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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
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><TITLE
>SDL_SetVideoMode</TITLE
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><H1
><A
NAME="SDLSETVIDEOMODE"
></A
>SDL_SetVideoMode</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN1239"
></A
><H2
>Name</H2
>SDL_SetVideoMode&nbsp;--&nbsp;Set up a video mode with the specified width, height and bits-per-pixel.</DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN1242"
></A
><H2
>Synopsis</H2
><DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN1243"
></A
><P
></P
><PRE
CLASS="FUNCSYNOPSISINFO"
>#include "SDL.h"</PRE
><P
><CODE
><CODE
CLASS="FUNCDEF"
>SDL_Surface *<B
CLASS="FSFUNC"
>SDL_SetVideoMode</B
></CODE
>(int width, int height, int bpp, Uint32 flags);</CODE
></P
><P
></P
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN1249"
></A
><H2
>Description</H2
><P
>Set up a video mode with the specified width, height and bits-per-pixel.</P
><P
>If <TT
CLASS="PARAMETER"
><I
>bpp</I
></TT
> is 0, it is treated as the 
current display bits per pixel.</P
><P
>The <TT
CLASS="PARAMETER"
><I
>flags</I
></TT
> parameter is the same as the <TT
CLASS="STRUCTFIELD"
><I
>flags</I
></TT
> field of the <A
HREF="sdlsurface.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_Surface</SPAN
></A
> structure. OR'd combinations of the following values are valid.</P
><DIV
CLASS="INFORMALTABLE"
><A
NAME="AEN1259"
></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"
>Create the video surface in system memory</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_HWSURFACE</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Create the video surface in video memory</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_ASYNCBLIT</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Enables the use of asynchronous updates of the display surface. This will
usually slow down blitting on single CPU machines, but may provide a speed
increase on SMP systems.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_ANYFORMAT</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Normally, if a video surface of the requested bits-per-pixel (<TT
CLASS="PARAMETER"
><I
>bpp</I
></TT
>) is not available, SDL will emulate one with a shadow surface. Passing <TT
CLASS="LITERAL"
>SDL_ANYFORMAT</TT
> prevents this and causes SDL to use the video surface, regardless of its pixel depth.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_HWPALETTE</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Give SDL exclusive palette access. Without this flag you may not always get the the colors you request with <A
HREF="sdlsetcolors.html"
><TT
CLASS="FUNCTION"
>SDL_SetColors</TT
></A
> or <A
HREF="sdlsetpalette.html"
><TT
CLASS="FUNCTION"
>SDL_SetPalette</TT
></A
>.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_DOUBLEBUF</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Enable hardware double buffering; only valid with SDL_HWSURFACE. Calling
<A
HREF="sdlflip.html"
><TT
CLASS="FUNCTION"
>SDL_Flip</TT
></A
> will flip the
buffers and update the screen. All drawing will take place on the surface
that is not displayed at the moment. If double buffering could not be enabled
then <TT
CLASS="FUNCTION"
>SDL_Flip</TT
> will just perform a
<A
HREF="sdlupdaterect.html"
><TT
CLASS="FUNCTION"
>SDL_UpdateRect</TT
></A
>
on the entire screen.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_FULLSCREEN</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>SDL will attempt to use a fullscreen mode. If a hardware resolution change is
not possible (for whatever reason), the next higher resolution will be used and
the display window centered on a black background.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_OPENGL</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Create an OpenGL rendering context. You should have previously set OpenGL video attributes with <A
HREF="sdlglsetattribute.html"
><TT
CLASS="FUNCTION"
>SDL_GL_SetAttribute</TT
></A
>.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_OPENGLBLIT</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Create an OpenGL rendering context, like above, but allow normal blitting
operations. The screen (2D) surface may have an alpha channel, and
<A
HREF="sdlupdaterects.html"
><TT
CLASS="FUNCTION"
>SDL_UpdateRects</TT
></A
>
must be used for updating changes to the screen surface. NOTE: This option
is kept for compatibility only, and is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> recommended for
new code.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_RESIZABLE</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>Create a resizable window. When the window is resized by the user a <A
HREF="sdlresizeevent.html"
><TT
CLASS="LITERAL"
>SDL_VIDEORESIZE</TT
></A
> event is generated and <TT
CLASS="FUNCTION"
>SDL_SetVideoMode</TT
> can be called again with the new size.</TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
><TT
CLASS="LITERAL"
>SDL_NOFRAME</TT
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
>If possible, <TT
CLASS="LITERAL"
>SDL_NOFRAME</TT
> causes SDL to create a window with no title bar or frame decoration. Fullscreen modes automatically have this flag set.</TD
></TR
></TBODY
></TABLE
><P
></P
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>Whatever <TT
CLASS="PARAMETER"
><I
>flags</I
></TT
> <TT
CLASS="FUNCTION"
>SDL_SetVideoMode</TT
> could satisfy are set in the <TT
CLASS="STRUCTFIELD"
><I
>flags</I
></TT
> member of the returned surface.</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
>The <TT
CLASS="PARAMETER"
><I
>bpp</I
></TT
> parameter is the number of bits per pixel,
so a <TT
CLASS="PARAMETER"
><I
>bpp</I
></TT
> of 24 uses the packed representation of
3 bytes/pixel. For the more common 4 bytes/pixel mode, use a
<TT
CLASS="PARAMETER"
><I
>bpp</I
></TT
> of 32. Somewhat oddly, both 15 and 16 will
request a 2 bytes/pixel mode, but different pixel formats.</P
></BLOCKQUOTE
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN1336"
></A
><H2
>Return Value</H2
><P
>The framebuffer surface, or <SPAN
CLASS="RETURNVALUE"
>NULL</SPAN
> if it fails.
The surface returned is freed by SDL_Quit() and should nt be freed by
the caller.</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN1340"
></A
><H2
>See Also</H2
><P
><A
HREF="sdllocksurface.html"
><TT
CLASS="FUNCTION"
>SDL_LockSurface</TT
></A
>,
<A
HREF="sdlsetcolors.html"
><TT
CLASS="FUNCTION"
>SDL_SetColors</TT
></A
>,
<A
HREF="sdlflip.html"
><TT
CLASS="FUNCTION"
>SDL_Flip</TT
></A
>,
<A
HREF="sdlsurface.html"
><SPAN
CLASS="STRUCTNAME"
>SDL_Surface</SPAN
></A
></P
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