OpenMP reduction after parallel region declared outside function

Question

Apologies if this has already been asked, I can't find an answer to my specific question easily.

I have code that I am parallelising. I want to declare a parallel region outside a function call, but inside the function I need to do some reduction operations.

The basic form of the code is:

#pragma omp parallel
{
    for(j=0;j<time_limit;j++)
    {
        //do some parallel loops
        do_stuff(arg1, arg2)
    }
}
...
... 
void do_stuff(int arg1, int arg2)
{
    int sum=0;
    #pragma omp for reduction(+:sum) //the sum must be shared between all threads
    for(int i=0; i<arg1;i++)
    {
        sum += something;
    }
  }

When I try to compile, the reduction clause throws an error because the variable sum is private for each thread (obviously since it is declared inside the parallel region).

Is there a way to do this reduction (or something with the same end result) without having to declare the parallel region inside the function do_stuff?

Answer 1

If you only want the reduction in the function you can use static storage. From 2.14.1.2 of the OpenMP 4.0.0 specification

Variables with static storage duration that are declared in called routines in the region are shared.

#include <stdio.h>

void do_stuff(int arg1, int arg2)
{
    static int sum = 0;
    #pragma omp for reduction(+:sum)
    for(int i=0; i<arg1;i++) sum += arg2;
    printf("sum %d\n", sum);
}

int main(void) {
    const int time_limit = 10;
    int x[time_limit]; for(int i=0; i<time_limit; i++) x[i] = i;
    #pragma omp parallel
    {
        for(int j=0;j<time_limit;j++) do_stuff(10,x[j]);
    }
}