使用pthreads计算列表中的数字。C

Question

我在使用pthreads计算列表中3的数量时遇到麻烦。 使用我的代码的串行版本可以正常工作，但是尝试使用pthread_create给我带来麻烦。 当前的问题是count3s_thread_2（int id）没有为我提供与串行版本相同的值。 我需要更改什么？ PS，对不起，一团糟。 我是C语言编程的新手。

// Declares some global variables we will use throughout the
// program with all versions.
#define NUM_THREADS      4
int Length = 1000;
int array[1000];
int count;
long i;
pthread_mutex_t m;
pthread_t threads[NUM_THREADS];

void create_list(int *array)
{
   srand(time(NULL));
   for (i = 0; i < Length; i++)
   {
      int r = rand();
      r = (r % 10) + 1;
      array[i] = r;
   }
}

void* count3s(void* threadid)
{
   // This is the function that counts the number of threes for
   // the first threaded version.
   //int i = (intptr_t)threadid;
   int i = (intptr_t)threadid;
   long tid = (long)threadid;
   int length_per_thread = Length / NUM_THREADS;
   long start = tid * (long)length_per_thread;

   for (i = start; i < start + length_per_thread; i++)
   {
      if (array[i] == 3)
      {
         count++;
      }
   }
   pthread_exit(NULL);
}

void* count3s_v2(void* threadid)
{
   // This is the function that counts the number of threes for
   // the second threaded version.
   //int serial = count3s_serial();
   //printf("Number of threes: %d\n", serial);
   int i = (intptr_t)threadid;
   long tid = (long)threadid;
   int length_per_thread = Length / NUM_THREADS;
   long start = tid * (long)length_per_thread;

   for (i = start; i < start + length_per_thread; i++)
   {
      if (array[i] == 3)
      {
         pthread_mutex_lock(&m);
         count++;
         pthread_mutex_unlock(&m);
      }
   }
   pthread_exit(NULL);
}

int count3s_serial()
{
   // This is the serial version of count3s. No threads are
   // created and run separately from other threads.
       count = 0;

   for (i = 0; i < Length; i++)
   {
      if (array[i] == 3)
      {
         count++;
      }
   }

   return count;
}

int count3s_thread(int id)
{
   clock_t begin, end;
   double time_spent;

   begin = clock();
   //pthread_attr_init();
   for (i = 0; i < NUM_THREADS; i++)
   {
      pthread_create(&threads[i], NULL, count3s, (void *)i);
   }
   //pthread_attr_destroy();
   end = clock();
   time_spent = (double)(end - begin) / CLOCKS_PER_SEC;

   return count;
}

int count3s_thread_2(int id)
{
   clock_t begin, end;
   double time_spent;
   begin = clock();
   pthread_attr_init(&something);
   for (i = 0; i < NUM_THREADS; i++)
   {
      pthread_create(&threads[i], NULL, count3s_v2, (void *)i);
   }
   pthread_attr_destroy(&something);
   end = clock();
   time_spent = (double)(end - begin) / CLOCKS_PER_SEC;

   return count;
   //printf("Thread Version 2: Number of threes = %d\nThread Version 2: Time Spent = %f\n", count, time_spent);
}

int main()
{
   create_list(array);
   clock_t begin, end;
   double time_spent;

   for (i = 0; i < Length; i++)
   {
      printf("%d\n", array[i]);
   }

   // Beginning of serial version. Timer begins, serial version
   // is ran and after it's done, the timer stops.
   begin = clock();
   int serial = count3s_serial();
   end = clock();

   time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
   printf("Serial Version: Number of threes = %d\nSerial Version: Time Spent = %f\n", serial, time_spent);
   // End of serial version.
/*
*********************************************************************
*/
   // Beginning of first theaded version. Timer begins, first
   // threaded version is ran and after it's done, the timer stops.
   int the_thing = 0;
   count = 0;
   the_thing = count3s_thread(i);

   printf("Thread Version 1: Number of threes = %d\nThread Version 1: Time Spent = %f\n", the_thing, time_spent);
   // End of first threaded version.
/*
*********************************************************************
*/
   // Beginning of second theaded version. Timer begins, second
   // threaded version is ran and after it's done, the timer stops.
   int the_other_thing = 0;
   count = 0;
   the_other_thing = count3s_thread_2(i);

   printf("Thread Version 2: Number of threes = %d\nThread Version 2: Time Spent = %f\n", the_other_thing, time_spent);

   pthread_exit(NULL);
}

Answer 1

问题是您生成了线程，但是在打印结果之前不要等待它们完成。 两个线程版本都有相同的问题。 使用pthread_join等待线程退出或实现一些其他同步，以使父级知道线程何时完成工作。

例如，下面的代码块添加到两者的端count3s_thread和count3s_thread_2 。 在打印结果之前，它将等待线程完成。 注意：您必须将其添加到两个功能（即使您可以将第一个计数错误也可以）。 否则，当您运行第二个线程版本时，第一组线程可能仍在执行，并且会弄乱全局count 。

for (i = 0; i < NUM_THREADS; i++) {
    pthread_join(threads[i], NULL);
}

Answer 2

好的，仔细研究一下您的代码并进行编译。 在宣布结果之前，您不必等待线程返回。 返回之前，您需要在count3s_thread() pthread_join()每个线程。