OpenMP Pi example outcome always changes rather than 3.1415

问题

I'm new to openMP and C, I tried the "Introduction to OpenMP - Tim Mattson (Intel)" Pi example but the outcome is not 3.14. I compare the code with teacher. They are same. But the result is different

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>

//OpenMP example program: hello;
static long num_steps = 100000;
#define NUM_THREADS 2
double step;

int main()
{
    int nnum,i,j=0;
    step= 1.0/(double)num_steps;
    double sum[NUM_THREADS];
    double x,pi,result=0.0;

    omp_set_num_threads(NUM_THREADS);

    #pragma omp parallel
    {
        int id=omp_get_thread_num();
        int num=omp_get_num_threads();
        if(id==0) nnum = num;

        for(i=id,sum[id]=0.0;i<num_steps;i=i+num)
        {       
            x=(i+0.5)*step;
            sum[id]=sum[id]+(4.0/(1.0+x*x));
        }
    }

    while(j<nnum)
    {
        printf(" is %2.4f \n",sum[j]);
        result=result+sum[j];
        j++;
    }

    pi = step*result;
    printf("the result is %f \n",pi);

    return 0;
}

回答1:

The code is wrong. The "i" variable is being shared by the threads and both increment it, so that effectively you're doing only 1/NUM_THREADS of the intended iterations.

There are three different way to fix it. The first is to write

#pragma omp parallel private(i)

This makes each thread use a separate copy of the variable. The second is to declare i inside the #pragma omp parallel, which has the same effect (see how id is already private in your code).

The third and more interesting is to change the for statement to

#pragma omp for 
    for(i=0;i<num_steps;i++)

This makes the OpenMP compiler look at the loop and say "ok, this is a loop with num_steps iterations". It will then produce code to split the 0..num_steps-1 range into one or more chunks and pass each of them to one of NUM_THREADS threads. One thread for example will process 0 to 49999, the other will process 50000 to 99999. It is important to notice that:

without #pragma omp for, the for loop specifies the indices for each separate thread, hence the iteration variable i must be private
with #pragma omp for, the for loop specifies the indices for the whole loop, and the iteration variable i does not have to be private because OpenMP will create a separate thread-private iteration variable on its own.

来源：https://stackoverflow.com/questions/31992781/openmp-pi-example-outcome-always-changes-rather-than-3-1415

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