Mercurial > sdl-ios-xcode
view include/SDL_loadso.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 | d3baf5ac4e37 |
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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 */ /** * \file SDL_loadso.h * * System dependent library loading routines * * Some things to keep in mind: * - These functions only work on C function names. Other languages may * have name mangling and intrinsic language support that varies from * compiler to compiler. * - Make sure you declare your function pointers with the same calling * convention as the actual library function. Your code will crash * mysteriously if you do not do this. * - Avoid namespace collisions. If you load a symbol from the library, * it is not defined whether or not it goes into the global symbol * namespace for the application. If it does and it conflicts with * symbols in your code or other shared libraries, you will not get * the results you expect. :) */ #ifndef _SDL_loadso_h #define _SDL_loadso_h #include "SDL_stdinc.h" #include "SDL_error.h" #include "begin_code.h" /* Set up for C function definitions, even when using C++ */ #ifdef __cplusplus /* *INDENT-OFF* */ extern "C" { /* *INDENT-ON* */ #endif /* This function dynamically loads a shared object and returns a pointer * to the object handle (or NULL if there was an error). * The 'sofile' parameter is a system dependent name of the object file. */ extern DECLSPEC void *SDLCALL SDL_LoadObject(const char *sofile); /* Given an object handle, this function looks up the address of the * named function in the shared object and returns it. This address * is no longer valid after calling SDL_UnloadObject(). */ extern DECLSPEC void *SDLCALL SDL_LoadFunction(void *handle, const char *name); /* Unload a shared object from memory */ extern DECLSPEC void SDLCALL SDL_UnloadObject(void *handle); /* Ends C function definitions when using C++ */ #ifdef __cplusplus /* *INDENT-OFF* */ } /* *INDENT-ON* */ #endif #include "close_code.h" #endif /* _SDL_loadso_h */ /* vi: set ts=4 sw=4 expandtab: */