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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 | 355632dca928 |
children |
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<HTML ><HEAD ><TITLE >CD-ROM</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+ "><LINK REL="HOME" TITLE="SDL Library Documentation" HREF="index.html"><LINK REL="UP" TITLE="SDL Reference" HREF="reference.html"><LINK REL="PREVIOUS" TITLE="SDL_CloseAudio" HREF="sdlcloseaudio.html"><LINK REL="NEXT" TITLE="SDL_CDNumDrives" HREF="sdlcdnumdrives.html"><META NAME="KEYWORD" CONTENT="cdrom"><META NAME="KEYWORD" CONTENT="function"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFF8DC" TEXT="#000000" LINK="#0000ee" VLINK="#551a8b" ALINK="#ff0000" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >SDL Library Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="sdlcloseaudio.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="sdlcdnumdrives.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><H1 ><A NAME="CDROM" ></A >Chapter 11. CD-ROM</H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT ><A HREF="sdlcdnumdrives.html" >SDL_CDNumDrives</A > -- Returns the number of CD-ROM drives on the system.</DT ><DT ><A HREF="sdlcdname.html" >SDL_CDName</A > -- Returns a human-readable, system-dependent identifier for the CD-ROM.</DT ><DT ><A HREF="sdlcdopen.html" >SDL_CDOpen</A > -- Opens a CD-ROM drive for access.</DT ><DT ><A HREF="sdlcdstatus.html" >SDL_CDStatus</A > -- Returns the current status of the given drive.</DT ><DT ><A HREF="sdlcdplay.html" >SDL_CDPlay</A > -- Play a CD</DT ><DT ><A HREF="sdlcdplaytracks.html" >SDL_CDPlayTracks</A > -- Play the given CD track(s)</DT ><DT ><A HREF="sdlcdpause.html" >SDL_CDPause</A > -- Pauses a CDROM</DT ><DT ><A HREF="sdlcdresume.html" >SDL_CDResume</A > -- Resumes a CDROM</DT ><DT ><A HREF="sdlcdstop.html" >SDL_CDStop</A > -- Stops a CDROM</DT ><DT ><A HREF="sdlcdeject.html" >SDL_CDEject</A > -- Ejects a CDROM</DT ><DT ><A HREF="sdlcdclose.html" >SDL_CDClose</A > -- Closes a SDL_CD handle</DT ><DT ><A HREF="sdlcd.html" >SDL_CD</A > -- CDROM Drive Information</DT ><DT ><A HREF="sdlcdtrack.html" >SDL_CDtrack</A > -- CD Track Information Structure</DT ></DL ></DIV ><P >SDL supports audio control of up to 32 local CD-ROM drives at once.</P ><P >You use this API to perform all the basic functions of a CD player, including listing the tracks, playing, stopping, and ejecting the CD-ROM. (Currently, multi-changer CD drives are not supported.)</P ><P >Before you call any of the SDL CD-ROM functions, you must first call "<TT CLASS="FUNCTION" >SDL_Init(SDL_INIT_CDROM)</TT >", which scans the system for CD-ROM drives, and sets the program up for audio control. Check the return code, which should be <SPAN CLASS="RETURNVALUE" >0</SPAN >, to see if there were any errors in starting up.</P ><P >After you have initialized the library, you can find out how many drives are available using the <TT CLASS="FUNCTION" >SDL_CDNumDrives()</TT > function. The first drive listed is the system default CD-ROM drive. After you have chosen a drive, and have opened it with <TT CLASS="FUNCTION" >SDL_CDOpen()</TT >, you can check the status and start playing if there's a CD in the drive.</P ><P >A CD-ROM is organized into one or more tracks, each consisting of a certain number of "frames". Each frame is ~2K in size, and at normal playing speed, a CD plays 75 frames per second. SDL works with the number of frames on a CD, but this can easily be converted to the more familiar minutes/seconds format by using the <TT CLASS="FUNCTION" >FRAMES_TO_MSF()</TT > macro.</P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="sdlcloseaudio.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="sdlcdnumdrives.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SDL_CloseAudio</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="reference.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >SDL_CDNumDrives</TD ></TR ></TABLE ></DIV ></BODY ></HTML >