dispatch_queue_set_specific就是向指定队列里面设置一个标识 如：

dispatch_queue_set_specific(queue1, queueKey1, &queueKey1,NULL);

就是向queue1对了中设置一个queueKey1标识。

dispatch_get_specific就是在当前队列中取出标识，注意iOS中线程和队列的关系，所有的动作都是在队列中执行的！

#import <Foundation/Foundation.h>
 
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        static void *queueKey1 = "queueKey1";
        
        dispatch_queue_t queue1 = dispatch_queue_create(queueKey1, DISPATCH_QUEUE_SERIAL);
        dispatch_queue_set_specific(queue1, queueKey1, &queueKey1, NULL);
        
        NSLog(@"1. 当前线程是: %@, 当前队列是: %@ 。",[NSThread currentThread],dispatch_get_current_queue());
        
        if (dispatch_get_specific(queueKey1)) {
            //当前队列是主队列，不是queue1队列，所以取不到queueKey1对应的值，故而不执行
            NSLog(@"2. 当前线程是: %@, 当前队列是: %@ 。",[NSThread currentThread],dispatch_get_current_queue());
            [NSThread sleepForTimeInterval:1];
        }else{
            NSLog(@"3. 当前线程是: %@, 当前队列是: %@ 。",[NSThread currentThread],dispatch_get_current_queue());
            [NSThread sleepForTimeInterval:1];
        }
        
        dispatch_sync(queue1, ^{
            NSLog(@"4. 当前线程是: %@, 当前队列是: %@ 。",[NSThread currentThread],dispatch_get_current_queue());
            [NSThread sleepForTimeInterval:1];
            
            if (dispatch_get_specific(queueKey1)) {
                 //当前队列是queue1队列，所以能取到queueKey1对应的值，故而执行
                NSLog(@"5. 当前线程是: %@, 当前队列是: %@ 。",[NSThread currentThread],dispatch_get_current_queue());
                [NSThread sleepForTimeInterval:1];
            }else{
                NSLog(@"6. 当前线程是: %@, 当前队列是: %@ 。",[NSThread currentThread],dispatch_get_current_queue());
                [NSThread sleepForTimeInterval:1];
            }
        });
        dispatch_async(queue1, ^{
            NSLog(@"7. t当前线程是: %@, 当前队列是: %@ 。",[NSThread currentThread],dispatch_get_current_queue());
            [NSThread sleepForTimeInterval:1];
        });
        
        [NSThread sleepForTimeInterval:5];
    }
    return 0;
}

输出结果：

2016-02-19 14:31:23.390 gcd[96865:820267] 1.当前线程是: <NSThread: 0x1001053e0>{number = 1, name = main},当前队列是: <OS_dispatch_queue: com.apple.main-thread[0x100059ac0]>。

2016-02-19 14:31:23.391 gcd[96865:820267] 3.当前线程是: <NSThread: 0x1001053e0>{number = 1, name = main},当前队列是: <OS_dispatch_queue: com.apple.main-thread[0x100059ac0]>。

2016-02-19 14:31:24.396 gcd[96865:820267] 4.当前线程是: <NSThread: 0x1001053e0>{number = 1, name = main},当前队列是: <OS_dispatch_queue: queueKey1[0x103000000]>。

2016-02-19 14:31:25.400 gcd[96865:820267] 5.当前线程是: <NSThread: 0x1001053e0>{number = 1, name = main},当前队列是: <OS_dispatch_queue: queueKey1[0x103000000]>。

2016-02-19 14:31:26.402 gcd[96865:820367] 7. t当前线程是: <NSThread: 0x100105e10>{number = 2, name = (null)},当前队列是: <OS_dispatch_queue: queueKey1[0x103000000]>。

Program ended with exit code: 0