Pthreads：分段错误

Question

我有几个问题。这是我第一次真正尝试制作多线程程序。

注意 - 完整的程序位于页面底部

（编译，使用

g++ -pthread -o <executable file name> <sourcefile>.cpp -fpermissive

）

我使用 Ubuntu Studio 10.10 64 位编译它。

这个程序最大的问题是它给我带来了分段错误。

这似乎是由我在 int main() 中注释的行引起的。如果我注释掉该行，它不会给我一个分段错误错误。

为了方便起见，这里单独使用 int main() ：

int main()
{
    pthread_attr_t attr;
    pthread_t threads[30];

    /* Initialize mutex and condition variable objects */
    pthread_mutex_init(&direction_mutex, NULL); 
    pthread_mutex_init(&arrive_mutex,NULL);


    pthread_cond_init (&count_threshold, NULL); 
    pthread_cond_init(&arrive_done, NULL);

    /*
     For portability, explicitly create threads in a joinable state

     I'll take your word for it on that one.
     */
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

   for( int x = 0 ; x < 30 ; x++)
   {
      long random = rand();
      int direction;

       if (random < RAND_MAX/2)
       {
            direction = 0;
       }

       else
       {
            direction = 1;
       }

       directions[x] = direction;
       printf("%d",direction);
    }

    printf("\n");

    currdir = directions[0];

    for(int j = 0 ; j < 30 ; j++)
    {
        if(j != 0)
        {
            pthread_cond_wait(&arrive_done, NULL); // THIS line of code is what is causing the problem
        }



        pthread_create(&threads[j], &attr, OneCar, (void *)&Thread_IDs[j]);
    }

    /* Wait for all threads to complete */
    for (j = 0; j < 30; j++) 
    {
        pthread_join(threads[j], NULL);
    }

    printf("test\n");

    /* Clean up and exit */
    pthread_attr_destroy(&attr);
    pthread_mutex_destroy(&direction_mutex);
    pthread_cond_destroy(&count_threshold);
    pthread_exit (NULL);

