c - 多线程 - 只有在所有线程都完成任务后才继续

Question

我正在尝试使用多线程实现多线程(用户可以在启动程序时输入工作线程数 = 线程数)每个线程调用 functionA 和之后的 functionB。但是在 functionB 之前应该只在所有线程之后执行调用了函数A。那是我的伪代码:

void* worker_do(void* worker_id)
{
  functionA((size_t) worker_id);
  // First thread should only start functionB after ALL threads
  // are finished with functionA
  functionB((size_t) worker_id);
  return NULL;
}

// I am not allowed to change pthread_create and pthread_join here
int main()
{
  // should be a flexible value
  ssize_t num_workers = 20;
  pthread_t* workers  = malloc(num_workers*sizeof(pthread_t));

  for(ssize_t i = 0; i < num_workers; i++)
    pthread_create(&workers[i], NULL, worker_do, (void*) i);

  for(ssize_t i = 0; i < num_workers; i++)
    pthread_join(workers[i], NULL);

  free(workers);

  return 0;
}

我用谷歌搜索并找到了“条件变量”的可能性。但我不确定表明他们必须为条件实现

IF last_thread_has_called_functionA THEN start_calling_fuctionB

或者条件变量不是解决这个问题的正确工具？

非常感谢提示我如何实现它...

罗伯特

Answer 1

我假设 functionA() 和 functionB() 可以由线程并行执行，因为您当前的代码中没有互斥锁保护。

为了解决您的问题，您可以使用简单的轮询机制。 functionA() 执行后，每个线程都会递增一个计数器。所有线程都将等待，直到计数器等于创建的线程数。

对于这种方法，您需要有一个在所有线程中通用的互斥锁和计数器。为了简化代码，我使用了一个全局变量。

unsigned int num_threads = 0;
unsigned int num_threads_completed_functionA = 0;
pthread_mutex_t lock;

void* worker_do(void* worker_id)
{
  functionA((size_t) worker_id);
  // First thread should only start functionB after ALL threads are finished with functionA

  //Lock the mutex and update the counter
  pthread_mutex_lock(&lock);
  num_threads_completed_functionA++;
  pthread_mutex_unlock(&lock);

  while(1)
  {
    //Lock mutex and check how many threads completed execution of functionA()
    pthread_mutex_lock(&lock);
    if(num_threads_completed_functionA == num_threads)
    {
       //If all threads completed, then break the loop and proceed executing functionB()
       pthread_mutex_unlock(&lock);
       break;
    }
    pthread_mutex_unlock(&lock);
    usleep(1); //Sleep for some time
  }

  //ALL threads are finished with functionA
  functionB((size_t) worker_id);
  return NULL;
}