlockedDecrement(&sem->count); retval = 0; break; case WAIT_TIMEOUT: retval = SDL_MUTEX_TIMEDOUT; break; default: SDL_SetError("WaitForSingleObject() failed"); retval = -1; break; } return retval; } int SDL_SemTryWait(SDL_sem * sem) { return SDL_SemWaitTimeout(sem, 0); } int SDL_SemWait(SDL_sem * sem) { return SDL_SemWaitTimeout(sem, SDL_MUTEX_MAXWAIT); } /* Returns the current count of the semaphore */ Uint32 SDL_SemValue(SDL_sem * sem) { if (!sem) { SDL_SetError("Passed a NULL sem"); return 0; } return sem->count; } int SDL_SemPost(SDL_sem * sem) { if (!sem) { SDL_SetError("Passed a NULL sem"); return -1; } /* Increase the counter in the first place, because * after a successful release the semaphore may * immediately get destroyed by another thread which * is waiting for this semaphore. */ InterlockedIncrement(&sem->count); #if defined(_WIN32_WCE) && (_WIN32_WCE < 300) if (ReleaseSemaphoreCE(sem->id, 1, NULL) == FALSE) { #else if (ReleaseSemaphore(sem->id, 1, NULL) == FALSE) { #endif InterlockedDecrement(&sem->count); /* restore */ SDL_SetError("ReleaseSemaphore() failed"); return -1; } return 0; } /* vi: set ts=4 sw=4 expandtab: */