/* pthread version */
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

static int num_steps=1000000000;
double pi, step;
struct start_arg {
	int		i_start;
	int		i_end;
	double	sum;
} start_arg[2];

void *start_func(void *arg)
{
	int		i;
	double 	x, sum = 0.0;
	struct start_arg *p_arg = (struct start_arg *) arg;

	for (i=p_arg->i_start; i<p_arg->i_end; i++) {
		x = (i+0.5) * step;
		sum += 4.0/(1.0 + x*x);
	}
	p_arg->sum = sum;
	return NULL;
}

int main()
{
	int 	i;
	double  sum;
	pthread_t	pt_id[2];

	step = 1.0/(double) num_steps;
	start_arg[0].i_start = 0;
	start_arg[0].i_end   = num_steps>>1;
	start_arg[1].i_start = start_arg[0].i_end + 1;
	start_arg[1].i_end   = num_steps;

	printf("%d~%d, %d~%d\n", 
			start_arg[0].i_start, start_arg[0].i_end ,
			start_arg[1].i_start, start_arg[1].i_end );

	for (i=2; i--; ) { /* create pthread */
		if (pthread_create(&pt_id[i], NULL, start_func, &start_arg[i])){
			perror("pthread_create");
			return 1;
		}
	}
	for (i=2; i--; ) { /* join pthread : explicit barrier */
		if (pthread_join(pt_id[i], NULL)){
			perror("pthread_join");
			return 1;
		}
	}
	sum = start_arg[0].sum + start_arg[1].sum;
	printf("PI = %.8f (sum = %.8f)\n", step*sum, sum);
	return EXIT_SUCCESS;
}