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.

.TH "SDL_CreateRGBSurfaceFrom" "3" "Tue 11 Sep 2001, 23:01" "SDL" "SDL API Reference" 
.SH "NAME"
SDL_CreateRGBSurfaceFrom \- Create an SDL_Surface from pixel data
.SH "SYNOPSIS"
.PP
\fB#include "SDL\&.h"
.sp
\fBSDL_Surface *\fBSDL_CreateRGBSurfaceFrom\fP\fR(\fBvoid *pixels, int width, int height, int depth, int pitch, Uint32 Rmask, Uint32 Gmask, Uint32 Bmask, Uint32 Amask\fR);
.SH "DESCRIPTION"
.PP
Creates an SDL_Surface from the provided pixel data\&.
.PP
The data stored in \fBpixels\fR is assumed to be of the \fBdepth\fR specified in the parameter list\&. The pixel data is not copied into the \fBSDL_Surface\fR structure so it should not be freed until the surface has been freed with a called to \fISDL_FreeSurface\fR\&. \fBpitch\fR is the length of each scanline in bytes\&. 
.PP
See \fI\fBSDL_CreateRGBSurface\fP\fR for a more detailed description of the other parameters\&.
.SH "RETURN VALUE"
.PP
Returns the created surface, or \fBNULL\fR upon error\&.
.SH "SEE ALSO"
.PP
\fI\fBSDL_CreateRGBSurface\fP\fR, \fI\fBSDL_FreeSurface\fP\fR
...\" created by instant / docbook-to-man, Tue 11 Sep 2001, 23:01