问题如下:

• 同一时间, 只能有 1 个线程进行写的操作
• 同一时间, 允许多个线程进行读的操作
• 同一时间, 不允许既有写的操作, 又有读的操作
  有专门的锁来处理以上问题:
• pthread_rwlock
• dispatch_barrier_async

pthread_rwlock(等待锁)

pthread_rwlock_t lock;
- (void)_rwlock_read {
    pthread_rwlock_rdlock(&lock);
    NSLog(@"%s", __func__);
    pthread_rwlock_unlock(&lock);
}
- (void)_rwlock_write {
    pthread_rwlock_wrlock(&lock);
    sleep(1);
    NSLog(@"%s", __func__);
    pthread_rwlock_unlock(&lock);
}
- (void)_rwlock {
    pthread_rwlock_init(&lock, NULL);
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    for (int i = 0; i < 10; ++i) {
        dispatch_async(queue, ^{
            [self _interview08_read];
        });
        dispatch_async(queue, ^{
            [self _interview08_write];
        });
    }
    
}

dispatch_barrier_async

会创建一个线程围栏, 保证其内部的操作执行不会被别人抢了, 也就是说, 两个 read 的 log 会同时打印, 而两次 write 的 log 会隔一秒在进行打印

注意:要想使用这个围栏必须由 dispatch_queue_create 来创建这个队列

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
    for (int i = 0; i < 10; ++i) {
        dispatch_async(queue, ^{
            sleep(1);
            NSLog(@"%s", "read");
        });
        dispatch_async(queue, ^{
            sleep(1);
            NSLog(@"%s", "read");
        });
        dispatch_barrier_async(queue, ^{
            sleep(1);
            NSLog(@"%s", "write");
        });
        dispatch_barrier_async(queue, ^{
            sleep(1);
            NSLog(@"%s", "write");
        });
    }