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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_ConvertAudio</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="Audio" HREF="audio.html"><LINK REL="PREVIOUS" TITLE="SDL_BuildAudioCVT" HREF="sdlbuildaudiocvt.html"><LINK REL="NEXT" TITLE="SDL_MixAudio" HREF="sdlmixaudio.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="sdlbuildaudiocvt.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="sdlmixaudio.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="SDLCONVERTAUDIO" ></A >SDL_ConvertAudio</H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN7048" ></A ><H2 >Name</H2 >SDL_ConvertAudio -- Convert audio data to a desired audio format.</DIV ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN7051" ></A ><H2 >Synopsis</H2 ><DIV CLASS="FUNCSYNOPSIS" ><A NAME="AEN7052" ></A ><P ></P ><PRE CLASS="FUNCSYNOPSISINFO" >#include "SDL.h"</PRE ><P ><CODE ><CODE CLASS="FUNCDEF" >int <B CLASS="FSFUNC" >SDL_ConvertAudio</B ></CODE >(SDL_AudioCVT *cvt);</CODE ></P ><P ></P ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN7058" ></A ><H2 >Description</H2 ><P ><TT CLASS="FUNCTION" >SDL_ConvertAudio</TT > takes one parameter, <TT CLASS="PARAMETER" ><I >cvt</I ></TT >, which was previously initilized. Initilizing a <A HREF="sdlaudiocvt.html" ><SPAN CLASS="STRUCTNAME" >SDL_AudioCVT</SPAN ></A > is a two step process. First of all, the structure must be passed to <A HREF="sdlbuildaudiocvt.html" ><TT CLASS="FUNCTION" >SDL_BuildAudioCVT</TT ></A > along with source and destination format parameters. Secondly, the <SPAN CLASS="STRUCTNAME" >cvt</SPAN >-><TT CLASS="STRUCTFIELD" ><I >buf</I ></TT > and <SPAN CLASS="STRUCTNAME" >cvt</SPAN >-><TT CLASS="STRUCTFIELD" ><I >len</I ></TT > fields must be setup. <SPAN CLASS="STRUCTNAME" >cvt</SPAN >-><TT CLASS="STRUCTFIELD" ><I >buf</I ></TT > should point to the audio data and <SPAN CLASS="STRUCTNAME" >cvt</SPAN >-><TT CLASS="STRUCTFIELD" ><I >len</I ></TT > should be set to the length of the audio data in bytes. Remember, the length of the buffer pointed to by <TT CLASS="STRUCTFIELD" ><I >buf</I ></TT > show be <TT CLASS="STRUCTFIELD" ><I >len</I ></TT >*<TT CLASS="STRUCTFIELD" ><I >len_mult</I ></TT > bytes in length.</P ><P >Once the <SPAN CLASS="STRUCTNAME" >SDL_AudioCVT</SPAN >structure is initilized then we can pass it to <TT CLASS="FUNCTION" >SDL_ConvertAudio</TT >, which will convert the audio data pointer to by <SPAN CLASS="STRUCTNAME" >cvt</SPAN >-><TT CLASS="STRUCTFIELD" ><I >buf</I ></TT >. If <TT CLASS="FUNCTION" >SDL_ConvertAudio</TT > returned <SPAN CLASS="RETURNVALUE" >0</SPAN > then the conversion was completed successfully, otherwise <SPAN CLASS="RETURNVALUE" >-1</SPAN > is returned.</P ><P >If the conversion completed successfully then the converted audio data can be read from <SPAN CLASS="STRUCTNAME" >cvt</SPAN >-><TT CLASS="STRUCTFIELD" ><I >buf</I ></TT >. The amount of valid, converted, audio data in the buffer is equal to <SPAN CLASS="STRUCTNAME" >cvt</SPAN >-><TT CLASS="STRUCTFIELD" ><I >len</I ></TT >*<TT CLASS="STRUCTFIELD" ><I >cvt</I ></TT >-><SPAN CLASS="STRUCTNAME" >len_ratio</SPAN >.</P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN7093" ></A ><H2 >Examples</H2 ><PRE CLASS="PROGRAMLISTING" >/* Converting some WAV data to hardware format */ void my_audio_callback(void *userdata, Uint8 *stream, int len); SDL_AudioSpec *desired, *obtained; SDL_AudioSpec wav_spec; SDL_AudioCVT wav_cvt; Uint32 wav_len; Uint8 *wav_buf; int ret; /* Allocated audio specs */ desired = malloc(sizeof(SDL_AudioSpec)); obtained = malloc(sizeof(SDL_AudioSpec)); /* Set desired format */ desired->freq=22050; desired->format=AUDIO_S16LSB; desired->samples=8192; desired->callback=my_audio_callback; desired->userdata=NULL; /* Open the audio device */ if ( SDL_OpenAudio(desired, obtained) < 0 ){ fprintf(stderr, "Couldn't open audio: %s\n", SDL_GetError()); exit(-1); } free(desired); /* Load the test.wav */ if( SDL_LoadWAV("test.wav", &wav_spec, &wav_buf, &wav_len) == NULL ){ fprintf(stderr, "Could not open test.wav: %s\n", SDL_GetError()); SDL_CloseAudio(); free(obtained); exit(-1); } /* Build AudioCVT */ ret = SDL_BuildAudioCVT(&wav_cvt, wav_spec.format, wav_spec.channels, wav_spec.freq, obtained->format, obtained->channels, obtained->freq); /* Check that the convert was built */ if(ret==-1){ fprintf(stderr, "Couldn't build converter!\n"); SDL_CloseAudio(); free(obtained); SDL_FreeWAV(wav_buf); } /* Setup for conversion */ wav_cvt.buf = malloc(wav_len * wav_cvt.len_mult); wav_cvt.len = wav_len; memcpy(wav_cvt.buf, wav_buf, wav_len); /* We can delete to original WAV data now */ SDL_FreeWAV(wav_buf); /* And now we're ready to convert */ SDL_ConvertAudio(&wav_cvt); /* do whatever */ . . . . </PRE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN7096" ></A ><H2 >See Also</H2 ><P ><A HREF="sdlbuildaudiocvt.html" ><TT CLASS="FUNCTION" >SDL_BuildAudioCVT</TT ></A >, <A HREF="sdlaudiocvt.html" ><TT CLASS="FUNCTION" >SDL_AudioCVT</TT ></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="sdlbuildaudiocvt.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="sdlmixaudio.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SDL_BuildAudioCVT</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="audio.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >SDL_MixAudio</TD ></TR ></TABLE ></DIV ></BODY ></HTML >