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view docs/html/sdlkeysym.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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<HTML ><HEAD ><TITLE >SDL_keysym</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="SDL Event Structures." HREF="eventstructures.html"><LINK REL="PREVIOUS" TITLE="SDL_QuitEvent" HREF="sdlquitevent.html"><LINK REL="NEXT" TITLE="SDLKey" HREF="sdlkey.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="sdlquitevent.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="sdlkey.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="SDLKEYSYM" ></A >SDL_keysym</H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN4659" ></A ><H2 >Name</H2 >SDL_keysym -- Keysym structure</DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN4662" ></A ><H2 >Structure Definition</H2 ><PRE CLASS="PROGRAMLISTING" >typedef struct{ Uint8 scancode; SDLKey sym; SDLMod mod; Uint16 unicode; } SDL_keysym;</PRE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN4665" ></A ><H2 >Structure Data</H2 ><DIV CLASS="INFORMALTABLE" ><A NAME="AEN4667" ></A ><P ></P ><TABLE BORDER="0" CLASS="CALSTABLE" ><TBODY ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >scancode</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Hardware specific scancode</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >sym</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >SDL virtual keysym</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >mod</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Current key modifiers</TD ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" ><TT CLASS="STRUCTFIELD" ><I >unicode</I ></TT ></TD ><TD ALIGN="LEFT" VALIGN="TOP" >Translated character</TD ></TR ></TBODY ></TABLE ><P ></P ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN4686" ></A ><H2 >Description</H2 ><P >The <SPAN CLASS="STRUCTNAME" >SDL_keysym</SPAN > structure is used by reporting key presses and releases since it is a part of the <A HREF="sdlkeyboardevent.html" ><SPAN CLASS="STRUCTNAME" >SDL_KeyboardEvent</SPAN ></A >.</P ><P >The <TT CLASS="STRUCTFIELD" ><I >scancode</I ></TT > field should generally be left alone, it is the hardware dependent scancode returned by the keyboard. The <TT CLASS="STRUCTFIELD" ><I >sym</I ></TT > field is extremely useful. It is the SDL-defined value of the key (see <A HREF="sdlkey.html" >SDL Key Syms</A >. This field is very useful when you are checking for certain key presses, like so: <PRE CLASS="PROGRAMLISTING" >. . while(SDL_PollEvent(&event)){ switch(event.type){ case SDL_KEYDOWN: if(event.key.keysym.sym==SDLK_LEFT) move_left(); break; . . . } } . .</PRE > <TT CLASS="STRUCTFIELD" ><I >mod</I ></TT > stores the current state of the keyboard modifiers as explained in <A HREF="sdlgetmodstate.html" ><TT CLASS="FUNCTION" >SDL_GetModState</TT ></A >. The <TT CLASS="STRUCTFIELD" ><I >unicode</I ></TT > is only used when UNICODE translation is enabled with <A HREF="sdlenableunicode.html" ><TT CLASS="FUNCTION" >SDL_EnableUNICODE</TT ></A >. If <TT CLASS="STRUCTFIELD" ><I >unicode</I ></TT > is non-zero then this a the UNICODE character corresponding to the keypress. If the high 9 bits of the character are 0, then this maps to the equivalent ASCII character: <PRE CLASS="PROGRAMLISTING" >char ch; if ( (keysym.unicode & 0xFF80) == 0 ) { ch = keysym.unicode & 0x7F; } else { printf("An International Character.\n"); }</PRE > UNICODE translation does have a slight overhead so don't enable it unless its needed.</P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN4705" ></A ><H2 >See Also</H2 ><P ><A HREF="sdlkey.html" ><SPAN CLASS="STRUCTNAME" >SDLKey</SPAN ></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="sdlquitevent.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="sdlkey.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SDL_QuitEvent</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="eventstructures.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >SDLKey</TD ></TR ></TABLE ></DIV ></BODY ></HTML >