2.8 Pragmas
1
A pragma is a compiler directive.
There are language-defined pragmas that give instructions for optimization,
listing control, etc. An implementation may support additional (implementation-defined)
pragmas.
Syntax
2
pragma ::=
pragma identifier [(
pragma_argument_association {,
pragma_argument_association})];
3
pragma_argument_association ::=
[
pragma_argument_identifier =>]
name
| [
pragma_argument_identifier =>]
expression
4
In a pragma, any
pragma_argument_associations without a pragma_argument_identifier
shall precede any associations with a pragma_argument_identifier.
5
Pragmas
are only allowed at the following places in a program:
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- After a semicolon delimiter,
but not within a formal_part or discriminant_part.
7
- At any place where the
syntax rules allow a construct defined by a syntactic category whose
name ends with "declaration", "statement",
"clause", or "alternative",
or one of the syntactic categories variant
or exception_handler; but not in place of
such a construct. Also at any place where a compilation_unit
would be allowed.
8
Additional syntax rules and placement restrictions
exist for specific pragmas.
9
The
name
of a
pragma is the identifier following the
reserved word
pragma.
The
name or
expression
of a
pragma_argument_association is a
pragma
argument.
10
An
identifier
specific to a pragma is an identifier that is used in a pragma argument
with special meaning for that pragma.
Static Semantics
11
If an implementation does not recognize the name
of a pragma, then it has no effect on the
semantics of the program. Inside such a pragma,
the only rules that apply are the Syntax Rules.
Dynamic Semantics
12
Any
pragma
that appears at the place of an executable construct is executed. Unless
otherwise specified for a particular pragma, this execution consists
of the evaluation of each evaluable pragma argument in an arbitrary order.
Implementation Requirements
13
The implementation shall give a warning message for
an unrecognized pragma name.
Implementation Permissions
14
An implementation may provide implementation-defined
pragmas; the name of an implementation-defined pragma shall differ from
those of the language-defined pragmas.
15
An implementation may ignore an unrecognized pragma
even if it violates some of the Syntax Rules, if detecting the syntax
error is too complex.
Implementation Advice
16
Normally, implementation-defined pragmas should have
no semantic effect for error-free programs; that is, if the implementation-defined
pragmas are removed from a working program, the program should still
be legal, and should still have the same semantics.
17
Normally, an implementation
should not define pragmas that can make an illegal program legal, except
as follows:
18
- A pragma
used to complete a declaration, such as a pragma
Import;
19
- A pragma
used to configure the environment by adding, removing, or replacing library_items.
Syntax
20
The forms of
List, Page, and Optimize pragmas are as follows:
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22
23
pragma Optimize(
identifier);
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Other pragmas are defined throughout this
International Standard, and are summarized in
Annex
L.
Static Semantics
25
A pragma List takes one
of the identifiers On or Off as the single
argument. This pragma is allowed anywhere a pragma
is allowed. It specifies that listing of the compilation is to be continued
or suspended until a List pragma with the
opposite argument is given within the same compilation. The pragma
itself is always listed if the compiler is producing a listing.
26
A pragma Page is allowed
anywhere a pragma is allowed. It specifies
that the program text which follows the pragma
should start on a new page (if the compiler is currently producing a
listing).
27
A pragma Optimize takes
one of the identifiers Time, Space, or Off
as the single argument. This pragma is allowed
anywhere a pragma is allowed, and it applies
until the end of the immediately enclosing declarative region, or for
a pragma at the place of a compilation_unit,
to the end of the compilation. It gives advice
to the implementation as to whether time or space is the primary optimization
criterion, or that optional optimizations should be turned off. It is
implementation defined how this advice is followed.
Examples
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Examples of pragmas:
29/2
pragma List(Off); -- turn off listing generation
pragma Optimize(Off); -- turn off optional optimizations
pragma Inline(Set_Mask); -- generate code for Set_Mask inline
pragma Import(C, Put_Char, External_Name => "putchar"); -- import C putchar function
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