OpenMP, for loop inside section

Question

I would like to run the following code (below). I want to spawn two independent threads, each one would run a parallel for loop. Unfortunately, I get an error. Apparently, parallel for cannot be spawned inside section. How to solve that?

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

int main()
{

omp_set_num_threads(10);

#pragma omp parallel    
#pragma omp sections
  {
#pragma omp section
#pragma omp for
    for(int i=0; i<5; i++) {
        printf("x %d\n", i);
    }

#pragma omp section
#pragma omp for
    for(int i=0; i<5; i++) {
        printf(". %d\n", i);
    }
  } // end parallel and end sections
}

And the error:

main.cpp: In function ‘int main()’:
main.cpp:14:9: warning: work-sharing region may not be closely nested inside of work-sharing, critical, ordered, master or explicit task region [enabled by default]
main.cpp:20:9: warning: work-sharing region may not be closely nested inside of work-sharing, critical, ordered, master or explicit task region [enabled by default]

Answer 1

Here you have to use nested parallelism. The problem with the omp for in the sections is that all the threads in scope have to take part in the omp for, and they clearly don't — they're broken up by sections. So you have to introduce functions, and do nested paralleism within the functions.

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

void doTask1(const int gtid) {
    omp_set_num_threads(5);
#pragma omp parallel 
    {
        int tid = omp_get_thread_num();
        #pragma omp for
        for(int i=0; i<5; i++) {
            printf("x %d %d %d\n", i, tid, gtid);
        }
    }
}

void doTask2(const int gtid) {
    omp_set_num_threads(5);
#pragma omp parallel 
    {
        int tid = omp_get_thread_num();
        #pragma omp for
        for(int i=0; i<5; i++) {
            printf(". %d %d %d\n", i, tid, gtid);
        }
    }
}


int main()
{
    omp_set_num_threads(2);
    omp_set_nested(1);

#pragma omp parallel    
    {
        int gtid = omp_get_thread_num();
#pragma omp sections
        {
#pragma omp section
            doTask1(gtid);

#pragma omp section
            doTask2(gtid);
        } // end parallel and end sections
    }
}