Mercurial > sdl-ios-xcode
view sdl.m4 @ 4105:84882a89ca50 SDL-1.2
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 | Fri, 28 Dec 2007 22:05:17 +0000 |
parents | 91063c450171 |
children | e9bcf004c324 |
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# Configure paths for SDL # Sam Lantinga 9/21/99 # stolen from Manish Singh # stolen back from Frank Belew # stolen from Manish Singh # Shamelessly stolen from Owen Taylor dnl AM_PATH_SDL([MINIMUM-VERSION, [ACTION-IF-FOUND [, ACTION-IF-NOT-FOUND]]]) dnl Test for SDL, and define SDL_CFLAGS and SDL_LIBS dnl AC_DEFUN([AM_PATH_SDL], [dnl dnl Get the cflags and libraries from the sdl-config script dnl AC_ARG_WITH(sdl-prefix,[ --with-sdl-prefix=PFX Prefix where SDL is installed (optional)], sdl_prefix="$withval", sdl_prefix="") AC_ARG_WITH(sdl-exec-prefix,[ --with-sdl-exec-prefix=PFX Exec prefix where SDL is installed (optional)], sdl_exec_prefix="$withval", sdl_exec_prefix="") AC_ARG_ENABLE(sdltest, [ --disable-sdltest Do not try to compile and run a test SDL program], , enable_sdltest=yes) if test x$sdl_exec_prefix != x ; then sdl_args="$sdl_args --exec-prefix=$sdl_exec_prefix" if test x${SDL_CONFIG+set} != xset ; then SDL_CONFIG=$sdl_exec_prefix/bin/sdl-config fi fi if test x$sdl_prefix != x ; then sdl_args="$sdl_args --prefix=$sdl_prefix" if test x${SDL_CONFIG+set} != xset ; then SDL_CONFIG=$sdl_prefix/bin/sdl-config fi fi if test "x$prefix" != xNONE; then PATH="$prefix/bin:$prefix/usr/bin:$PATH" fi AC_PATH_PROG(SDL_CONFIG, sdl-config, no, [$PATH]) min_sdl_version=ifelse([$1], ,0.11.0,$1) AC_MSG_CHECKING(for SDL - version >= $min_sdl_version) no_sdl="" if test "$SDL_CONFIG" = "no" ; then no_sdl=yes else SDL_CFLAGS=`$SDL_CONFIG $sdlconf_args --cflags` SDL_LIBS=`$SDL_CONFIG $sdlconf_args --libs` sdl_major_version=`$SDL_CONFIG $sdl_args --version | \ sed 's/\([[0-9]]*\).\([[0-9]]*\).\([[0-9]]*\)/\1/'` sdl_minor_version=`$SDL_CONFIG $sdl_args --version | \ sed 's/\([[0-9]]*\).\([[0-9]]*\).\([[0-9]]*\)/\2/'` sdl_micro_version=`$SDL_CONFIG $sdl_config_args --version | \ sed 's/\([[0-9]]*\).\([[0-9]]*\).\([[0-9]]*\)/\3/'` if test "x$enable_sdltest" = "xyes" ; then ac_save_CFLAGS="$CFLAGS" ac_save_CXXFLAGS="$CXXFLAGS" ac_save_LIBS="$LIBS" CFLAGS="$CFLAGS $SDL_CFLAGS" CXXFLAGS="$CXXFLAGS $SDL_CFLAGS" LIBS="$LIBS $SDL_LIBS" dnl dnl Now check if the installed SDL is sufficiently new. (Also sanity dnl checks the results of sdl-config to some extent dnl rm -f conf.sdltest AC_TRY_RUN([ #include <stdio.h> #include <stdlib.h> #include <string.h> #include "SDL.h" char* my_strdup (char *str) { char *new_str; if (str) { new_str = (char *)malloc ((strlen (str) + 1) * sizeof(char)); strcpy (new_str, str); } else new_str = NULL; return new_str; } int main (int argc, char *argv[]) { int major, minor, micro; char *tmp_version; /* This hangs on some systems (?) system ("touch conf.sdltest"); */ { FILE *fp = fopen("conf.sdltest", "a"); if ( fp ) fclose(fp); } /* HP/UX 9 (%@#!) writes to sscanf strings */ tmp_version = my_strdup("$min_sdl_version"); if (sscanf(tmp_version, "%d.%d.%d", &major, &minor, µ) != 3) { printf("%s, bad version string\n", "$min_sdl_version"); exit(1); } if (($sdl_major_version > major) || (($sdl_major_version == major) && ($sdl_minor_version > minor)) || (($sdl_major_version == major) && ($sdl_minor_version == minor) && ($sdl_micro_version >= micro))) { return 0; } else { printf("\n*** 'sdl-config --version' returned %d.%d.%d, but the minimum version\n", $sdl_major_version, $sdl_minor_version, $sdl_micro_version); printf("*** of SDL required is %d.%d.%d. If sdl-config is correct, then it is\n", major, minor, micro); printf("*** best to upgrade to the required version.\n"); printf("*** If sdl-config was wrong, set the environment variable SDL_CONFIG\n"); printf("*** to point to the correct copy of sdl-config, and remove the file\n"); printf("*** config.cache before re-running configure\n"); return 1; } } ],, no_sdl=yes,[echo $ac_n "cross compiling; assumed OK... $ac_c"]) CFLAGS="$ac_save_CFLAGS" CXXFLAGS="$ac_save_CXXFLAGS" LIBS="$ac_save_LIBS" fi fi if test "x$no_sdl" = x ; then AC_MSG_RESULT(yes) ifelse([$2], , :, [$2]) else AC_MSG_RESULT(no) if test "$SDL_CONFIG" = "no" ; then echo "*** The sdl-config script installed by SDL could not be found" echo "*** If SDL was installed in PREFIX, make sure PREFIX/bin is in" echo "*** your path, or set the SDL_CONFIG environment variable to the" echo "*** full path to sdl-config." else if test -f conf.sdltest ; then : else echo "*** Could not run SDL test program, checking why..." CFLAGS="$CFLAGS $SDL_CFLAGS" CXXFLAGS="$CXXFLAGS $SDL_CFLAGS" LIBS="$LIBS $SDL_LIBS" AC_TRY_LINK([ #include <stdio.h> #include "SDL.h" int main(int argc, char *argv[]) { return 0; } #undef main #define main K_and_R_C_main ], [ return 0; ], [ echo "*** The test program compiled, but did not run. This usually means" echo "*** that the run-time linker is not finding SDL or finding the wrong" echo "*** version of SDL. If it is not finding SDL, you'll need to set your" echo "*** LD_LIBRARY_PATH environment variable, or edit /etc/ld.so.conf to point" echo "*** to the installed location Also, make sure you have run ldconfig if that" echo "*** is required on your system" echo "***" echo "*** If you have an old version installed, it is best to remove it, although" echo "*** you may also be able to get things to work by modifying LD_LIBRARY_PATH"], [ echo "*** The test program failed to compile or link. See the file config.log for the" echo "*** exact error that occured. This usually means SDL was incorrectly installed" echo "*** or that you have moved SDL since it was installed. In the latter case, you" echo "*** may want to edit the sdl-config script: $SDL_CONFIG" ]) CFLAGS="$ac_save_CFLAGS" CXXFLAGS="$ac_save_CXXFLAGS" LIBS="$ac_save_LIBS" fi fi SDL_CFLAGS="" SDL_LIBS="" ifelse([$3], , :, [$3]) fi AC_SUBST(SDL_CFLAGS) AC_SUBST(SDL_LIBS) rm -f conf.sdltest ])