Suspending GCD query problem

Any blocks that have been dispatched to your queue asynchronously before you actually call dispatch_suspend() will get run before the suspend takes effect. In your code you are firing off a bunch of blocks asynchronously, so some are probably still in the queue when you call test(2), and those blocks will be executed.

If you want to be able to cancel your running jobs, you’ll need to do so in your own logic. GCD purposefully doesn’t expose a true cancellation API. You could do something like this:

@interface Canceller
{
    BOOL _shouldCancel;
}
- (void)setShouldCancel:(BOOL)shouldCancel;
- (BOOL)shouldCancel;
@end

@implementation Canceller
- (void)setShouldCancel:(BOOL)shouldCancel {
    _shouldCancel = shouldCancel;
}
- (BOOL)shouldCancel {
    return _shouldCancel;
}
@end

static void test(int a){
    static Canceller * canceller = nil;

    if(q){
        [canceller setShouldCancel:YES];
        [canceller release];
        dispatch_suspend(q);
        dispatch_release(q);
        q=nil;
    }
    canceller = [[Canceller alloc] init];
    q=dispatch_get_global_queue(0,0);
    dispatch_async(q,^ {
        while(![canceller shouldCancel]){NSLog(@"query %d",a);sleep(2);}
    });

}

In this way, each block will keep a reference to an object that knows if it should stop doing work.