Mercurial > sdl-ios-xcode
view include/begin_code.h @ 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 | a67a961e2171 |
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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 Library General Public License as published by the Free Software Foundation; either version 2 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 Library General Public License for more details. You should have received a copy of the GNU Library General Public License along with this library; if not, write to the Free Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA Sam Lantinga slouken@libsdl.org */ /* This file sets things up for C dynamic library function definitions, static inlined functions, and structures aligned at 4-byte alignment. If you don't like ugly C preprocessor code, don't look at this file. :) */ /* This shouldn't be nested -- included it around code only. */ #ifdef _begin_code_h #error Nested inclusion of begin_code.h #endif #define _begin_code_h /* Some compilers use a special export keyword */ #ifndef DECLSPEC # if defined(__BEOS__) # if defined(__GNUC__) # define DECLSPEC __declspec(dllexport) # else # define DECLSPEC __declspec(export) # endif # elif defined(__WIN32__) # ifdef __BORLANDC__ # ifdef BUILD_SDL # define DECLSPEC # else # define DECLSPEC __declspec(dllimport) # endif # else # define DECLSPEC __declspec(dllexport) # endif # elif defined(__OS2__) # ifdef __WATCOMC__ # ifdef BUILD_SDL # define DECLSPEC __declspec(dllexport) # else # define DECLSPEC # endif # else # define DECLSPEC # endif # else # if defined(__GNUC__) && __GNUC__ >= 4 # define DECLSPEC __attribute__ ((visibility("default"))) # else # define DECLSPEC # endif # endif #endif /* By default SDL uses the C calling convention */ #ifndef SDLCALL #if defined(__WIN32__) && !defined(__GNUC__) #define SDLCALL __cdecl #else #ifdef __OS2__ /* But on OS/2, we use the _System calling convention */ /* to be compatible with every compiler */ #define SDLCALL _System #else #define SDLCALL #endif #endif #endif /* SDLCALL */ /* Removed DECLSPEC on Symbian OS because SDL cannot be a DLL in EPOC */ #ifdef __SYMBIAN32__ #undef DECLSPEC #define DECLSPEC #endif /* __SYMBIAN32__ */ /* Force structure packing at 4 byte alignment. This is necessary if the header is included in code which has structure packing set to an alternate value, say for loading structures from disk. The packing is reset to the previous value in close_code.h */ #if defined(_MSC_VER) || defined(__MWERKS__) || defined(__BORLANDC__) #ifdef _MSC_VER #pragma warning(disable: 4103) #endif #ifdef __BORLANDC__ #pragma nopackwarning #endif #pragma pack(push,4) #endif /* Compiler needs structure packing set */ /* Set up compiler-specific options for inlining functions */ #ifndef SDL_INLINE_OKAY #ifdef __GNUC__ #define SDL_INLINE_OKAY #else /* Add any special compiler-specific cases here */ #if defined(_MSC_VER) || defined(__BORLANDC__) || \ defined(__DMC__) || defined(__SC__) || \ defined(__WATCOMC__) || defined(__LCC__) || \ defined(__DECC) #ifndef __inline__ #define __inline__ __inline #endif #define SDL_INLINE_OKAY #else #if !defined(__MRC__) && !defined(_SGI_SOURCE) #ifndef __inline__ #define __inline__ inline #endif #define SDL_INLINE_OKAY #endif /* Not a funky compiler */ #endif /* Visual C++ */ #endif /* GNU C */ #endif /* SDL_INLINE_OKAY */ /* If inlining isn't supported, remove "__inline__", turning static inlined functions into static functions (resulting in code bloat in all files which include the offending header files) */ #ifndef SDL_INLINE_OKAY #define __inline__ #endif /* Apparently this is needed by several Windows compilers */ #if !defined(__MACH__) #ifndef NULL #ifdef __cplusplus #define NULL 0 #else #define NULL ((void *)0) #endif #endif /* NULL */ #endif /* ! Mac OS X - breaks precompiled headers */