C - 线程在等待时无限期卡住答案

【问题标题】：C - Thread stuck indefinitely while waitingC - 线程在等待时无限期卡住
【发布时间】：2021-10-13 14:13:34
【问题描述】：

我在实施“餐饮哲学家”问题时遇到了麻烦。基本上我的程序在线程等待时陷入无限循环。我正在尝试以某种方式实现它，以便它强制执行进食顺序。所以哲学家 0 会先吃饭，然后是哲学家 1，依此类推。仅当我尝试强制执行进食顺序时才会出现问题，而当我让进食顺序随机时不会出现问题。

这是我的实现，重点是吃的顺序（陷入无限循环的那个）：

void *philosopherThread(void *threadIndex){
    int philID = (int)threadIndex;
    int nextIndex = 0;
    printf("This is philosopher %d\n", philID);

    // waiting status
    thinking(philID);
    pthread_mutex_lock(&lock);

    // when the ith philosopher thread completes "thinking," the thread should check whether it is the ith philosopher's turn to eat or not prior to obtaining the chopsticks
    // If nextIndex==philID, the ith philosopher will grab both chopsticks and begin eating
    if(philID ==nextIndex){
        pickUpChopsticks(philID);
        eating(philID);
        putDownChopsticks(philID);
    }
    // waits on conditional variable
    while(nextIndex != philID){
        pthread_cond_wait(&var, &lock);
    }

    pickUpChopsticks(philID);
    eating(philID);
    putDownChopsticks(philID);

    // after finished eating it will  increment the value of nextIndex and release the lock to wake up all threads that are waiting on the conditional variable
    nextIndex++;
    pthread_cond_broadcast(&var);
    return(NULL);
}

作为参考，这是我的实现，其中不强调进食顺序（这个可行，没有无限循环）

void *philosopherThread(void *threadIndex){
    int philID = (int)threadIndex;
    printf("This is philosopher %d\n", philID);

    // waiting status
    thinking();

    // acquire locks
    pickUpChopsticks(philID);
    eating();

    // release lock
    putDownChopsticks(philID);

    return(NULL);
}

所以我需要关于第一个代码的帮助，以便它不再陷入无限循环。任何帮助都将不胜感激。

【问题讨论】：

mutex_lock 和 cond_wait 之间的“距离”令人担忧。中间是什么？？？此外，您在互斥锁锁定的情况下退出线程。
最下面的可能会死锁。它绝对不会“工作”。

标签： c multithreading pthreads thread-synchronization

【解决方案1】：

三个问题：

每个线程都有自己的nextIndex。应该只有一个。
线程退出时仍然持有互斥锁。
mutex_lock 和 cond_wait 之间的“距离”令人担忧。中间是什么？？？你在等你吃饭，所以不应该有任何其他的表演。

static int nextIndex = 0;

void *philosopherThread(void *threadIndex){
    int philID = (intptr_t)threadIndex;           // Use intptr_t on the outside too.
    printf("This is philosopher %d\n", philID);

    // We're thinking.
    thinking(philID);

    // Wait for our turn.
    pthread_mutex_lock(&lock);
    while(nextIndex != philID)
        pthread_cond_wait(&var, &lock);

    // We're eating.
    pickUpChopsticks(philID);
    eating(philID);
    putDownChopsticks(philID);

    // We're back to thinking.
    thinking(philID);

    // Make it someone else's turn, and have all of the
    // waiting philosophers check if its their turn to eat.
    nextIndex++;
    pthread_cond_broadcast(&var);
    pthread_mutex_unlock(&lock);

    return NULL;
}

【讨论】：