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view docs/html/sdllistmodes.html @ 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 | 355632dca928 |
children |
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<HTML ><HEAD ><TITLE >SDL_ListModes</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="Video" HREF="video.html"><LINK REL="PREVIOUS" TITLE="SDL_VideoDriverName" HREF="sdlvideodrivername.html"><LINK REL="NEXT" TITLE="SDL_VideoModeOK" HREF="sdlvideomodeok.html"></HEAD ><BODY CLASS="REFENTRY" 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="sdlvideodrivername.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="sdlvideomodeok.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="SDLLISTMODES" ></A >SDL_ListModes</H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN1159" ></A ><H2 >Name</H2 >SDL_ListModes -- Returns a pointer to an array of available screen dimensions for the given format and video flags</DIV ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN1162" ></A ><H2 >Synopsis</H2 ><DIV CLASS="FUNCSYNOPSIS" ><A NAME="AEN1163" ></A ><P ></P ><PRE CLASS="FUNCSYNOPSISINFO" >#include "SDL.h"</PRE ><P ><CODE ><CODE CLASS="FUNCDEF" >SDL_Rect **<B CLASS="FSFUNC" >SDL_ListModes</B ></CODE >(SDL_PixelFormat *format, Uint32 flags);</CODE ></P ><P ></P ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN1169" ></A ><H2 >Description</H2 ><P >Return a pointer to an array of available screen dimensions for the given format and video flags, sorted largest to smallest. Returns <TT CLASS="LITERAL" >NULL</TT > if there are no dimensions available for a particular format, or <SPAN CLASS="RETURNVALUE" >-1</SPAN > if any dimension is okay for the given format.</P ><P >If <TT CLASS="PARAMETER" ><I >format</I ></TT > is <TT CLASS="LITERAL" >NULL</TT >, the mode list will be for the format returned by <A HREF="sdlgetvideoinfo.html" >SDL_GetVideoInfo()</A >-><TT CLASS="STRUCTFIELD" ><I >vfmt</I ></TT >. The <TT CLASS="PARAMETER" ><I >flag</I ></TT > parameter is an OR'd combination of <A HREF="sdlsurface.html" >surface</A > flags. The flags are the same as those used <A HREF="sdlsetvideomode.html" ><TT CLASS="FUNCTION" >SDL_SetVideoMode</TT ></A > and they play a strong role in deciding what modes are valid. For instance, if you pass <TT CLASS="LITERAL" >SDL_HWSURFACE</TT > as a flag only modes that support hardware video surfaces will be returned.</P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN1184" ></A ><H2 >Example</H2 ><PRE CLASS="PROGRAMLISTING" >SDL_Rect **modes; int i; . . . /* Get available fullscreen/hardware modes */ modes=SDL_ListModes(NULL, SDL_FULLSCREEN|SDL_HWSURFACE); /* Check is there are any modes available */ if(modes == (SDL_Rect **)0){ printf("No modes available!\n"); exit(-1); } /* Check if our resolution is restricted */ if(modes == (SDL_Rect **)-1){ printf("All resolutions available.\n"); } else{ /* Print valid modes */ printf("Available Modes\n"); for(i=0;modes[i];++i) printf(" %d x %d\n", modes[i]->w, modes[i]->h); } . .</PRE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN1187" ></A ><H2 >See Also</H2 ><P ><A HREF="sdlsetvideomode.html" ><TT CLASS="FUNCTION" >SDL_SetVideoMode</TT ></A >, <A HREF="sdlgetvideoinfo.html" ><TT CLASS="FUNCTION" >SDL_GetVideoInfo</TT ></A >, <A HREF="sdlrect.html" ><SPAN CLASS="STRUCTNAME" >SDL_Rect</SPAN ></A >, <A HREF="sdlpixelformat.html" ><SPAN CLASS="STRUCTNAME" >SDL_PixelFormat</SPAN ></A ></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="sdlvideodrivername.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="sdlvideomodeok.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SDL_VideoDriverName</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="video.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >SDL_VideoModeOK</TD ></TR ></TABLE ></DIV ></BODY ></HTML >