3.2  Clause Format

This section defines the format and categories of OpenMP clauses. OpenMP clauses are specified as part of a directive-specification. Clauses are optional and, thus, may be omitted from a directive-specification unless otherwise specified. The order in which clauses appear on directives is not significant unless otherwise specified. A clause-specification specifies each OpenMP clause in a directive-specification where clause-specification for inarguable clauses is simply:  

 
clause-name
  

Inarguable clauses often form natural groupings that have similar semantic effect and so are frequently specified as a clause grouping. For argument-modified clauses, clause-specification is:  

 
clause-name[(clause-argument-specification [; clause-argument-specification [;...]])]
  

An implementation may allow clauses with clause names that start with the ompx_ prefix for use on any OpenMP directive, and the format and semantics of any such clause is implementation defined. All other clause names are reserved.

For argument-modified clauses, the first clause-argument-specification is required unless otherwise explicitly stated while additional ones are only permitted on clauses that explicitly allow them. When the first one is omitted, the syntax is identical to an inarguable clause. Clause arguments may be unmodified or modified. For an unmodified argument, clause-argument-specification is:  

 
clause-argument-list
  

Unless otherwise specified, modified arguments are pre-modified, for which the format is:  

 
[modifier-specification [[, modifier-specification] ,... ] :]clause-argument-list
  

A few modified arguments are explicitly specified as post-modified, for which the format is:  

 
clause-argument-list[: modifier-specification [[, modifier-specification] ,... ]]
  

For many OpenMP clauses, clause-argument-list is an OpenMP argument list, which is a comma-separated list of a specific kind of list items (see Section 3.2.1), in which case the format of clause-argument-list is:  

 
argument-name
  

For all other OpenMP clauses, clause-argument-list is a comma-separated list of arguments so the format is:  

 
argument-name [, argument-name [,... ]]
  

In most of these cases, the list only has a single item so the format of clause-argument-list is again:  

 
argument-name
  

In all cases, white space in clause-argument-list is optional.

Clause argument modifiers may be simple or complex. Almost all clause arguments are simple, for which the format of modifier-specification is:  

 
modifier-name
  

The format of a complex modifier is:  

 
modifier-name(modifier-parameter-specification)
  

where modifier-parameter-specification is a comma-separated list of arguments as defined above for clause-argument-list. The position of each modifier-argument-name in the list is significant.

Each argument-name and modifier-name is an OpenMP term that may be used in the definitions of the clause and any directives on which the clause may appear. Syntactically, each of these terms is one of the following:

• keyword: An OpenMP keyword
• OpenMP identifier: An OpenMP identifier
• OpenMP argument list: An OpenMP argument list
• expression: An expression of some OpenMP type
• OpenMP stylized expression: An OpenMP stylized expression

A particular lexical instantiation of an argument specifies a parameter of the clause, while a lexical instantiation of a modifier and its parameters affects how or when the argument is applied.

The order of arguments must match the order in the clause-specification. The order of modifiers in a clause-argument-specification is not significant unless otherwise specified.

General syntactic properties govern the use of clauses, clause and directive arguments, and modifiers in an OpenMP directive. These properties are summarized in 3.1, along with the respective default properties for clauses, arguments and modifiers.

A clause, argument or modifier with a given property implies that it does not have the corresponding inverse property, and vice versa. The ultimate property implies the unique property. If all arguments and modifiers of an argument-modified clause or directive are optional and omitted then the parentheses of the syntax for the clause or directive is also omitted.

Arguments and modifiers that are expressions may additionally have any of the following value properties: constant, positive, non-negative, and region-invariant.

 
#pragma omp depobj(o) depend(inout: d)
  

The clauses that a directive accepts may form sets. These sets may imply restrictions on their use on that directive or may otherwise capture properties for the clauses on the directive. While specific properties may be defined for a clause set on a particular directive, the following clause-set properties have general meanings and implications as indicated by the restrictions below: required, unique, and exclusive.

All clauses that are specified as a clause grouping form a clause set for which properties are specified with the specification of the grouping. Some directives accept a clause grouping for which each member is a directive-name of a directive that has a specific property. These groupings are required, unique and exclusive unless otherwise specified.

Restrictions

• A required clause for a directive must appear on the directive.
• A unique clause for a directive may appear at most once on the directive.
• An exclusive clause for a directive must not appear if a clause with a different clause-name also appears on the directive.
• An ultimate clause for a directive must be the lexically last clause to appear on the directive.
• If a clause set has the required property, at least one clause in the set must be present on the directive for which the clause set is specified.
• If a clause is a member of a set that has the unique property for a directive then the clause has the unique property for that directive regardless of whether it has the unique property when it is not part of such a set.
• If one clause of a clause set with the exclusive property appears on a directive, no other clauses with a different clause-name in that set may appear on the directive.
• A required argument must appear in the clause-specification.
• A unique argument may appear at most once in a clause-argument-specification.
• An exclusive argument must not appear if an argument with a different argument-name appears in the clause-argument-specification.
• A required modifier must appear in the clause-argument-specification.
• A unique modifier may appear at most once in a clause-argument-specification.
• An exclusive modifier must not appear if a modifier with a different modifier-name also appears in the clause-argument-specification.
• If a clause is pre-modified, an ultimate modifier must be the last modifier in a clause-argument-specification in which any modifier appears.
• If a clause is post-modified, an ultimate modifier must be the first modifier in a clause-argument-specification in which any modifier appears.
• A modifier that is an expression must neither lexically match the name of a simple modifier defined for the clause that is an OpenMP keyword nor modifier-name parenthesized-tokens, where modifier-name is the modifier-name of a complex modifier defined for the clause and parenthesized-tokens is a token sequence that starts with ( and ends with ).
• A constant argument or parameter must be a compile-time constant.
• A positive argument or parameter must be greater than zero; a non-negative argument or parameter must be greater than or equal to zero.
• A region-invariant argument or parameter must have the same value throughout any given execution of the construct or, for declarative directives, execution of the function or subroutine with which the declaration is associated.

Cross References

• Directive Format, see Section 3.1
• OpenMP Argument Lists, see Section 3.2.1
• OpenMP Stylized Expressions, see Section 4.2
• OpenMP Types and Identifiers, see Section 4.1