Getting ready

C and C++ programs can access OpenMP functionality by including the omp.h header file and by linking to the correct library. The compiler will generate parallel code according to preprocessor directives preceding the performance-critical sections. In this recipe, we will build the following example source code (example.cpp). This code sums integers from 1 to N, where N is given as a command-line argument:

#include <iostream>
#include <omp.h>
#include <string>

int main(int argc, char *argv[]) {
  std::cout << "number of available processors: " << omp_get_num_procs()
            << std::endl;
  std::cout << "number of threads: " << omp_get_max_threads() << std::endl;

  auto n = std::stol(argv[1]);
  std::cout << "we will form sum of numbers from 1 to " << n << std::endl;

  // start timer
  auto t0 = omp_get_wtime();

  auto s = 0LL;
#pragma omp parallel for reduction(+ : s)
  for (auto i = 1; i <= n; i++) {
    s += i;
  }

  // stop timer
  auto t1 = omp_get_wtime();

  std::cout << "sum: " << s << std::endl;
  std::cout << "elapsed wall clock time: " << t1 - t0 << " seconds" << std::endl;

  return 0;
}

In Fortran, one needs to use the omp_lib module and link to the correct library. Use of parallel directives is once again possible in code comments preceding the performance-critical sections. The corresponding example.F90 contains the following:

program example
  
  use omp_lib

  implicit none

  integer(8) :: i, n, s
  character(len=32) :: arg
  real(8) :: t0, t1

  print *, "number of available processors:", omp_get_num_procs()
  print *, "number of threads:", omp_get_max_threads()

  call get_command_argument(1, arg)
  read(arg , *) n

  print *, "we will form sum of numbers from 1 to", n

  ! start timer
  t0 = omp_get_wtime()

  s = 0
!$omp parallel do reduction(+:s)
  do i = 1, n
    s = s + i
  end do

  ! stop timer
  t1 = omp_get_wtime()

  print *, "sum:", s
  print *, "elapsed wall clock time (seconds):", t1 - t0

end program