view docs/html/cdrom.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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>CD-ROM</TITLE
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>Chapter 11. CD-ROM</H1
><DIV
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><DL
><DT
><B
>Table of Contents</B
></DT
><DT
><A
HREF="sdlcdnumdrives.html"
>SDL_CDNumDrives</A
>&nbsp;--&nbsp;Returns the number of CD-ROM drives on the system.</DT
><DT
><A
HREF="sdlcdname.html"
>SDL_CDName</A
>&nbsp;--&nbsp;Returns a human-readable, system-dependent identifier for the CD-ROM.</DT
><DT
><A
HREF="sdlcdopen.html"
>SDL_CDOpen</A
>&nbsp;--&nbsp;Opens a CD-ROM drive for access.</DT
><DT
><A
HREF="sdlcdstatus.html"
>SDL_CDStatus</A
>&nbsp;--&nbsp;Returns the current status of the given drive.</DT
><DT
><A
HREF="sdlcdplay.html"
>SDL_CDPlay</A
>&nbsp;--&nbsp;Play a CD</DT
><DT
><A
HREF="sdlcdplaytracks.html"
>SDL_CDPlayTracks</A
>&nbsp;--&nbsp;Play the given CD track(s)</DT
><DT
><A
HREF="sdlcdpause.html"
>SDL_CDPause</A
>&nbsp;--&nbsp;Pauses a CDROM</DT
><DT
><A
HREF="sdlcdresume.html"
>SDL_CDResume</A
>&nbsp;--&nbsp;Resumes a CDROM</DT
><DT
><A
HREF="sdlcdstop.html"
>SDL_CDStop</A
>&nbsp;--&nbsp;Stops a CDROM</DT
><DT
><A
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>SDL_CDEject</A
>&nbsp;--&nbsp;Ejects a CDROM</DT
><DT
><A
HREF="sdlcdclose.html"
>SDL_CDClose</A
>&nbsp;--&nbsp;Closes a SDL_CD handle</DT
><DT
><A
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>SDL_CD</A
>&nbsp;--&nbsp;CDROM Drive Information</DT
><DT
><A
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>SDL_CDtrack</A
>&nbsp;--&nbsp;CD Track Information Structure</DT
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><P
>SDL supports audio control of up to 32 local CD-ROM drives at once.</P
><P
>You use this API to perform all the basic functions of a CD player,
including listing the tracks, playing, stopping, and ejecting the CD-ROM.
(Currently, multi-changer CD drives are not supported.)</P
><P
>Before you call any of the SDL CD-ROM functions, you must first call
"<TT
CLASS="FUNCTION"
>SDL_Init(SDL_INIT_CDROM)</TT
>", which scans the system for
CD-ROM drives, and sets the program up for audio control.  Check the 
return code, which should be <SPAN
CLASS="RETURNVALUE"
>0</SPAN
>, to see if there 
were any errors in starting up.</P
><P
>After you have initialized the library, you can find out how many drives
are available using the <TT
CLASS="FUNCTION"
>SDL_CDNumDrives()</TT
> function.  
The first drive listed is the system default CD-ROM drive.  After you have 
chosen a drive, and have opened it with <TT
CLASS="FUNCTION"
>SDL_CDOpen()</TT
>, 
you can check the status and start playing if there's a CD in the drive.</P
><P
>A CD-ROM is organized into one or more tracks, each consisting of a certain
number of "frames".  Each frame is ~2K in size, and at normal playing speed,
a CD plays 75 frames per second.  SDL works with the number of frames on a
CD, but this can easily be converted to the more familiar minutes/seconds
format by using the <TT
CLASS="FUNCTION"
>FRAMES_TO_MSF()</TT
> macro.</P
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