[GCD] Use semaphore as lock

main.m

#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) {
	@autoreleasepool {
		dispatch_semaphore_t lock =  dispatch_semaphore_create(0);
		
		dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_global_queue(QOS_CLASS_DEFAULT, 0), ^{
				NSLog(@"unlock");
				dispatch_semaphore_signal(lock);
			});
		
		long timeout = dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
		NSLog(@"done");
	}
}