信号量的锁如果多释放信号会怎么样

dispatch_semaphore_t lock = dispatch_semaphore_create(1);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务1");
        dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
        NSLog(@"任务1 over");
        dispatch_semaphore_signal(lock);
        NSLog(@"任务1 ----->1");
        dispatch_semaphore_signal(lock);
        NSLog(@"任务1 ----->2");
        dispatch_semaphore_signal(lock);
        NSLog(@"任务1 ----->3");
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务2");
        dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
        NSLog(@"任务2 over");
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务3");
        dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
        NSLog(@"任务3 over");
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务4");
        dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
        NSLog(@"任务4 over");
    });

2020-09-10 19:18:26.347357+0800 FindClassMethods[6856:271676] 任务1
2020-09-10 19:18:26.347364+0800 FindClassMethods[6856:271671] 任务2
2020-09-10 19:18:26.347387+0800 FindClassMethods[6856:271675] 任务3
2020-09-10 19:18:26.347400+0800 FindClassMethods[6856:271672] 任务4
2020-09-10 19:18:26.347539+0800 FindClassMethods[6856:271676] 任务1 over
2020-09-10 19:18:26.347656+0800 FindClassMethods[6856:271676] 任务1 ----->1
2020-09-10 19:18:26.347667+0800 FindClassMethods[6856:271671] 任务2 over
2020-09-10 19:18:26.347743+0800 FindClassMethods[6856:271676] 任务1 ----->2
2020-09-10 19:18:26.347755+0800 FindClassMethods[6856:271675] 任务3 over
2020-09-10 19:18:26.347995+0800 FindClassMethods[6856:271676] 任务1 ----->3
2020-09-10 19:18:26.348008+0800 FindClassMethods[6856:271672] 任务4 over

如上所示，如果运气好第一个先执行，那么就会出现1得到锁，2，3，4必须等待，1执行任务，1放开一个锁，另一个线程2立即得到锁，2执行任务，1继续放开一个锁，然后被3得到了... （所以唤起另一个线程之后会立即执行这个线程？）

 

 

 

 

 

如果说，一开始先执行了多次释放信号量，后面加锁但是没有释放信号量也可以有多个线程得到锁往下执行，说明每释放一次就会+1信号量，多释放可能会出现自己不希望的结果，所以对信号量值的掌握要相当有把握才行

 

 dispatch_semaphore_t lock = dispatch_semaphore_create(1);
//    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务1");
        NSLog(@"任务1     %ld",dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER));
        NSLog(@"任务1 over");
        NSLog(@"任务1 ----->%ld",dispatch_semaphore_signal(lock));
        NSLog(@"任务1 ----->%ld",dispatch_semaphore_signal(lock));
        NSLog(@"任务1 ----->%ld",dispatch_semaphore_signal(lock));
//    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务2");
        NSLog(@"任务2     %ld",dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER));
        NSLog(@"任务2 over");
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务3");
        NSLog(@"任务3     %ld",dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER));
        NSLog(@"任务3 over");
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务4");
        NSLog(@"任务4     %ld",dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER));
        NSLog(@"任务4 over");
        NSLog(@"任务4 ----->%ld",dispatch_semaphore_signal(lock));
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务5");
        NSLog(@"任务5     %ld",dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER));
        NSLog(@"任务5 over");
    });
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"任务6");
        NSLog(@"任务6     %ld",dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER));
        NSLog(@"任务6 over");
    });

 

2020-09-10 19:31:07.739063+0800 FindClassMethods[6937:278894] 任务1
2020-09-10 19:31:07.739182+0800 FindClassMethods[6937:278894] 任务1     0
2020-09-10 19:31:07.739307+0800 FindClassMethods[6937:278894] 任务1 over
2020-09-10 19:31:07.739441+0800 FindClassMethods[6937:278894] 任务1 ----->0
2020-09-10 19:31:07.739518+0800 FindClassMethods[6937:278894] 任务1 ----->0
2020-09-10 19:31:07.739604+0800 FindClassMethods[6937:278894] 任务1 ----->0
2020-09-10 19:31:07.739742+0800 FindClassMethods[6937:278954] 任务2
2020-09-10 19:31:07.739751+0800 FindClassMethods[6937:278950] 任务3
2020-09-10 19:31:07.739763+0800 FindClassMethods[6937:278952] 任务4
2020-09-10 19:31:07.739772+0800 FindClassMethods[6937:278951] 任务6
2020-09-10 19:31:07.739779+0800 FindClassMethods[6937:278953] 任务5
2020-09-10 19:31:07.739836+0800 FindClassMethods[6937:278954] 任务2     0
2020-09-10 19:31:07.739856+0800 FindClassMethods[6937:278950] 任务3     0
2020-09-10 19:31:07.739863+0800 FindClassMethods[6937:278952] 任务4     0
2020-09-10 19:31:07.740217+0800 FindClassMethods[6937:278954] 任务2 over
2020-09-10 19:31:07.740366+0800 FindClassMethods[6937:278952] 任务4 over
2020-09-10 19:31:07.740689+0800 FindClassMethods[6937:278952] 任务4 ----->1
2020-09-10 19:31:07.740695+0800 FindClassMethods[6937:278951] 任务6     0
2020-09-10 19:31:07.740813+0800 FindClassMethods[6937:278950] 任务3 over
2020-09-10 19:31:07.742692+0800 FindClassMethods[6937:278951] 任务6 over

 

dispatch_semaphore_wait返回0标识获得锁成功，往下执行，否则等待超时失败。

dispatch_semaphore_signal返回0标识当前没有等待的线程，否则标识跳转到其他线程执行任务