[学习]多线程锁

信号量使用

在多线程条件下，完成对🥧的计算。实验提供有单线程版本，要求进行修改，并在代码空白处实现多线程计算。学习使用信号量，防止在多线程计算最终结果时出错。

一、原理

利用级数的思想，将计算过程分为多个线程，每个线程计算🥧的一部分，最后将结果相加。 求🥧的问题是一个累加问题，不同线程计算自己的项数和，然后输出汇总到一个解上，这里限定为一块共享内存。 因为不同的线程运行速度有快有慢，如果碰上两个线程对同一个共享内存进行加法操作，就会出现计算错误，为此需要引入信号量sem_t。

sem_t的基本使用方法如下。

#include <semaphore>	# 引入这个库

sem_t sem				# 新建一个信号量
sem_init(&sem ,0 ,1)	# 在main中调用将信号量初始化为1
sem_wait(&sem)			# 在进入临界区之前，等待锁释放
sem_post(&sem)			# 临界区代码运行完毕后释放掉信号量
sem_destory(&sem)		# 在主线程运行结束后，调用释放掉信号量

二、源代码

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <pthread.h>
#include <semaphore.h>
#include <sys/time.h>

#define GET_TIME(now)                         \
   {                                          \
      struct timeval t;                       \
      gettimeofday(&t, NULL);                 \
      now = t.tv_sec + t.tv_usec / 1000000.0; \
   }

const int MAX_THREADS = 1024;

long thread_count;
long long n;
double sum;

sem_t sem;

void *Thread_sum(void *rank);

/* Only executed by main thread */
void Get_args(int argc, char *argv[]);
void Usage(char *prog_name);
double Serial_pi(long long n);

int main(int argc, char *argv[])
{
   long thread; /* Use long in case of a 64-bit system */
   pthread_t *thread_handles;
   double start, finish, elapsed;

   /* please choose terms 'n', and the threads 'thread_count' here. */
   n = 10000000000;		// 控制计算项数
   thread_count = 4;	// 线程数

   /* You can also get number of threads from command line */
   //Get_args(argc, argv);

   thread_handles = (pthread_t *)malloc(thread_count * sizeof(pthread_t));
   sem_init(&sem, 0, 1);
   sum = 0.0;

   GET_TIME(start);
   for (thread = 0; thread < thread_count; thread++)
      pthread_create(&thread_handles[thread], NULL,
                     Thread_sum, (void *)thread);

   for (thread = 0; thread < thread_count; thread++)
      pthread_join(thread_handles[thread], NULL);
   GET_TIME(finish);
   elapsed = finish - start;

   sum = 4.0 * sum;
   printf("With n = %lld terms,\n", n);
   printf("   Our estimate of pi = %.15f\n", sum);
   printf("The elapsed time is %e seconds\n", elapsed);
   GET_TIME(start);
   sum = Serial_pi(n);
   GET_TIME(finish);
   elapsed = finish - start;
   printf("   Single thread est  = %.15f\n", sum);
   printf("The elapsed time is %e seconds\n", elapsed);
   printf("                   pi = %.15f\n", 4.0 * atan(1.0));

   sem_destroy(&sem);
   free(thread_handles);
   return 0;
} /* main */

/*------------------------------------------------------------------*/
void *Thread_sum(void *rank)
{
   long my_rank = (long long)rank;
   double my_sum = 0.0;

   /*******************************************************************/
   double factor = 1;
   long long max_size = 0;
   if (my_rank == 3)	// 平均分配计算量，最后一个线程计算剩余部分。
   {
      max_size = n;
   }
   else
   {
      max_size = my_rank * n / 4 + n / 4;
   }
   for (long long i = my_rank * n / 4; i < max_size; i++, factor = -factor)
   {
      my_sum += factor / (2 * (i + my_rank) + 1);
   }
   //printf("%.15lf\n", my_sum);

   sem_wait(&sem);	// 等待锁
   sum += my_sum;	// 临界状态
   sem_post(&sem);	// 释放锁

   /******************************************************************/
   return NULL;
} /* Thread_sum */

/*------------------------------------------------------------------
 * Function:   Serial_pi
 * Purpose:    Estimate pi using 1 thread
 * In arg:     n
 * Return val: Estimate of pi using n terms of Maclaurin series
 */
double Serial_pi(long long n)
{
   double sum = 0.0;
   long long i;
   double factor = 1.0;

   for (i = 0; i < n; i++, factor = -factor)
   {
      sum += factor / (2 * i + 1);
   }
   return 4.0 * sum;

} /* Serial_pi */

/*------------------------------------------------------------------
 * Function:    Get_args
 * Purpose:     Get the command line args
 * In args:     argc, argv
 * Globals out: thread_count, n
 */
void Get_args(int argc, char *argv[])
{
   if (argc != 3)
      Usage(argv[0]);
   thread_count = strtol(argv[1], NULL, 10);
   if (thread_count <= 0 || thread_count > MAX_THREADS)
      Usage(argv[0]);
   n = strtoll(argv[2], NULL, 10);
   if (n <= 0)
      Usage(argv[0]);
} /* Get_args */

/*------------------------------------------------------------------
 * Function:  Usage
 * Purpose:   Print a message explaining how to run the program
 * In arg:    prog_name
 */
void Usage(char *prog_name)
{
   fprintf(stderr, "usage: %s <number of threads> <n>\n", prog_name);
   fprintf(stderr, "   n is the number of terms and should be >= 1\n");
   fprintf(stderr, "   n should be evenly divisible by the number of threads\n");
   exit(0);
} /* Usage */