GCD

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

sleep(3);

NSLog(@"dispatch_sync(dispatch_get_global_queue  同步  全局并发队列");

});

dispatch_sync(dispatch_get_main_queue(), ^{

NSLog(@"dispatch_sync在等待block语句执行完成，而block语句需要在主线程里执行，所以dispatch_sync如果在主线程调用就会造成死锁");

NSLog(@"dispatch_sync是同步的，本身就会阻塞当前线程，也即主线程。而又往主线程里塞进去一个block，所以就会发生死锁");

});

dispatch_async(dispatch_get_main_queue(), ^{

sleep(3);

NSLog(@"dispatch_async(dispatch_get_main_queue()  异步 主队列");

});

NSLog(@"1234");

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

sleep(10);

NSLog(@" dispatch_async(dispatch_get_global_queue  异步  全局并发队列");

});

NSLog(@"52431");