GCD - Dispatch Semaphore

 

先看代码

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    NSMutableArray *array = [[NSMutableArray alloc]init];
    
        for (int i = 0; i != 10000; ++i)
        {
            dispatch_async(queue, ^{
               [array addObject:[NSNumber numberWithInt:i]];
            });
        }

 这段代码90%会崩溃，原因是多线程导致内存错误。

看下面的代码

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    NSMutableArray *array = [[NSMutableArray alloc]init];
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);//信号量为1
    
        for (int i = 0; i != 10000; ++i)
        {
            dispatch_async(queue, ^{
                dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);//等待返回值为1或者大于1，获得执行权利后信号量－1
               [array addObject:[NSNumber numberWithInt:i]];
                dispatch_semaphore_signal(semaphore);//信号量+1
            });
        }

 百分之百不崩溃，semaphore的值表示由多少线程可以操作，根据我们的设置，永远只有一个线程可以操作数组。