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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
>Video</TITLE
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><DIV
CLASS="CHAPTER"
><H1
><A
NAME="VIDEO"
></A
>Chapter 6. Video</H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
><A
HREF="sdlgetvideosurface.html"
>SDL_GetVideoSurface</A
>&nbsp;--&nbsp;returns a pointer to the current display surface</DT
><DT
><A
HREF="sdlgetvideoinfo.html"
>SDL_GetVideoInfo</A
>&nbsp;--&nbsp;returns a pointer to information about the video hardware</DT
><DT
><A
HREF="sdlvideodrivername.html"
>SDL_VideoDriverName</A
>&nbsp;--&nbsp;Obtain the name of the video driver</DT
><DT
><A
HREF="sdllistmodes.html"
>SDL_ListModes</A
>&nbsp;--&nbsp;Returns a pointer to an array of available screen dimensions for 
the given format and video flags</DT
><DT
><A
HREF="sdlvideomodeok.html"
>SDL_VideoModeOK</A
>&nbsp;--&nbsp;Check to see if a particular video mode is supported.</DT
><DT
><A
HREF="sdlsetvideomode.html"
>SDL_SetVideoMode</A
>&nbsp;--&nbsp;Set up a video mode with the specified width, height and bits-per-pixel.</DT
><DT
><A
HREF="sdlupdaterect.html"
>SDL_UpdateRect</A
>&nbsp;--&nbsp;Makes sure the given area is updated on the given screen.</DT
><DT
><A
HREF="sdlupdaterects.html"
>SDL_UpdateRects</A
>&nbsp;--&nbsp;Makes sure the given list of rectangles is updated on the given screen.</DT
><DT
><A
HREF="sdlflip.html"
>SDL_Flip</A
>&nbsp;--&nbsp;Swaps screen buffers</DT
><DT
><A
HREF="sdlsetcolors.html"
>SDL_SetColors</A
>&nbsp;--&nbsp;Sets a portion of the colormap for the given 8-bit surface.</DT
><DT
><A
HREF="sdlsetpalette.html"
>SDL_SetPalette</A
>&nbsp;--&nbsp;Sets the colors in the palette of an 8-bit surface.</DT
><DT
><A
HREF="sdlsetgamma.html"
>SDL_SetGamma</A
>&nbsp;--&nbsp;Sets the color gamma function for the display</DT
><DT
><A
HREF="sdlgetgammaramp.html"
>SDL_GetGammaRamp</A
>&nbsp;--&nbsp;Gets the color gamma lookup tables for the display</DT
><DT
><A
HREF="sdlsetgammaramp.html"
>SDL_SetGammaRamp</A
>&nbsp;--&nbsp;Sets the color gamma lookup tables for the display</DT
><DT
><A
HREF="sdlmaprgb.html"
>SDL_MapRGB</A
>&nbsp;--&nbsp;Map a RGB color value to a pixel format.</DT
><DT
><A
HREF="sdlmaprgba.html"
>SDL_MapRGBA</A
>&nbsp;--&nbsp;Map a RGBA color value to a pixel format.</DT
><DT
><A
HREF="sdlgetrgb.html"
>SDL_GetRGB</A
>&nbsp;--&nbsp;Get RGB values from a pixel in the specified pixel format.</DT
><DT
><A
HREF="sdlgetrgba.html"
>SDL_GetRGBA</A
>&nbsp;--&nbsp;Get RGBA values from a pixel in the specified pixel format.</DT
><DT
><A
HREF="sdlcreatergbsurface.html"
>SDL_CreateRGBSurface</A
>&nbsp;--&nbsp;Create an empty SDL_Surface</DT
><DT
><A
HREF="sdlcreatergbsurfacefrom.html"
>SDL_CreateRGBSurfaceFrom</A
>&nbsp;--&nbsp;Create an SDL_Surface from pixel data</DT
><DT
><A
HREF="sdlfreesurface.html"
>SDL_FreeSurface</A
>&nbsp;--&nbsp;Frees (deletes) a SDL_Surface</DT
><DT
><A
HREF="sdllocksurface.html"
>SDL_LockSurface</A
>&nbsp;--&nbsp;Lock a surface for directly access.</DT
><DT
><A
HREF="sdlunlocksurface.html"
>SDL_UnlockSurface</A
