Question

I have a single block enclosed in a sections block like this

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

int main (int argc, char *argv[]) 
{

int nthreads, tid;

/* Fork a team of threads giving them their own copies of variables */
#pragma omp parallel private(tid) 
{

#pragma omp sections
{


#pragma omp section
{
printf("First section %d \n" , tid);
}

#pragma omp section
{


#pragma omp single
{
printf("Second Section block %d \n" , tid);
}

}

}

}  /* All threads join master thread and disband */

printf("Outside parallel block \n");


}

When i compile this code the compiler gives the following warning

work-sharing region may not be closely nested inside of work-sharing, critical, ordered or master region

Why is that ?

Answer 1

It gives you this warning because you have an openmp single region nested inside an openmp sections region without an openmp parallel region nested between them.

This is known as a closely nested region.

In C, the worksharing constructs are for, sections, and single.

For further information see the OpenMP Specification or see Intel’s Documentation on Improper nesting of OpenMP* constructs.

In order to have the code compile cleanly, try replacing your #pragma omp sections with #pragma omp parallel sections
or enclosing #pragma omp sections with #pragma omp parallel.

See Guide into OpenMP: Easy multithreading programming for C++ for more information and examples.