关于 semaphore 信号量

2017-01-14 10:20:44来源:http://www.jianshu.com/p/4a2c4fc48ed2作者:sea777777人点击

dispatch_semaphore 信号量基于计数器的一种多线程同步机制。在多个线程访问共有资源时候,会因为多线程的特性而引发数据出错的问题。


和普通锁不一样,这个可以精细的控制并发量;


dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);
会创建三个信号量
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
每次wait都会消耗一个信号量,等待时间可以设置,这里设置为一直等待,如果当前信号量(semaphore)小于1,则一直等待
dispatch_semaphore_signal(semaphore);
发送信号量,发送一次,信号量+1


下面的demo可以并发执行 ,但每次只能并发3个线程;
因为消耗3次信号量后,就进入等待状态,直到sleep 2s完成后,会发送信号量dispatch_semaphore_signal
然后继续执行;


所以每次输出3个,直到循环完成;


dispatch_group_t group = dispatch_group_create();
dispatch_semaphore_t semaphore = dispatch_semaphore_create(3);
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
for (int i = 0; i < 90; i++)
{
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
dispatch_group_async(group, queue, ^{
NSLog(@"%i",i);
sleep(2);
dispatch_semaphore_signal(semaphore);
});
}

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