[c++] OpenMP set_num_threads() is not working

I am writing a parallel program using OpenMP in C++.

I want to control the number of threads in the program using omp_set_num_threads(), but it does not work.

#include <iostream>
#include <omp.h>
#include "mpi.h"

using namespace std;

int myrank;
int groupsize;
double sum;
double t1,t2;
int n = 10000000;

int main(int argc, char *argv[])
{
    MPI_Init( &argc, &argv);
    MPI_Comm_rank( MPI_COMM_WORLD, &myrank );
    MPI_Comm_size(MPI_COMM_WORLD,&groupsize);

    omp_set_num_threads(4);

    sum = 0;
    #pragma omp for  reduction(+:sum)
    for (int i = 0; i < n; i++)
        sum+= i/(n/10);

    cout<<"sum="<<sum<<endl;
    cout<<"threads="<<omp_get_num_threads()<<endl;

    MPI_Finalize();
    return 0;
}

The program outputs:

sum = 4.5e+007
threads=1

How to control the number of threads?

Try setting your num_threads inside your omp parallel code, it worked for me. This will give output as 4

#pragma omp parallel
{
   omp_set_num_threads(4);
   int id = omp_get_num_threads();
   #pragma omp for
   for (i = 0:n){foo(A);}
}

printf("Number of threads: %d", id);

The omp_get_num_threads() function returns the number of threads that are currently in the team executing the parallel region from which it is called. You are calling it outside of the parallel region, which is why it returns 1.

According to the GCC manual for omp_get_num_threads:

In a sequential section of the program omp_get_num_threads returns 1

So this:

cout<<"sum="<<sum<<endl;
cout<<"threads="<<omp_get_num_threads()<<endl;

Should be changed to something like:

#pragma omp parallel
{
    cout<<"sum="<<sum<<endl;
    cout<<"threads="<<omp_get_num_threads()<<endl;
}

The code I use follows Hristo's advice of disabling dynamic teams, too.

I was facing the same problem . Solution is given below

Right click on Source Program > Properties > Configuration Properties > C/C++ > Language > Now change Open MP support flag to Yes....

You will get the desired result.