>&nbsp;--&nbsp;Unlocks a previously locked surface.</DT
><DT
><A
HREF="sdlloadbmp.html"
>SDL_LoadBMP</A
>&nbsp;--&nbsp;Load a Windows BMP file into an SDL_Surface.</DT
><DT
><A
HREF="sdlsavebmp.html"
>SDL_SaveBMP</A
>&nbsp;--&nbsp;Save an SDL_Surface as a Windows BMP file.</DT
><DT
><A
HREF="sdlsetcolorkey.html"
>SDL_SetColorKey</A
>&nbsp;--&nbsp;Sets the color key (transparent pixel) in a blittable surface and
RLE acceleration.</DT
><DT
><A
HREF="sdlsetalpha.html"
>SDL_SetAlpha</A
>&nbsp;--&nbsp;Adjust the alpha properties of a surface</DT
><DT
><A
HREF="sdlsetcliprect.html"
>SDL_SetClipRect</A
>&nbsp;--&nbsp;Sets the clipping rectangle for a surface.</DT
><DT
><A
HREF="sdlgetcliprect.html"
>SDL_GetClipRect</A
>&nbsp;--&nbsp;Gets the clipping rectangle for a surface.</DT
><DT
><A
HREF="sdlconvertsurface.html"
>SDL_ConvertSurface</A
>&nbsp;--&nbsp;Converts a surface to the same format as another surface.</DT
><DT
><A
HREF="sdlblitsurface.html"
>SDL_BlitSurface</A
>&nbsp;--&nbsp;This performs a fast blit from the source surface to the destination surface.</DT
><DT
><A
HREF="sdlfillrect.html"
>SDL_FillRect</A
>&nbsp;--&nbsp;This function performs a fast fill of the given rectangle with some color</DT
><DT
><A
HREF="sdldisplayformat.html"
>SDL_DisplayFormat</A
>&nbsp;--&nbsp;Convert a surface to the display format</DT
><DT
><A
HREF="sdldisplayformatalpha.html"
>SDL_DisplayFormatAlpha</A
>&nbsp;--&nbsp;Convert a surface to the display format</DT
><DT
><A
HREF="sdlwarpmouse.html"
>SDL_WarpMouse</A
>&nbsp;--&nbsp;Set the position of the mouse cursor.</DT
><DT
><A
HREF="sdlcreatecursor.html"
>SDL_CreateCursor</A
>&nbsp;--&nbsp;Creates a new mouse cursor.</DT
><DT
><A
HREF="sdlfreecursor.html"
>SDL_FreeCursor</A
>&nbsp;--&nbsp;Frees a cursor created with SDL_CreateCursor.</DT
><DT
><A
HREF="sdlsetcursor.html"
>SDL_SetCursor</A
>&nbsp;--&nbsp;Set the currently active mouse cursor.</DT
><DT
><A
HREF="sdlgetcursor.html"
>SDL_GetCursor</A
>&nbsp;--&nbsp;Get the currently active mouse cursor.</DT
><DT
><A
HREF="sdlshowcursor.html"
>SDL_ShowCursor</A
>&nbsp;--&nbsp;Toggle whether or not the cursor is shown on the screen.</DT
><DT
><A
HREF="sdlglloadlibrary.html"
>SDL_GL_LoadLibrary</A
>&nbsp;--&nbsp;Specify an OpenGL library</DT
><DT
><A
HREF="sdlglgetprocaddress.html"
>SDL_GL_GetProcAddress</A
>&nbsp;--&nbsp;Get the address of a GL function</DT
><DT
><A
HREF="sdlglgetattribute.html"
>SDL_GL_GetAttribute</A
>&nbsp;--&nbsp;Get the value of a special SDL/OpenGL attribute</DT
><DT
><A
HREF="sdlglsetattribute.html"
>SDL_GL_SetAttribute</A
>&nbsp;--&nbsp;Set a special SDL/OpenGL attribute</DT
><DT
><A
HREF="sdlglswapbuffers.html"
>SDL_GL_SwapBuffers</A
>&nbsp;--&nbsp;Swap OpenGL framebuffers/Update Display</DT
><DT
><A
HREF="sdlcreateyuvoverlay.html"
>SDL_CreateYUVOverlay</A
>&nbsp;--&nbsp;Create a YUV video overlay</DT
><DT
><A
HREF="sdllockyuvoverlay.html"
>SDL_LockYUVOverlay</A
>&nbsp;--&nbsp;Lock an overlay</DT
><DT
><A
HREF="sdlunlockyuvoverlay.html"
>SDL_UnlockYUVOverlay</A
>&nbsp;--&nbsp;Unlock an overlay</DT
><DT
><A
HREF="sdldisplayyuvoverlay.html"
>SDL_DisplayYUVOverlay</A
