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:

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

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