From 67255d8294bdb8f01ead1838f0dcf9cf3ce3ab8a Mon Sep 17 00:00:00 2001
From: Sam Lantinga <slouken@libsdl.org>
Date: Mon, 21 Sep 2009 07:32:26 +0000
Subject: [PATCH] Fixed bug #570

SDL_SemWaitTimeout in src/thread/generic/SDL_syssem.c line 179 (SVN trunk):

--sem->count;

should be

if (retval == 0) {
    --sem->count;
}

Without this, sem->count will underflow on timeout effectively breaking the
semaphore. It appears that the implementation has been wrong since the initial
revision.
---
 src/thread/generic/SDL_syssem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/thread/generic/SDL_syssem.c b/src/thread/generic/SDL_syssem.c
index 041d64f20..ee73d442c 100644
--- a/src/thread/generic/SDL_syssem.c
+++ b/src/thread/generic/SDL_syssem.c
@@ -165,7 +165,9 @@ int SDL_SemWaitTimeout(SDL_sem *sem, Uint32 timeout)
 		                             sem->count_lock, timeout);
 	}
 	--sem->waiters_count;
-	--sem->count;
+	if (retval == 0) {
+		--sem->count;
+	}
 	SDL_UnlockMutex(sem->count_lock);
 
 	return retval;