Modified version of Steven Frank's example here: http://stevenf.com/notes/index.php/?Running+blocks+on+Grand+Central+Dispatch

gcd-test.m

#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) {
	@autoreleasepool {
		
		NSArray *array = @[@"one", @"two", @"three", @"four", @"five", @"six", @"seven", @"eight", @"nine", @"ten", @"eleven", @"twelve"];
		
		dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0);
		dispatch_group_t group = dispatch_group_create();
		dispatch_semaphore_t semaphore = dispatch_semaphore_create(6);

		for (NSString *string in array) {
			
		    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
			
			dispatch_group_async(group, queue, ^{
		        NSLog(@"%@", string);
		        dispatch_semaphore_signal(semaphore);
		    });
		}
		
		dispatch_group_t group2 = dispatch_group_create();
		dispatch_group_enter(group2);

		dispatch_group_notify(group, queue, ^{
			NSLog(@"Group finished");
		    dispatch_release(semaphore);
			dispatch_group_leave(group2);
		});
		
		dispatch_group_wait(group2, DISPATCH_TIME_FOREVER);
		dispatch_release(group);
		dispatch_release(group2);
		NSLog(@"END");
	}
}