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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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>SDL_PeepEvents</H1
><DIV
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><A
NAME="AEN5421"
></A
><H2
>Name</H2
>SDL_PeepEvents&nbsp;--&nbsp;Checks the event queue for messages and optionally returns them.</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN5424"
></A
><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN5425"
></A
><P
></P
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include "SDL.h"</PRE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int <B
CLASS="FSFUNC"
>SDL_PeepEvents</B
></CODE
>(SDL_Event *events, int numevents, SDL_eventaction action, Uint32 mask);</CODE
></P
><P
></P
></DIV
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><DIV
CLASS="REFSECT1"
><A
NAME="AEN5431"
></A
><H2
>Description</H2
><P
>Checks the event queue for messages and optionally returns them.</P
><P
>If <TT
CLASS="PARAMETER"
><I
>action</I
></TT
> is <TT
CLASS="LITERAL"
>SDL_ADDEVENT</TT
>, up to 
<TT
CLASS="PARAMETER"
><I
>numevents</I
></TT
> events will be added to the back of the event
 queue.</P
><P
>If <TT
CLASS="PARAMETER"
><I
>action</I
></TT
> is <TT
CLASS="LITERAL"
>SDL_PEEKEVENT</TT
>, up to
<TT
CLASS="PARAMETER"
><I
>numevents</I
></TT
> events at the front of the event queue, 
matching <A
HREF="sdlevent.html"
><TT
CLASS="PARAMETER"
><I
>mask</I
></TT
></A
>, 
will be returned and will not be removed from the queue.</P
><P
>If <TT
CLASS="PARAMETER"
><I
>action</I
></TT
> is <TT
CLASS="LITERAL"
>SDL_GETEVENT</TT
>, up to 
<TT
CLASS="PARAMETER"
><I
>numevents</I
></TT
> events at the front of the event queue, 
matching <A
HREF="sdlevent.html"
><TT
CLASS="PARAMETER"
><I
>mask</I
></TT
></A
>, 
will be returned and will be removed from the queue.</P
><P
>The <TT
CLASS="PARAMETER"
><I
>mask</I
></TT
> parameter is an bitwise OR of
<TT
CLASS="LITERAL"
>SDL_EVENTMASK</TT
>(<TT
CLASS="PARAMETER"
><I
>event_type</I
></TT
>), for all
event types you are interested in.</P
><P
>This function is thread-safe.</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN5455"
></A
><H2
>Return Value</H2
><P
>This function returns the number of events actually stored, or 
<SPAN
CLASS="RETURNVALUE"
>-1</SPAN
> if there was an error.  </P
></DIV
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><A
NAME="AEN5459"
></A
><H2
>See Also</H2
><P
><A
HREF="sdlevent.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_Event</SPAN
></A
>,
<A
HREF="sdlpollevent.html"
><TT
CLASS="FUNCTION"
>SDL_PollEvent</TT
></A
>,
<A
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><TT
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>SDL_PushEvent</TT
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