[NSThread isMainThread] is probably not what you want!

main.m

#import <Foundation/Foundation.h>

int main (int argc, const char **argv) {
  @autoreleasepool {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      dispatch_sync(dispatch_get_main_queue(), ^{
        NSLog(@"is main thread? %i", (int)[NSThread isMainThread]);
      });
    });

    dispatch_main();
  }

  return 0;
}

output.txt

2015-11-24 16:08:39.545 main[88889:21446128] is main thread? 0

@adamkaplan Good point, thanks.

No problem :)

Interesting learnings from this: main thread pumps the main queue – not the other way around – since GCD only has a main queue, which by default, does nothing.

the main thread pumps the main queue – not the other way around – since GCD only has a main queue, which by default, does nothing.

What's that mean? Sorry, a bit of newb here.