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OpenMP Directives: Work-Sharing Constructs: SECTIONS Directive

Purpose:

The SECTIONS directive is a non-iterative work-sharing construct. It specifies that the enclosed section(s) of code are to be divided among the threads in the team.

Independent SECTION directives are nested within a SECTIONS directive. Each SECTION is executed once by a thread in the team. Different sections may be executed by different threads. It is possible for a thread to execute more than one section if it is quick enough and the implementation permits such.

Format:

Fortran:

!$OMP SECTIONS [clause ...] 
               PRIVATE (list) 
               FIRSTPRIVATE (list) 
               LASTPRIVATE (list) 
               REDUCTION (operator | intrinsic : list) 

!$OMP  SECTION 

   block

!$OMP  SECTION 

    block 

!$OMP END SECTIONS  [ NOWAIT ]

C/C++:

#pragma omp sections [clause ...]  newline 
                     private (list) 
                     firstprivate (list) 
                     lastprivate (list) 
                     reduction (operator: list) 
                     nowait
  {

  #pragma omp section   newline 

     structured_block

  #pragma omp section   newline 

     structured_block

  }

Clauses:

There is an implied barrier at the end of a SECTIONS directive, unless the NOWAIT/nowait clause is used. Clauses are described in detail later, in the Data Scope Attribute Clauses section.

Questions:

What happens if the number of threads and the number of SECTIONs are different? More threads than SECTIONs? Less threads than SECTIONs?

#### ANSWER: If there are more threads than sections, some threads will not execute a section and some will. If there are more sections than threads, the implementation defines how the extra sections are executed.

Which thread executes which SECTION?

#### ANSWER: It is up to the implementation to decide which threads will execute a section and which threads will not, and it can vary from execution to execution.

Restrictions:

It is illegal to branch (goto) into or out of section blocks.

SECTION directives must occur within the lexical extent of an enclosing SECTIONS directive (no orphan SECTIONs).

Example: SECTIONS Directive

Simple program demonstrating that different blocks of work will be done by different threads.

Fortran - SECTIONS Directive Example

      PROGRAM VEC_ADD_SECTIONS

      INTEGER N, I
      PARAMETER (N=1000)
      REAL A(N), B(N), C(N), D(N)

!     Some initializations
      DO I = 1, N
        A(I) = I * 1.5
        B(I) = I + 22.35
      ENDDO

!$OMP PARALLEL SHARED(A,B,C,D), PRIVATE(I)

!$OMP SECTIONS

!$OMP SECTION
      DO I = 1, N
         C(I) = A(I) + B(I)
      ENDDO

!$OMP SECTION
      DO I = 1, N
         D(I) = A(I) * B(I)
      ENDDO

!$OMP END SECTIONS NOWAIT

!$OMP END PARALLEL

      END

C/C++ - sections Directive Example

#include <omp.h>
#define N     1000

main ()
{

int i;
float a[N], b[N], c[N], d[N];

/* Some initializations */
for (i=0; i < N; i++) {
  a[i] = i * 1.5;
  b[i] = i + 22.35;
  }

#pragma omp parallel shared(a,b,c,d) private(i)
  {

  #pragma omp sections nowait
    {

    #pragma omp section
    for (i=0; i < N; i++)
      c[i] = a[i] + b[i];

    #pragma omp section
    for (i=0; i < N; i++)
      d[i] = a[i] * b[i];

    }  /* end of sections */

  }  /* end of parallel section */

}