Question

我正在研究线程和并发编程。从课堂上提供的内容中尝试了这个基本的例子：

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define NUM_THREADS 8

void *printGoGators(void *noArg)
{
  printf("Go Gators!\n");
  return NULL;
}

int main(int argc, char *argv[])
{
  pthread_t threads[NUM_THREADS];
  int rc, t;
  for (t = 0; t < NUM_THREADS; t++)
    {
      printf("Creating thread %d\n", t);
      rc = pthread_create(&threads[t],
              NULL,
              printGoGators,
              NULL);
      if(rc)
    {
      printf("ERROR %d", rc);
      exit(-1);
    }
    }
  pthread_exit(NULL);
}

此代码生成以下输出。：

Creating thread 0
Creating thread 1
Go Gators!
Creating thread 2
Go Gators!
Creating thread 3
Go Gators!
Creating thread 4
Go Gators!
Creating thread 5
Go Gators!
Creating thread 6
Go Gators!
Creating thread 7
Go Gators!
Go Gators!

为什么Go Gators!在所有线程的相应Creating thread #之后不能直接打印？请帮忙！

Answer 1

如果您的代码看起来像这样，那么输出将按您期望的顺序排列：

for (t = 0; t < NUM_THREADS; t++)
    {
      printf("Creating thread %d\n", t);
      printGoGators(NULL);
    }

因此，您假设线程将按照创建它们的顺序执行。但是，这种假设是不正确的 - 线程可以按任何顺序执行。

Answer 2

创建线程后，由操作系统决定执行哪些顺序线程。您可以使用互斥锁和条件来控制它，以锁定线程让另一个线程运行然后解锁它。

与接受的答案不同，此示例使用线程，而不仅仅是在循环中打印。

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define NUM_THREADS 8

pthread_mutex_t myMutex;                // Declere global mutex
pthread_cond_t myCondition;             // Declere global condition


void *printGoGators(void *arg)    {         

  printf("Go Gators! %i\n", *(int*)arg);
  delete (int*) arg;

   pthread_cond_signal(&myCondition);    // Signal that a condition is met

  return NULL;
}

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

  pthread_mutex_init(&myMutex, NULL);              // Initialize mutex
  pthread_cond_init (&myCondition, NULL);          // Initialize condition

  pthread_t threads[NUM_THREADS];
  int rc, t;
  for (t = 0; t < NUM_THREADS; t++)    {

        int* n = new int;
        *n = t;

      printf("Creating thread %d\n", t);
      rc = pthread_create(&threads[t],
              NULL,
              printGoGators,
              n);            

      if(rc)   {
      printf("ERROR %d", rc);
      exit(-1);
    }


  pthread_cond_wait(&myCondition, &myMutex);         // waite for condition

    }
  pthread_exit(NULL);
}

结果：

Creating thread 0
Go Gators! 0
Creating thread 1
Go Gators! 1
Creating thread 2
Go Gators! 2
Creating thread 3
Go Gators! 3
Creating thread 4
Go Gators! 4
Creating thread 5
Go Gators! 5
Creating thread 6
Go Gators! 6
Creating thread 7
Go Gators! 7

主线程创建循环：创建一个线程，然后等待条件。

新主题：打印一条消息，然后发出满足条件的信号。

这样就可以管理执行顺序。

无法理解这种基本的线程行为

2 个答案: