OpenMP Directives: Directive Binding and Nesting Rules

This section is provided mainly as a quick reference on rules which govern OpenMP directives and binding. Users should consult their implementation documentation and the OpenMP standard for other rules and restrictions.
Unless indicated otherwise, rules apply to both Fortran and C/C++ OpenMP implementations.
Note: the Fortran API also defines a number of Data Environment rules. Those have not been reproduced here.

Directive Binding:

The DO/for, SECTIONS, SINGLE, MASTER and BARRIER directives bind to the dynamically enclosing PARALLEL, if one exists. If no parallel region is currently being executed, the directives have no effect.
The ORDERED directive binds to the dynamically enclosing DO/for.
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.

A worksharing region may not be closely nested inside a worksharing, explicit task, critical, ordered, atomic, or master region.
A barrier region may not be closely nested inside a worksharing, explicit task, critical, ordered, atomic, or master region.
A master region may not be closely nested inside a worksharing, atomic, or explicit task region.
An ordered region may not be closely nested inside a critical, atomic, or explicit task region.
An ordered region must be closely nested inside a loop region (or parallel loop region) with an ordered clause.
A critical region may not be nested (closely or otherwise) inside a critical region with the same name. Note that this restriction is not sufficient to prevent deadlock.
parallel, flush, critical, atomic, taskyield, and explicit task regions may not be closely nested inside an atomic region.