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

OpenMP cannot create parallel regions inside parallel regions. This is due to the fact that OpenMP create at the beginning of the program num_threads parallel threads, in non parallel regions the others are not used and sleep. They have done this, as the frequent generation of new threads is quite slow compared to waking sleeping threads.

Therefore you should parallelize only the loops:

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

int main()
{

omp_set_num_threads(10);

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

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