僵局 - 转让计划 [英] Deadlock - Transfer Program

问题描述

我想知道为什么这个程序可以有死锁

I want to know why this program can have a deadlock

void transfer(int from, into to, double amount) {
     sem_t *sem_from, *sem_to;
     sem_from=get_sem(from); //function that obtains the semaphore from bank account argument
     sem_to=get_sem(to);
     sem_wait(sem_from);
     sem_wait(sem_to);
     withdraw(from, amount); 
     deposit(to, amount); 
     sem_post(sem_to);
     sem_post(sem_from);
}

感谢。

推荐答案

说，有同时运行功能的两个实例。第一个是从一个账户转移一定量的帐户B，而第二个是从帐户B，转一些其他的量如果碰巧一审获得A的锁，并在同一时间二审获得占A. B的锁，死锁的发生。

Say that there are two instances of the function running concurrently. The first one is transferring some amount from account A to account B, while the second is transferring some other amount from account B to account A. If it happens that the first instance acquires the lock of A and in the same time the second instance acquires the lock of B, a deadlock happens.

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