iOS信号量的使用

并发多线程，多个资源的情况下，使用信号量。

并发多线程，单个资源的情况下，使用mutex。

6辆车，3个停车位的问题：

-(void)testsemph{
    
    dispatch_semaphore_t semaphores = dispatch_semaphore_create(3);
    
    for(int i =0;i<6; i++)
    {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0), ^{
               dispatch_semaphore_wait( semaphores, DISPATCH_TIME_FOREVER);
               NSLog(@" car %d input the plsition",i);
               sleep(2);
               dispatch_semaphore_signal(semaphores);
               NSLog(@" car %d leave the plsition",i);
               
           });
    }
    
    
}

2020-12-16 16:17:43.889732+0800 sempher[76427:2940388]  car 0 input the plsition
2020-12-16 16:17:43.889811+0800 sempher[76427:2940389]  car 1 input the plsition
2020-12-16 16:17:43.890044+0800 sempher[76427:2940390]  car 2 input the plsition
2020-12-16 16:17:45.894960+0800 sempher[76427:2940389]  car 1 leave the plsition
2020-12-16 16:17:45.895105+0800 sempher[76427:2940390]  car 2 leave the plsition
2020-12-16 16:17:45.895478+0800 sempher[76427:2940391]  car 4 input the plsition
2020-12-16 16:17:45.895506+0800 sempher[76427:2940387]  car 3 input the plsition
2020-12-16 16:17:45.894954+0800 sempher[76427:2940388]  car 0 leave the plsition
2020-12-16 16:17:45.895907+0800 sempher[76427:2940399]  car 5 input the plsition
2020-12-16 16:17:47.900862+0800 sempher[76427:2940391]  car 4 leave the plsition
2020-12-16 16:17:47.900900+0800 sempher[76427:2940399]  car 5 leave the plsition
2020-12-16 16:17:47.901183+0800 sempher[76427:2940387]  car 3 leave the plsition