3.11. firstprivate Clause and sections Construct

In the following example of the sections construct the firstprivate clause is used to initialize the private copy of section_count of each thread. The problem is that the section constructs modify section_count, which breaks the independence of the section constructs. When different threads execute each section, both sections will print the value 1. When the same thread executes the two sections, one section will print the value 1 and the other will print the value 2. Since the order of execution of the two sections in this case is unspecified, it is unspecified which section prints which value.

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

/*
* name: fpriv_sections.1
* type: C
*/
#include <omp.h>
#include <stdio.h>
#define NT 4
int main( ) {
    int section_count = 0;
    omp_set_dynamic(0);
    omp_set_num_threads(NT);
#pragma omp parallel
#pragma omp sections firstprivate( section_count )
{
#pragma omp section
    {
        section_count++;
        /* may print the number one or two */
        printf( "section_count %d\n", section_count );
    }
#pragma omp section
    {
        section_count++;
        /* may print the number one or two */
        printf( "section_count %d\n", section_count );
    }
}
    return 0;
}

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

! name: fpriv_sections.1
! type: F-free
program section
    use omp_lib
    integer :: section_count = 0
    integer, parameter :: NT = 4
    call omp_set_dynamic(.false.)
    call omp_set_num_threads(NT)
!$omp parallel
!$omp sections firstprivate ( section_count )
!$omp section
    section_count = section_count + 1
! may print the number one or two
    print *, 'section_count', section_count
!$omp section
    section_count = section_count + 1
! may print the number one or two
    print *, 'section_count', section_count
!$omp end sections
!$omp end parallel
end program section