Question

In the GCD documentation it’s quite clear that to submit work to the main queue, you need to either be working within an NSApplication (or UIApplication) or call dispatch_main() to act as a run loop of sorts. However, do I need to do anything to set up the global concurrent queue?

Basically what I am asking is: If I write a plain simple C program, do I need to perform any special setup before I go dispatch_get_global_queue() and start giving it work?

Answer 1

No, you don’t need any additional setup. But you need to call dispatch_main() to start the GCD dispatcher.
As dispatch_main() never returns, this will also prevent your main function from reaching it’s return.

Example for a minimal C program that uses GCD & a global queue (based on http://wiki.freebsd.org/GCD):

#include <dispatch/dispatch.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_time_t dispatchTime = dispatch_time(DISPATCH_TIME_NOW, 5LL * NSEC_PER_SEC);
    dispatch_after(dispatchTime, globalQueue, ^{
        printf("Dispatched on global queue\n");
        exit(0);
    });
    dispatch_main();
    return (0);
}

To compile this, use:

clang -Wall -Werror -fblocks -L/usr/local/lib -I/usr/local/include -o test test.c