An OS X and iOS implementation of semaphores that uses OSAtomic functions (libkern/OSAtomic.h) to guarantee thread safety

OSAtomic-Semaphore.c

//
//  main.c
//
//  Copyright (c) 2014 Matt Rajca. All rights reserved.
//

#include <stdio.h>
#include <dispatch/dispatch.h>
#include <libkern/OSAtomic.h>
#include <sched.h>

typedef struct _MRSemaphore {
	volatile uint32_t lock;
	volatile uint32_t sem;
} MRSemaphore;

void MRSemaphoreInit(MRSemaphore *s, uint32_t count) {
	s->lock = 0;
	s->sem = count;
}

void MRSemaphoreLock(MRSemaphore *s) {
	while (1) {
		while (OSAtomicTestAndSet(1, &s->lock)) {
			sched_yield();
		}
		
		if (s->sem > 0) {
			// decrement and proceed
			s->sem--;
			s->lock = 0;
			return;
		}
		
		// wait
		s->lock = 0;
		usleep(10);
	}
}

void MRSemaphoreUnlock(MRSemaphore *s) {
	while (OSAtomicTestAndSet(1, &s->lock)) {
		sched_yield();
	}
	
	s->sem++;
	s->lock = 0;
}

int main(int argc, const char *argv[]) {
	dispatch_queue_t queue = dispatch_queue_create(NULL, DISPATCH_QUEUE_CONCURRENT);
	
	__block MRSemaphore s;
	__block int sharedCounter = 0;
	
	MRSemaphoreInit(&s, 1); // 1 = mutex
	
	for (int n = 0; n < 100000; n++) {
		dispatch_async(queue, ^{
			MRSemaphoreLock(&s);
			sharedCounter++;
			MRSemaphoreUnlock(&s);
		});
	}
	
	dispatch_barrier_sync(queue, ^{
		printf("%d (should be 100000)\n", sharedCounter);
	});
	
	dispatch_release(queue);
	queue = NULL;
	
	return 0;
}