c - 线程同时运行

Question

所以我想要 3 个线程，它们都递增一个全局整数。我认为，当创建一个线程时，它与主线程中的一个分支类似，将继续与新创建的线程同时执行代码。但这并不是实际发生的情况，因为直到第一个线程完成后才会创建第二个和第三个线程。

void * say_it( void * )
{
  int count = 0;

  while(BUFFER < 24)
    {

      //LOCK
      if (pthread_mutex_lock(&output_lock) != 0)
    {
      perror("Could not lock output: ");
      exit(-1);
    }

      //print message
      cout << "TID: " << pthread_self() << " PID: " << "WORK IN PROGRESS " << "Buffer: " << BUFFER << endl;
      BUFFER++;

      //UNLOCK
      if (pthread_mutex_unlock(&output_lock) != 0)
    {
      perror("Could not unlock output: ");
      exit(-1);
    }
      count++;
    }

  //print final message
  cout << "TID: " << pthread_self() << " worked on the buffer " << count << " times" << endl; 

}

int main(int argc, char *argv[])
{

  int num_threads = 3;
   pthread_t *thread_ids;
   void  *p_status;

   //Use unbuffered output on stdout
   setvbuf(stdout, (char *) NULL, _IONBF, 0);

   //Set up an output lock so that threads wait their turn to speak.
   if (pthread_mutex_init(&output_lock, NULL)!=0)
     {
       perror("Could not create mutex for output: ");
       return 1;
     }

   //Create 3 THREADS
   thread_ids = new pthread_t[num_threads];

   // generate threads 
   for (int i = 0; i < num_threads; i++)
     {

       int *arg = new int;
       *arg = i;
       if( pthread_create(&thread_ids[i],NULL,say_it,NULL) > 0)
       {
         perror("creating thread:");
         return 2;
       }

       if (pthread_join(thread_ids[i], &p_status) != 0)
       {
         perror("trouble joining thread: ");
         return 3;
       }
     }




   return 0;
}

我也曾尝试在程序的某些区域放置 sleep(1)，但没有成功。

谁能给我解释一下？谢谢!

Answer 1

当您调用 pthread_join 时，main 将阻塞直到该线程完成。在对线程调用 pthread_join 之前，您需要创建所有线程。

旁注:您在注释//join threads and print their return values 之后再次调用pthread_create。所有这些都需要删除。