3.2. parallel Construct

The parallel construct can be used in coarse-grain parallel programs. In the following example, each thread in the parallel region decides what part of the global array x to work on, based on the thread number:

//%compiler: clang
//%cflags: -fopenmp

/*
* name: parallel.1
* type: C
*/
#include <omp.h>

void subdomain(float *x, int istart, int ipoints)
{
  int i;

  for (i = 0; i < ipoints; i++)
      x[istart+i] = 123.456;
}

void sub(float *x, int npoints)
{
    int iam, nt, ipoints, istart;

#pragma omp parallel default(shared) private(iam,nt,ipoints,istart)
    {
        iam = omp_get_thread_num();
        nt =  omp_get_num_threads();
        ipoints = npoints / nt;    /* size of partition */
        istart = iam * ipoints;  /* starting array index */
        if (iam == nt-1)     /* last thread may do more */
          ipoints = npoints - istart;
        subdomain(x, istart, ipoints);
    }
}

int main()
{
    float array[10000];

    sub(array, 10000);

    return 0;
}

!!%compiler: gfortran
!!%cflags: -fopenmp

! name: parallel.1
! type: F-fixed
      SUBROUTINE SUBDOMAIN(X, ISTART, IPOINTS)
          INTEGER ISTART, IPOINTS
          REAL X(*)

          INTEGER I

          DO 100 I=1,IPOINTS
             X(ISTART+I) = 123.456
 100      CONTINUE

      END SUBROUTINE SUBDOMAIN

      SUBROUTINE SUB(X, NPOINTS)
          INCLUDE "omp_lib.h"     ! or USE OMP_LIB

          REAL X(*)
          INTEGER NPOINTS
          INTEGER IAM, NT, IPOINTS, ISTART

!$OMP PARALLEL DEFAULT(PRIVATE) SHARED(X,NPOINTS)

          IAM = OMP_GET_THREAD_NUM()
          NT =  OMP_GET_NUM_THREADS()
          IPOINTS = NPOINTS/NT
          ISTART = IAM * IPOINTS
          IF (IAM .EQ. NT-1) THEN
              IPOINTS = NPOINTS - ISTART
          ENDIF
          CALL SUBDOMAIN(X,ISTART,IPOINTS)

!$OMP END PARALLEL
      END SUBROUTINE SUB

      PROGRAM PAREXAMPLE
          REAL ARRAY(10000)
          CALL SUB(ARRAY, 10000)
      END PROGRAM PAREXAMPLE