OpenMP gathering data (join data?) after parallel for - c

Question

What I am looking for is what is the best way to gather all the data from the parallel for loops into one variable. OpenMP seems to have a different routine then I am used to seeing as I started learning OpenMPI first which has scatter and gather routines.
Calculating PI (embarrassingly parallel routine)
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_STEPS 100
#define CHUNKSIZE 20
int main(int argc, char *argv[])
{
double step, x, pi, sum=0.0;
int i, chunk;
chunk = CHUNKSIZE;
step = 1.0/(double)NUM_STEPS;
#pragma omp parallel shared(chunk) private(i,x,sum,step)
{
#pragma omp for schedule(dynamic,chunk)
for(i = 0; i < NUM_STEPS; i++)
{
x = (i+0.5)*step;
sum = sum + 4.0/(1.0+x*x);
printf("Thread %d: i = %i sum = %f \n",tid,i,sum);
}
pi = step * sum;
}
EDIT: It seems that I could use an array sum[*NUM_STEPS / CHUNKSIZE*] and sum the array into one value, or would it be better to use some sort of blocking routine to sum the product of each iteration

Answer 1

Add this clause to your #pragma omp parallel ... statement:
reduction(+ : pi)
Then just do pi += step * sum; at the end of the parallel region. (Notice the plus!) OpenMP will then automagically sum up the partial sums for you.

Answer 2

Lets see, I am not quite sure what happens, because I havn't got deterministic behaviour on the finished application, but I have something looks like it resembles π. I removed the #pragma omp parallel shared(chunk) and changed the #pragma omp for schedule(dynamic,chunk) to #pragma omp parallel for schedule(dynamic) reduction(+:sum).
#pragma omp parallel for schedule(dynamic) reduction(+:sum)
This requires some explanation, I removed the schedules chunk just to make it all simpler (for me). The part that you are interested in is the reduction(+:sum) which is a normal reduce opeartion with the operator + and using the variable sum.
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_STEPS 100
int main(int argc, char *argv[])
{
double step, x, pi, sum=0.0;
int i;
step = 1.0/(double)NUM_STEPS;
#pragma omp parallel for schedule(dynamic) reduction(+:sum)
for(i = 0; i < NUM_STEPS; i++)
{
x = (i+0.5)*step;
sum +=4.0/(1.0+x*x);
printf("Thread %%d: i = %i sum = %f \n",i,sum);
}
pi = step * sum;
printf("pi=%lf\n", pi);
}