在不同线程中运行的函数产生奇怪的输出

Question

Hello I am new to multithreading in C++. I am using a thread class available in C++11 in order to run a function in a different thread but somehow the output I am getting from the function is very awkward. This might be because different threads might be trying to execute the same variable at the same time and thus causing conflicts. Please suggest how should I modify my code so that I can get correct output. I am posting a sample code of what I am trying to do. This is not the original code but it just shows the flow of my original code since my original code is too long to be posted, But the problem remains same for both the cases.

    #include<iostream>
    #include<thread>    

    using namespace std;

    typedef struct {
         int thread_id;
         char *message;
    }threadData;

    int display(threadData *tData){
         threadData *my_data;
         my_data = (threadData *) tData;

         cout << "Thread ID: " << my_data -> thread_id << endl; 
         cout << "Message: " << my_data -> message << endl; 

         return 0;
    }

  int main(){

      threadData *data;

      data = (threadData *)malloc(sizeof(threadData));
      data->thread_id = 12;
      data->message = "This is the message"; 
      for (int i = 0; i<10; i++)
      {
          std::thread t1(display, data);    
          t1.detach();
      }
      return 0;
  }

Output:

    Thread ID: 12
    Message: This is the messageThread ID: 
    12
    Message: This is the message
    Thread ID: 12
    Message: This is the message
    Thread ID: 12
    Message: This is the message
    Thread ID: 12
    Message: This is the message

Answer 1

What I read is the for loop expected to run 10 times, but it only ran 4 times, the reason is because in the main function you didn't wait for all the thread done, so the main process exit before other threads having a chance to run. 'main' need to sleep for a while to wait for all the thread done their work.

And I didn't see a race condition here because all the thread just reading, no one writing to threadData.

Answer 2

As thread does not guaruntee which runs first, you need to protect access to the shared resource. The easiest way to do so, is through a mutex.

std::mutex g_i_mutex;  // protects g_i

typedef struct {
     int thread_id;
     string message;
}threadData;

int display(threadData *tData)
{
 std::lock_guard<std::mutex> lock(g_i_mutex);
 threadData *my_data;
 my_data = (threadData *) tData;

 cout << "Thread ID: " << my_data -> thread_id << endl; 
 cout << "Message: " << my_data -> message << endl; 

 return 0;

}

Output:

Thread ID: 12 Message: This is the message Thread ID: 12 Message: This is the message Thread ID: 12 Message: This is the message Thread ID: 12 Message: This is the message

I'd suggest you read a bit more about threading concepts. The concepts behind it are not simple and just getting a ready made solution will not help you in the long run.