Simple parallel example

Syntax

  • #pragma omp parallel indicates that the following block shall be executed by all the threads.

  • int omp_get_num_threads (void) : returns the number of the threads working on the parallel region (aka team of threads).

  • int omp_get_thread_num (void) : returns the identifier of the calling thread (ranges from 0 to N-1 where N is bounded to omp_get_num_threads()).

You can use the OMP_NUM_THREADS environment variable or the num_threads directive within the #pragma parallel to indicate the number of executing threads for the whole application or for the specified region, respectively.

Parallel hello world using OpenMP

The following C code uses the OpenMP parallel programming model to write the thread ID and number of threads to stdout using multiple threads.

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

int main ()
{
    #pragma omp parallel
    {   
        // ID of the thread in the current team
        int thread_id = omp_get_thread_num();
        // Number of threads in the current team
        int nthreads = omp_get_num_threads();

        printf("I'm thread %d out of %d threads.\n", thread_id, nthreads);
    }
    return 0;
}

In Fortran 90+ the equivalent program looks like:

program Hello
  use omp_lib, only: omp_get_thread_num, omp_get_num_threads

  implicit none

  integer :: thread_id
  integer :: nthreads

  !$omp parallel private( thread_id, nthreads )

  ! ID of the thread in the current team
  thread_id = omp_get_thread_num()
  ! Number of threads in the current team
  nthreads = omp_get_num_threads()

  print *, "I'm thread", thread_id, "out of", nthreads, "threads."
  !$omp end parallel
end program Hello