3.12. single Construct

The following example demonstrates the single construct. In the example, only one thread prints each of the progress messages. All other threads will skip the single region and stop at the barrier at the end of the single construct until all threads in the team have reached the barrier. If other threads can proceed without waiting for the thread executing the single region, a nowait clause can be specified, as is done in the third single construct in this example. The user must not make any assumptions as to which thread will execute a single region.

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

/*
* name: single.1
* type: C
*/
#include <stdio.h>

void work1() {}
void work2() {}

void single_example()
{
  #pragma omp parallel
  {
    #pragma omp single
      printf("Beginning work1.\n");

    work1();

    #pragma omp single
      printf("Finishing work1.\n");

    #pragma omp single nowait
      printf("Finished work1 and beginning work2.\n");

    work2();
  }
}

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

! name: single.1
! type: F-fixed
      SUBROUTINE WORK1()
      END SUBROUTINE WORK1

      SUBROUTINE WORK2()
      END SUBROUTINE WORK2

      PROGRAM SINGLE_EXAMPLE
!$OMP PARALLEL

!$OMP SINGLE
        print *, "Beginning work1."
!$OMP END SINGLE

        CALL WORK1()

!$OMP SINGLE
        print *, "Finishing work1."
!$OMP END SINGLE

!$OMP SINGLE
        print *, "Finished work1 and beginning work2."
!$OMP END SINGLE NOWAIT

        CALL WORK2()

!$OMP END PARALLEL

      END PROGRAM SINGLE_EXAMPLE