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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 Tue, 17 Feb 2009 05:39:18 +0000
parents 99210400e8b9
children dc1eb82ffdaa
line wrap: on
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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
*/

/* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 
 * Default cursor - it happens to be the Mac cursor, but could be anything   */

#define DEFAULT_CWIDTH	16
#define DEFAULT_CHEIGHT	16
#define DEFAULT_CHOTX	0
#define DEFAULT_CHOTY	0

/* Added a real MacOS cursor, at the request of Luc-Olivier de Charrière */
#define USE_MACOS_CURSOR

#ifdef USE_MACOS_CURSOR

static unsigned char default_cdata[] = {
    0x00, 0x00,
    0x40, 0x00,
    0x60, 0x00,
    0x70, 0x00,
    0x78, 0x00,
    0x7C, 0x00,
    0x7E, 0x00,
    0x7F, 0x00,
    0x7F, 0x80,
    0x7C, 0x00,
    0x6C, 0x00,
    0x46, 0x00,
    0x06, 0x00,
    0x03, 0x00,
    0x03, 0x00,
    0x00, 0x00
};

static unsigned char default_cmask[] = {
    0xC0, 0x00,
    0xE0, 0x00,
    0xF0, 0x00,
    0xF8, 0x00,
    0xFC, 0x00,
    0xFE, 0x00,
    0xFF, 0x00,
    0xFF, 0x80,
    0xFF, 0xC0,
    0xFF, 0xE0,
    0xFE, 0x00,
    0xEF, 0x00,
    0xCF, 0x00,
    0x87, 0x80,
    0x07, 0x80,
    0x03, 0x00
};

#else

static unsigned char default_cdata[] = {
    0x00, 0x00,
    0x40, 0x00,
    0x60, 0x00,
    0x70, 0x00,
    0x78, 0x00,
    0x7C, 0x00,
    0x7E, 0x00,
    0x7F, 0x00,
    0x7F, 0x80,
    0x7C, 0x00,
    0x6C, 0x00,
    0x46, 0x00,
    0x06, 0x00,
    0x03, 0x00,
    0x03, 0x00,
    0x00, 0x00
};

static unsigned char default_cmask[] = {
    0x40, 0x00,
    0xE0, 0x00,
    0xF0, 0x00,
    0xF8, 0x00,
    0xFC, 0x00,
    0xFE, 0x00,
    0xFF, 0x00,
    0xFF, 0x80,
    0xFF, 0xC0,
    0xFF, 0x80,
    0xFE, 0x00,
    0xEF, 0x00,
    0x4F, 0x00,
    0x07, 0x80,
    0x07, 0x80,
    0x03, 0x00
};

#endif /* TRUE_MACINTOSH_CURSOR */
/* vi: set ts=4 sw=4 expandtab: */