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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. Below is a sample C++ app to demonstrate the problem (should output plain zeros when correct):

#include <SDL.h>
#include <iostream>

int main(int argc, char* argv[])
{
	SDL_Init(0);
	SDL_sem* sem = SDL_CreateSemaphore(0);
	std::cout << SDL_SemValue(sem) << std::endl;
	
	SDL_SemTryWait(sem);
	std::cout << SDL_SemValue(sem) << std::endl;
	
	SDL_SemTryWait(sem);
	std::cout << SDL_SemValue(sem) << std::endl;
	
	// Breaks here! Integer underflows!
	SDL_SemWaitTimeout(sem, 10);
	std::cout << SDL_SemValue(sem) << std::endl;
	
	SDL_SemTryWait(sem);
	std::cout << SDL_SemValue(sem) << std::endl;
	
	SDL_SemTryWait(sem);
	std::cout << SDL_SemValue(sem) << std::endl;

	SDL_DestroySemaphore(sem);
	sem = NULL;
	SDL_Quit();
	return 0;
}