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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 d72a0dd80e8b
children b196d2758026
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SDL on DirectFB

Supports:

- Hardware YUV overlays
- OpenGL - software only
- 2D/3D accelerations (depends on directfb driver)
- multiple displays
- windows

What you need:

DirectFB 1.0.1, 1.2.x, 1.3.0
Kernel-Framebuffer support: required: vesafb, radeonfb .... 
Mesa 7.0.x	   - optional for OpenGL

/etc/directfbrc

This file should contain the following lines to make
your joystick work and avoid crashes:
------------------------
disable-module=joystick
disable-module=cle266
disable-module=cyber5k
no-linux-input-grab
------------------------

To disable to use x11 backend when DISPLAY variable is found use

export SDL_DIRECTFB_X11_CHECK=0

To disable the use of linux input devices, i.e. multimice/multikeyboard support,
use

export SDL_DIRECTFB_LINUX_INPUT=0

To use hardware accelerated YUV-overlays for YUV-textures, use:

export SDL_DIRECTFB_YUV_DIRECT=1

This is disabled by default. It will only support one 
YUV texture, namely the first. Every other YUV texture will be
rendered in software.

In addition, you may use (directfb-1.2.x)

export SDL_DIRECTFB_YUV_UNDERLAY=1

to make the YUV texture an underlay. This will make the cursor to
be shown.

Simple Window Manager
=====================

The driver has support for a very, very basic window manager you may
want to use when runnning with "wm=default". Use

export SDL_DIRECTFB_WM=1

to enable basic window borders. In order to have the window title rendered,
you need to have the following font installed:

/usr/share/fonts/truetype/freefont/FreeSans.ttf

OPENGL Support
==============

As of this writing 20070810 you need to pull Mesa from git and do the following:

------------------------
cd mesa 
make linux-directfb
make

echo Installing - please enter sudo pw.

sudo make install INSTALL_DIR=/usr/local/dfb_GL
cd src/mesa/drivers/directfb
make
sudo make install INSTALL_DIR=/usr/local/dfb_GL
------------------------

To run the SDL - testprograms:

export SDL_VIDEODRIVER=directfb
export LD_LIBRARY_PATH=/usr/local/dfb_GL/lib
export LD_PRELOAD=/usr/local/dfb_GL/libGL.so.7

./testgl