}

如果没有该行，程序将运行，但问题是，它的线程顺序似乎相当随机。

我试图使用该互斥锁来阻止 int main() 启动新线程，直到最后一个线程完成，因为该程序应该让线程按 FIFO 顺序运行。

如果没有此代码，它的行为会有所不同。

大多数时候，它从线程 0 开始，然后转到线程 3,4，有时甚至是 5，然后再返回线程 1。

有时，它从线程 3 开始，然后转到线程 4，然后线程 0...我可以不明白为什么要这样做。

每次线程执行的顺序都不同，但它永远不会像需要的那样是 0,1,2,3,4

这是注释掉有问题的行的输出：

*** Output of program with "pthread_cond_wait(&arrive_done, NULL);" commented out:


101110010101011111010001000010
ArriveBridge(): Car 1 goes accross the bridge
ExitBridge(): car 1 has left the bridge
Arrivebridge(): Thead 0 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 0 goes accross the bridge
ExitBridge(): car 0 has left the bridge
Arrivebridge(): Thead 5 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 5 goes accross the bridge
ExitBridge(): car 5 has left the bridge
Arrivebridge(): Thead 3 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 3 goes accross the bridge
ExitBridge(): car 3 has left the bridge
ArriveBridge(): Car 2 goes accross the bridge
ExitBridge(): car 2 has left the bridge
ArriveBridge(): Car 4 goes accross the bridge
ExitBridge(): car 4 has left the bridge
Arrivebridge(): Thead 6 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 6 goes accross the bridge
ExitBridge(): car 6 has left the bridge
Arrivebridge(): Thead 7 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 7 goes accross the bridge
ExitBridge(): car 7 has left the bridge
Arrivebridge(): Thead 8 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 8 goes accross the bridge
ExitBridge(): car 8 has left the bridge
Arrivebridge(): Thead 9 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 9 goes accross the bridge
ExitBridge(): car 9 has left the bridge
Arrivebridge(): Thead 10 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 10 goes accross the bridge
ExitBridge(): car 10 has left the bridge
Arrivebridge(): Thead 11 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 11 goes accross the bridge
ExitBridge(): car 11 has left the bridge
ArriveBridge(): Car 13 goes accross the bridge
ExitBridge(): car 13 has left the bridge
Arrivebridge(): Thead 12 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 12 goes accross the bridge
ExitBridge(): car 12 has left the bridge
Arrivebridge(): Thead 14 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 14 goes accross the bridge
ExitBridge(): car 14 has left the bridge
ArriveBridge(): Car 15 goes accross the bridge
ExitBridge(): car 15 has left the bridge
ArriveBridge(): Car 16 goes accross the bridge
ExitBridge(): car 16 has left the bridge
Arrivebridge(): Thead 18 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 18 goes accross the bridge
ExitBridge(): car 18 has left the bridge
Arrivebridge(): Thead 17 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 17 goes accross the bridge
ExitBridge(): car 17 has left the bridge
ArriveBridge(): Car 19 goes accross the bridge
ExitBridge(): car 19 has left the bridge
Arrivebridge(): Thead 21 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 21 goes accross the bridge
ExitBridge(): car 21 has left the bridge
ArriveBridge(): Car 20 goes accross the bridge
ExitBridge(): car 20 has left the bridge
ArriveBridge(): Car 22 goes accross the bridge
ExitBridge(): car 22 has left the bridge
Arrivebridge(): Thead 23 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 23 goes accross the bridge
ExitBridge(): car 23 has left the bridge
Arrivebridge(): Thead 24 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 24 goes accross the bridge
ExitBridge(): car 24 has left the bridge
ArriveBridge(): Car 25 goes accross the bridge
ExitBridge(): car 25 has left the bridge
ArriveBridge(): Car 26 goes accross the bridge
ExitBridge(): car 26 has left the bridge
ArriveBridge(): Car 27 goes accross the bridge
ExitBridge(): car 27 has left the bridge
ArriveBridge(): Car 29 goes accross the bridge
ExitBridge(): car 29 has left the bridge
Arrivebridge(): Thead 28 is trying to go in the opposite direction, it must wait for traffic to clear
ArriveBridge(): Car 28 goes accross the bridge
ExitBridge(): car 28 has left the bridge
test

这是没有注释掉该行的输出

****output of program before commenting pthread_cond_wait(&arrive_done, NULL); out:


101110010101011111010001000010
Segmentation fault

正如您所看到的，在创建任何线程之前，它几乎立即失败。

我试图改进的另一件事是我的零和一序列不是很随机。有没有更好的方法来生成随机数？它不必非常随机，但这个序列每次都完全相同。

感谢您抽出时间

Answer 1

您实际上需要将互斥体传递给pthread_cond_wait，您正在传递NULL。

对于随机数据（至少在 Linux 上），从 /dev/random 或 /dev/urandom 读取。您还可以尝试： direction = (rand() >> 8) & 1

Answer 2

你的主循环应该是：

pthread_mutex_lock(&arrive_mutex);
for(int j = 0 ; j < 30 ; j++)
{
    if(j != 0)
    {
        // Pass a locked mutex as the second parameter.
        pthread_cond_wait(&arrive_done, &arrive_mutex);
        // This releases the lock and suspends the thread.
        // When the condition variable is signaled. It re-establishes the lock
        // then releases the thread. If another processes is holding the lock
        // the released thread is stalled until it can acquire the lock.
        //
        // This means the child thread should acquire the lock on the mutex.
        // call signal and then release the lock. If the child does not
        // aquire the lock first then there is the potential for the child to
        // reach the signal before the parent waits() if this happens then the
        // parent will suspend forever (as nobody else will signal).

    }

    pthread_create(&threads[j], &attr, OneCar, (void *)&Thread_IDs[j]);
}
// Unlock the mutex afterwords.
pthread_mutex_unlock(&arrive_mutex);

Answer 3

我认为您需要将互斥体而不是 NULL 传递给 pthread_cond_wait 。 手册页指出：

int pthread_cond_wait(pthread_cond_t *限制 cond,
pthread_mutex_t *限制互斥体）；

这些函数以原子方式释放互斥体并导致调用线程在条件变量cond上阻塞；原子地在这里意味着“原子地相对于另一个线程访问互斥体然后访问条件变量”。

它试图用代码原样释放一个空互斥体。