Linux上多线程通过蒙特卡洛法来求取pi值

Linux下多线程通过蒙特卡洛法来求取pi值

特卡洛法又称随机抽样技术

用该方法计算π的基本思路是： 

根据圆面积的公式： s=πR2 ,当R=1时，S=π。 

由于圆的方程是：x2+y2=1(X2为X的平方的意思),因此1/4圆面积为X轴、y轴和上述方程所包围的部分。 

如果在1*1的矩形中均匀地落入随机点，则落入1/4园中的点的概率就是1/4圆的面积。其4倍，就是圆面积。

由于半径为1，该面积的值为π的值。

#include <pthread.h>#include <sys/time.h>#include <stdlib.h>#include <stdio.h>#define MaxThreadNum 32#define kSamplePoints 1000#define kSpace 1void *compute_pi(void *);inline double WallTime();int total_hits, hits[MaxThreadNum][kSpace];int sample_points_per_thread, num_threads;int main(void){    int i;    double time_start, time_end;    pthread_t p_threads[MaxThreadNum];    pthread_attr_t attr;    pthread_attr_init(&attr);    pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);    printf("Enter num_threads\n");    scanf("%d", &num_threads);    time_start = WallTime();    total_hits = 0;    sample_points_per_thread = kSamplePoints / num_threads;    for(i=0; i<num_threads; i++)    {        hits[i][0] = i;        pthread_create(&p_threads[i], &attr, compute_pi, (void *)&hits[i]);    }    for(i=0; i<num_threads; i++)    {        pthread_join(p_threads[i], NULL);        total_hits += hits[i][0];    }    double pi = 4.0 * (double)total_hits / kSamplePoints;    time_end = WallTime();    printf("Elasped time: %lf, Pi: %lf\n", time_end - time_start, pi);    return 0;}void *compute_pi(void * s){    unsigned int seed;    int i;    int *hit_pointer;    double rand_no_x, rand_no_y;    hit_pointer = (int *)s;    seed = *hit_pointer;    local_hits = 0;    for(i=0; i<sample_points_per_thread; i++)    {        rand_no_x = (double)(rand_r(&seed))/(double)(RAND_MAX);        rand_no_y = (double)(rand_r(&seed))/(double)(RAND_MAX);        if((rand_no_x - 0.5)*(rand_no_x - 0.5) + (rand_no_y - 0.5) * (rand_no_y - 0.5) < 0.25)        {            (*hit_pointer)++;        }        seed *= i;    }    pthread_exit(0);}inline double WallTime(){    struct timeval tv;    struct timezone tz;    gettimeofday(&tv, &tz);    double currTime = (double)tv.tv_sec + (double)tv.tv_usec/1000000.0;    return currTime;}