Linux下多線程通過蒙特卡洛法來求取pi值

特卡洛法又稱隨機抽樣技術

是一種應用隨機數進行仿真試驗的方法。

用該方法計算π的基本思路是：

根據圓面積的公式： s=πR2 ,當R=1時，S=π。

由於圓的方程是：x2+y2=1(X2為X的平方的意思),因此1/4圓面積為X軸、y軸和上述方程所包圍的部分。

如果在1*1的矩形中均勻地落入隨機點，則落入1/4園中的點的概率就是1/4圓的面積。其4倍，就是圓面積。

由於半徑為1，該面積的值為π的值。

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

#define MaxThreadNum 32
#define kSamplePoints 1000
#define kSpace 1

void *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;
}