>&nbsp;--&nbsp;Blit the overlay to the display</DT
><DT
><A
HREF="sdlfreeyuvoverlay.html"
>SDL_FreeYUVOverlay</A
>&nbsp;--&nbsp;Free a YUV video overlay</DT
><DT
><A
HREF="sdlglattr.html"
>SDL_GLattr</A
>&nbsp;--&nbsp;SDL GL Attributes</DT
><DT
><A
HREF="sdlrect.html"
>SDL_Rect</A
>&nbsp;--&nbsp;Defines a rectangular area</DT
><DT
><A
HREF="sdlcolor.html"
>SDL_Color</A
>&nbsp;--&nbsp;Format independent color description</DT
><DT
><A
HREF="sdlpalette.html"
>SDL_Palette</A
>&nbsp;--&nbsp;Color palette for 8-bit pixel formats</DT
><DT
><A
HREF="sdlpixelformat.html"
>SDL_PixelFormat</A
>&nbsp;--&nbsp;Stores surface format information</DT
><DT
><A
HREF="sdlsurface.html"
>SDL_Surface</A
>&nbsp;--&nbsp;Graphical Surface Structure</DT
><DT
><A
HREF="sdlvideoinfo.html"
>SDL_VideoInfo</A
>&nbsp;--&nbsp;Video Target information</DT
><DT
><A
HREF="sdloverlay.html"
>SDL_Overlay</A
>&nbsp;--&nbsp;YUV video overlay</DT
></DL
></DIV
><P
>SDL presents a very simple interface to the display framebuffer.  The
framebuffer is represented as an offscreen surface to which you can write
directly.  If you want the screen to show what you have written, call the <A
HREF="sdlupdaterects.html"
>update</A
> function which will
guarantee that the desired portion of the screen is updated.</P
><P
>Before you call any of the SDL video functions, you must first call
<SPAN
CLASS="TOKEN"
>SDL_Init(SDL_INIT_VIDEO)</SPAN
>, which initializes the video
and events in the SDL library.  Check the return code, which should be
<SPAN
CLASS="RETURNVALUE"
>0</SPAN
>, to see if there were any errors in starting up.</P
><P
>If you use both sound and video in your application, you need to call
<SPAN
CLASS="TOKEN"
>SDL_Init(SDL_INIT_AUDIO | SDL_INIT_VIDEO)</SPAN
> before opening the
sound device, otherwise under Win32 DirectX, you won't be able to set
full-screen display modes.</P
><P
>After you have initialized the library, you can start up the video display in a
number of ways.  The easiest way is to pick a common screen resolution and
depth and just initialize the video, checking for errors.  You will probably
get what you want, but SDL may be emulating your requested mode and converting
the display on update.  The best way is to
<A
HREF="sdlgetvideoinfo.html"
>query</A
>, for the best
video mode closest to the desired one, and then
<A
HREF="sdldisplayformat.html"
>convert</A
>
your images to that pixel format.</P
><P
>SDL currently supports any bit depth &gt;= 8 bits per pixel.  8 bpp formats are
considered 8-bit palettized modes, while 12, 15, 16, 24, and 32 bits per pixel
are considered "packed pixel" modes, meaning each pixel contains the RGB color
components packed in the bits of the pixel.</P
><P
>After you have initialized your video mode, you can take the surface that was
returned, and write to it like any other framebuffer, calling the update
routine as you go.</P
><P
>When you have finished your video access and are ready to quit your
application, you should call "<SPAN
CLASS="TOKEN"
>SDL_Quit()</SPAN
>" to shutdown the
video and events.</P
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