C语言三个线程通信的问题（发生死锁）

Question

我正在创建一个程序，其中有 3 个链表，我正在尝试更新或删除这三个线程中这些链表的节点。但是死锁正在发生

插入和删除工作正常。这里的三个变量var1InUse,var2InUse,var3InUse分别表示3个链表是否被使用（不是所有线程都使用这三个）。正如您在代码中看到的那样，我根据 var1InUse、var2InUse 和 var3InUse 将线程置于等待状态。有时这工作正常但有时会发生死锁。我已经在 Internet 上搜索了解决方案，但可以找到。我是否正确使用了等待和信号方法？

    pthread variables declaration

    pthread_mutex_t myMutex = PTHREAD_MUTEX_INITIALIZER;

     pthread_cond_t t1cond = PTHREAD_COND_INITIALIZER;

     pthread_cond_t t2cond = PTHREAD_COND_INITIALIZER;  

     pthread_cond_t t3cond = PTHREAD_COND_INITIALIZER;

    int var1InUse=0,var2InUse=0,var3InUse=0;

线程 1

    void* thread1(void* args){
        while(var1InUse || var2InUse ) {
                pthread_cond_wait(&t1cond,&myMutex);}
        var1InUse=1,var2InUse=1;
        while(1){

                pthread_mutex_lock(&myMutex);
                /*
                some other code about adding and removing from                         
                       linkedlist
                */
        var1InUse=0,var2InUse=0;
        pthread_cond_signal(&t2cond);
        pthread_cond_signal(&t3cond);
        pthread_mutex_unlock(&myMutex);}
     }

线程 2

    void* thread2(void* args){
        while(var1InUse || var2InUse || var3InUse) {
                pthread_cond_wait(&t2cond,&myMutex);}
        var1InUse=1,var2InUse=1,var3InUse=1;
        while(1){

                pthread_mutex_lock(&myMutex);
                /*
                some other code adding and removing from linkedlist
                */
                var1InUse=0,var2InUse=0,var3InUse=0;
                pthread_cond_signal(&t1cond);
                pthread_cond_signal(&t3cond);
                pthread_mutex_unlock(&myMutex);}
                    }

线程 3

     void* thread3(void* args){
        while(var1InUse || var3InUse ) {
                pthread_cond_wait(&t3cond,&myMutex);}
        var1InUse=1,var3InUse=1;
        while(1){

                pthread_mutex_lock(&myMutex);
                /*
                some other code adding and removing from linkedlist
                */
        var1InUse=0,var3InUse=0;
        pthread_cond_signal(&t1cond);
        pthread_cond_signal(&t2cond);
        pthread_mutex_unlock(&myMutex);}
            }

主要方法

    int main(){
        pthread_t t1,t2,t3,t4;
        pthread_mutex_init(&myMutex,0);
        pthread_create(&t1,NULL,thread1,NULL);
        pthread_create(&t2,NULL,thread2,NULL);
        pthread_create(&t3,NULL,thread3,NULL);
        pthread_join(t1,NULL); 
        pthread_join(t2,NULL);
            pthread_join(t3,NULL);
        pthread_mutex_destroy(&myMutex);
        return 0
            }

我希望解除死锁。

Answer 1

pthread_cond_wait() 使用的互斥量需要在函数调用前锁定。这是手册页的摘录：

The pthread_cond_timedwait() and pthread_cond_wait() functions shall block on a condition variable. The application shall ensure that these functions are called with mutex locked by the calling thread; otherwise, an error (for PTHREAD_MUTEX_ERRORCHECK and robust mutexes) or undefined behavior (for other mutexes) results.

虽然pthread_cond_wait()在内部解锁互斥锁，但在函数returns成功之前又被锁定了：

Upon successful return, the mutex shall have been locked and shall be owned by the calling thread.

此外，您应该在锁定互斥锁的情况下访问共享变量 var1InUse、var2InUse 和 var3InUse。

这是您的 thread1() 的修改版本，它遵循这些规则。对其他线程启动例程的修改应该类似：

void* thread1(void* args){

    pthread_mutex_lock(&myMutex);

    while(var1InUse || var2InUse ) {
         pthread_cond_wait(&t1cond,&myMutex);
    }
    var1InUse=1,var2InUse=1;

    pthread_mutex_unlock(&myMutex);

    while(1){

        pthread_mutex_lock(&myMutex);
        /*
        some other code about adding and removing from linkedlist
        */
        var1InUse=0,var2InUse=0;
        pthread_cond_signal(&t2cond);
        pthread_cond_signal(&t3cond);
        pthread_mutex_unlock(&myMutex);
    }
    return NULL;
}

（我不完全确定上面的内容是否正确，因为从原始代码中还不完全清楚 while(1) 循环应该做什么。）

C语言三个线程通信的问题（发生死锁）

Problem in communication of three threads in C language (deadlock occurs)

c

multithreading

posix

deadlock

pthreads