Data Sharing Attribute

Shared variable¶

All the threads have access to the shared variable.
By default, in the parallel region, all the variables are considered shared variables except the loop iteration counter variables.

Note

Shared variables should be handled carefully; otherwise, they cause race conditions in the program.

Examples: Shared variable

(C/C++)(FORTRAN)

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

using namespace std;

int main()
{
  // Array size
  int N = 10;

  // Initialize the variables
  float *a;

  // Allocate the memory
  a  = (float*)malloc(sizeof(float) * N);

  //#pragma omp parallel for
  // or 
#pragma omp parallel for shared(a)
  for (int i = 0; i < N; i++)
    {
      a[i] = a[i] + i;  
      cout << "value of a in the parallel region" << a[i] << endl;
    }

  for (int i = 0; i < N; i++)
    cout << "value of a after the parallel region " << a[i] << endl;

  return 0;
}

program main
  use omp_lib
  implicit none

  ! Input vectors
  real(8), dimension(:), allocatable :: a

  integer :: n, i
  n=10

  ! Allocate memory for vector
  allocate(a(n))

  !$omp parallel shared(a)
  !$omp do
  do i = 1, n
      a(i) = a(i) + i
      print*, 'value of a in the parallel region', a(i)
  end do
  !$omp end do
  !$omp end parallel

  do i = 1, n
      a(i) = a(i) + i
      print*,'value of an after the parallel region', a(i)
  end do

  ! Delete the memory
  deallocate(a)

end program main

Questions

Does the value of vector a change after the parallel loop, if not why, think?
Do we really need to mention shared(a), is it neccessary?

Private variable¶

Each thread will have its own copy of the private variable.
And the private variable is only accessible within the parallel region, not outside of the parallel region.
By default, the loop iteration counters are considered as a private.
A change made by one thread is not visible to other threads.

Examples: Private variable

(C/C++)(FORTRAN)

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

using namespace std;

int main()
{
  // Array size
  int N = 10;

  // Initialize the variables
  float *a,b,c;
  b = 1.0;
  c = 2.0;

  // Allocate the memory
  a  = (float*)malloc(sizeof(float) * N);

#pragma omp parallel for private(b,c)
  for (int i = 0; i < N; i++)
    {
      b = a[i] + i;
      c = b + 10 * i;
      cout << "value of c in the parallel region " << c << endl;
    }

  cout << "value of c after the parallel region " << c << endl; 

  return 0;
}

program main
  use omp_lib
  implicit none

  ! Input vectors
  real(8), dimension(:), allocatable :: a

  real(8) :: b, c
  integer :: n, i  
  n=10
  b=1.0
  c=2.0

  ! Allocate memory for vector
  allocate(a(n))

  !$omp parallel private(b,c) shared(a)
  !$omp do
  do i = 1, n
      b = a(i) + i
      c = b + 10 * i
      print*, 'value of c in the parallel region', c
  end do
  !$omp end do
  !$omp end parallel
  print*, 'value of c after the parallel region', c

  ! Delete the memory
  deallocate(a)

end program main

Questions

What is the value of the variable c in the parallel region and after the parallel region?
After the parallel region, has variable c been updated or not?

Lastprivate¶

lastprivate: is also similar to a private clause
But each thread will have an uninitialized copy of the variables passed as lastprivate
At the end of the parallel loop or sections, the final variable value will be the last thread accessed value in the section or in a parallel loop.

Examples: Lastprivate variable

(C/C++)(FORTRAN)

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

using namespace std;

int main()
{
  int n = 10;
  int var = 5;
  omp_set_num_threads(10);
#pragma omp parallel for lastprivate(var)
  for(int i = 0; i < n; i++)
    {
      var += omp_get_thread_num();
      cout << " lastprivate in the parallel region " << var << endl;
    } /*-- End of parallel region --*/
  cout << "lastprivate after the parallel region " << var <<endl;

  return 0;
}

program main
  use omp_lib
  implicit none

  ! Initialise the variable
  real(8) :: var
  integer :: n, i  
  n = 10
  var = 5

  call omp_set_num_threads(10)

  !$omp parallel 
  !$omp do lastprivate(var)
  do i = 1, n
     var  =  var + omp_get_thread_num()
     print*, 'lastprivate in the parallel region ', var
  end do
  !$omp end do
  !$omp end parallel

  print*, 'lastprivate after the parallel region ', var

end program main

Questions

What is the value of the variable var in the parallel region and after the parallel region?
Do you think the initial value of variable var is considered within the parallel region?

Firstprivate¶

firstprivate: is similar to a private clause
But each thread will have an initialized copy of the variables passed as firstprivate
Available for parallel constructs, loops, sections and single constructs

Examples: Firstprivate variable

(C/C++)(FORTRAN)

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

using namespace std;

int main()
{
  int n = 10;
  int var = 5;
  omp_set_num_threads(10);
#pragma omp parallel for firstprivate(var)
  for(int i = 0; i < n; i++)
    {
      var += omp_get_thread_num();
      cout << " lastprivate in the parallel region " << var << endl;
    } /*-- End of parallel region --*/
  cout << "lastprivate after the parallel region " << var <<endl;

  return 0;
}

program main
  use omp_lib
  implicit none

  ! Initialise the variable
  real(8) :: var
  integer :: n, i  
  n = 10
  var = 5

  call omp_set_num_threads(10)

  !$omp parallel 
  !$omp do firstprivate(var)
  do i = 1, n
     var  =  var + omp_get_thread_num()
     print*, 'lastprivate in the parallel region ', var
  end do
  !$omp end do
  !$omp end parallel

  print*, 'lastprivate after the parallel region ', var

end program main

Questions

What is the value of the variable var in the parallel region and after the parallel region?
Is variable var has been updated after the parallel region, if not why, think?