//other necessary includes
#include <stdlib.h>
#include <stdio.h>
#include <dispatch/dispatch.h>

typedef void (^voidBlock)(void);
typedef int (^intBlock)(int);

void doAsyncWork(voidBlock theWork) {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, block);
}

int main(int argc, char** argv) {
    dispatch_semaphore_t workDone = dispatch_semaphore_create(0);
    doAsyncWork( ^ {
        //something expensive
        dispatch_semaphore_signal(workDone);
    });
    dispatch_semaphore_wait(workDone);
    dispatch_release(workDone);
    printf("yay work finished!\n");
    return EXIT_SUCCESS;
}