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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 >SDL_Surface</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_PixelFormat" HREF="sdlpixelformat.html"><LINK REL="NEXT" TITLE="SDL_VideoInfo" HREF="sdlvideoinfo.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="sdlpixelformat.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="sdlvideoinfo.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="SDLSURFACE" ></A >SDL_Surface</H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN3263" ></A ><H2 >Name</H2 >SDL_Surface -- Graphical Surface Structure</DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN3266" ></A ><H2 >Structure Definition</H2 ><PRE CLASS="PROGRAMLISTING" >typedef struct SDL_Surface { Uint32 flags; /* Read-only */ SDL_PixelFormat *format; /* Read-only */ int w, h; /* Read-only */ Uint16 pitch; /* Read-only */ void *pixels; /* Read-write */ /* clipping information */ SDL_Rect clip_rect; /* Read-only */ /* Reference count -- used when freeing surface */ int refcount; /* Read-mostly */ /* This structure also contains private fields not shown here */ } SDL_Surface;</PRE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN3269" ></A ><H2 >Structure Data</H2 ><DIV CLASS="INFORMALTABLE" ><A NAME="AEN3271" ></A ><P ></P ><TABLE BORDER="0" CLASS="CALSTABLE" ><TBODY ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >flags</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface flags</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >format</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Pixel <A HREF="sdlpixelformat.html" >format</A ></TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >w, h</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Width and height of the surface</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >pitch</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Length of a surface scanline in bytes</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >pixels</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Pointer to the actual pixel data</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >clip_rect</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >surface clip <A HREF="sdlrect.html" >rectangle</A ></TD ></TR ></TBODY ></TABLE ><P ></P ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN3300" ></A ><H2 >Description</H2 ><P ><SPAN CLASS="STRUCTNAME" >SDL_Surface</SPAN >'s represent areas of "graphical" memory, memory that can be drawn to. The video framebuffer is returned as a <SPAN CLASS="STRUCTNAME" >SDL_Surface</SPAN > by <A HREF="sdlsetvideomode.html" ><TT CLASS="FUNCTION" >SDL_SetVideoMode</TT ></A > and <A HREF="sdlgetvideosurface.html" ><TT CLASS="FUNCTION" >SDL_GetVideoSurface</TT ></A >. Most of the fields should be pretty obvious. <TT CLASS="STRUCTFIELD" ><I >w</I ></TT > and <TT CLASS="STRUCTFIELD" ><I >h</I ></TT > are the width and height of the surface in pixels. <TT CLASS="STRUCTFIELD" ><I >pixels</I ></TT > is a pointer to the actual pixel data, the surface should be <A HREF="sdllocksurface.html" >locked</A > before accessing this field. The <TT CLASS="STRUCTFIELD" ><I >clip_rect</I ></TT > field is the clipping rectangle as set by <A HREF="sdlsetcliprect.html" ><TT CLASS="FUNCTION" >SDL_SetClipRect</TT ></A >.</P ><P >The following are supported in the <TT CLASS="STRUCTFIELD" ><I >flags</I ></TT > field.</P ><DIV CLASS="INFORMALTABLE" ><A NAME="AEN3318" ></A ><P ></P ><TABLE BORDER="1" CLASS="CALSTABLE" ><TBODY ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_SWSURFACE</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface is stored in system memory</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_HWSURFACE</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface is stored in video memory</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_ASYNCBLIT</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface uses asynchronous blits if possible</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_ANYFORMAT</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Allows any pixel-format (Display surface)</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_HWPALETTE</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface has exclusive palette</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_DOUBLEBUF</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface is double buffered (Display surface)</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_FULLSCREEN</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface is full screen (Display Surface)</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_OPENGL</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface has an OpenGL context (Display Surface)</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_OPENGLBLIT</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface supports OpenGL blitting (Display Surface)</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_RESIZABLE</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface is resizable (Display Surface)</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_HWACCEL</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface blit uses hardware acceleration</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_SRCCOLORKEY</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface use colorkey blitting</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_RLEACCEL</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Colorkey blitting is accelerated with RLE</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_SRCALPHA</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface blit uses alpha blending</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="LITERAL" >SDL_PREALLOC</TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Surface uses preallocated memory</TD ></TR ></TBODY ></TABLE ><P ></P ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN3381" ></A ><H2 >See Also</H2 ><P ><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="sdlpixelformat.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="sdlvideoinfo.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SDL_PixelFormat</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_VideoInfo</TD ></TR ></TABLE ></DIV ></BODY ></HTML >