Nesting and Binding Rules

This section describes the dynamic nesting and binding rules for OpenMP* Fortran API directives.

Binding Rules

The following rules apply to dynamic binding:

The DO, SECTIONS, SINGLE, MASTER, and BARRIER directives bind to the dynamically enclosing PARALLEL directive, if one exists.
The ORDERED directive binds to the dynamically enclosing DO directive.
The ATOMIC directive enforces exclusive access with respect to ATOMIC directives in all threads, not just the current team.
The CRITICAL directive enforces exclusive access with respect to CRITICAL directives in all threads, not just the current team.
A directive can never bind to any directive outside the closest enclosing PARALLEL directive.

Nesting Rules

The following rules apply to dynamic nesting:

A PARALLEL directive dynamically inside another PARALLEL directive logically establishes a new team, which is composed of only the current thread unless nested parallelism is enabled.
DO, SECTIONS, and SINGLE directives that bind to the same PARALLEL directive are not allowed to be nested one inside the other.
DO, SECTIONS, and SINGLE directives are not permitted in the dynamic extent of CRITICAL and MASTER directives.
BARRIER directives are not permitted in the dynamic extent of DO, SECTIONS, SINGLE, MASTER, and CRITICAL directives.
MASTER directives are not permitted in the dynamic extent of DO, SECTIONS, and SINGLE directives.
ORDERED sections are not allowed in the dynamic extent of CRITICAL sections.
Any directive set that is legal when executed dynamically inside a PARALLEL region is also legal when executed outside a parallel region. When executed dynamically outside a user-specified parallel region, the directive is executed with respect to a team composed of only the master thread.

Examples

The following example shows nested PARALLEL regions:

  !$OMP PARALLEL DEFAULT(SHARED)
  !$OMP DO
          DO I =1, N
  !$OMP PARALLEL SHARED(I,N)
  !$OMP DO
          DO J =1, N
            CALL WORK(I,J)
          END DO
  !$OMP END PARALLEL
        END DO
  !$OMP END PARALLEL

Note that the inner and outer DO directives bind to different PARALLEL regions.

The following example shows a variation of the preceding example:

  !$OMP PARALLEL DEFAULT(SHARED)
  !$OMP DO
          DO I =1, N
             CALL SOME_WORK(I,N)
          END DO
  !$OMP END PARALLEL
        ...
        SUBROUTINE SOME_WORK(I,N)
  !$OMP PARALLEL DEFAULT(SHARED)
  !$OMP DO
          DO J =1, N
             CALL WORK(I,J)
          END DO
  !$OMP END PARALLEL
          RETURN
          END