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* extend.texi (Attribute Syntax): New section.

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From-SVN: r39305
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Joseph Myers 2001-01-27 21:29:53 +00:00 committed by Joseph Myers
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@ -1,3 +1,7 @@
2001-01-27 Joseph S. Myers <jsm28@cam.ac.uk>
* extend.texi (Attribute Syntax): New section.
2001-01-27 Michael Sokolov <msokolov@ivan.Harhan.ORG>
* fixproto: Correctly install synthesised unistd.h and stdlib.h when

186
gcc/extend.texi
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@ -56,6 +56,7 @@ C++ Language}, for extensions that apply @emph{only} to C++.
* Case Ranges:: `case 1 ... 9' and such.
* Function Attributes:: Declaring that functions have no side effects,
or that they can never return.
* Attribute Syntax:: Formal syntax for attributes.
* Function Prototypes:: Prototype declarations and old-style definitions.
* C++ Comments:: C++ comments are recognized.
* Dollar Signs:: Dollar sign is allowed in identifiers.
@ -105,6 +106,7 @@ C++ Language}, for extensions that apply @emph{only} to C++.
* Case Ranges:: `case 1 ... 9' and such.
* Function Attributes:: Declaring that functions have no side effects,
or that they can never return.
* Attribute Syntax:: Formal syntax for attributes.
* Function Prototypes:: Prototype declarations and old-style definitions.
* C++ Comments:: C++ comments are recognized.
* Dollar Signs:: Dollar sign is allowed in identifiers.
@ -1505,6 +1507,9 @@ each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use @code{__noreturn__} instead of @code{noreturn}.
@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.
@table @code
@cindex @code{noreturn} function attribute
@item noreturn
@ -1979,6 +1984,181 @@ attachment of attributes to their corresponding declarations, whereas
part of the grammar. @xref{Other Directives,,Miscellaneous
Preprocessing Directives, cpp, The C Preprocessor}.
@node Attribute Syntax
@section Attribute Syntax
@cindex attribute syntax
This section describes the syntax with which @code{__attribute__} may be
used, and the constructs to which attribute specifiers bind, for the C
language. Some details may vary for C++ and Objective C. Because of
infelicities in the grammar for attributes, some forms described here
may not be successfully parsed in all cases.
@xref{Function Attributes}, for details of the semantics of attributes
applying to functions. @xref{Variable Attributes}, for details of the
semantics of attributes applying to variables. @xref{Type Attributes},
for details of the semantics of attributes applying to structure, union
and enumerated types.
An @dfn{attribute specifier} is of the form
@code{__attribute__ ((@var{attribute-list}))}. An @dfn{attribute list}
is a possibly empty comma-separated sequence of @dfn{attributes}, where
each attribute is one of the following:
@itemize @bullet
@item
Empty. Empty attributes are ignored.
@item
A word (which may be an identifier such as @code{unused}, or a reserved
word such as @code{const}).
@item
A word, followed by, in parentheses, parameters for the attribute.
These parameters take one of the following forms:
@itemize @bullet
@item
An identifier. For example, @code{mode} attributes use this form.
@item
An identifier followed by a comma and a non-empty comma-separated list
of expressions. For example, @code{format} attributes use this form.
@item
A possibly empty comma-separated list of expressions. For example,
@code{format_arg} attributes use this form with the list being a single
integer constant expression, and @code{alias} attributes use this form
with the list being a single string constant.
@end itemize
@end itemize
An @dfn{attribute specifier list} is a sequence of one or more attribute
specifiers, not separated by any other tokens.
An attribute specifier list may appear after the colon following a
label, other than a @code{case} or @code{default} label. The only
attribute it makes sense to use after a label is @code{unused}. This
feature is intended for code generated by programs which contains labels
that may be unused but which is compiled with @option{-Wall}. It would
not normally be appropriate to use in it human-written code, though it
could be useful in cases where the code that jumps to the label is
contained within an @code{#ifdef} conditional.
An attribute specifier list may appear as part of a @code{struct},
@code{union} or @code{enum} specifier. It may go either immediately
after the @code{struct}, @code{union} or @code{enum} keyword, or after
the closing brace. It is ignored if the content of the structure, union
or enumerated type is not defined in the specifier in which the
attribute specifier list is used---that is, in usages such as
@code{struct __attribute__((foo)) bar} with no following opening brace.
Where attribute specifiers follow the closing brace, they are considered
to relate to the structure, union or enumerated type defined, not to any
enclosing declaration the type specifier appears in, and the type
defined is not complete until after the attribute specifiers.
@c Otherwise, there would be the following problems: a shift/reduce
@c conflict between attributes binding the the struct/union/enum and
@c binding to the list of specifiers/qualifiers; and "aligned"
@c attributes could use sizeof for the structure, but the size could be
@c changed later by "packed" attributes.
Otherwise, an attribute specifier appears as part of a declaration,
counting declarations of unnamed parameters and type names, and relates
to that declaration (which may be nested in another declaration, for
example in the case of a parameter declaration). In future, attribute
specifiers in some places may however apply to a particular declarator
within a declaration instead; these cases are noted below.
Any list of specifiers and qualifiers at the start of a declaration may
contain attribute specifiers, whether or not such a list may in that
context contain storage class specifiers. (Some attributes, however,
are essentially in the nature of storage class specifiers, and only make
sense where storage class specifiers may be used; for example,
@code{section}.) There is one necessary limitation to this syntax: the
first old-style parameter declaration in a function definition cannot
begin with an attribute specifier, because such an attribute applies to
the function instead by syntax described below (which, however, is not
yet implemented in this case). In some other cases, attribute
specifiers are permitted by this grammar but not yet supported by the
compiler. All attribute specifiers in this place relate to the
declaration as a whole. In the obsolencent usage where a type of
@code{int} is implied by the absence of type specifiers, such a list of
specifiers and qualifiers may be an attribute specifier list with no
other specifiers or qualifiers.
An attribute specifier list may appear immediately before a declarator
(other than the first) in a comma-separated list of declarators in a
declaration of more than one identifier using a single list of
specifiers and qualifiers. At present, such attribute specifiers apply
not only to the identifier before whose declarator they appear, but to
all subsequent identifiers declared in that declaration, but in future
they may apply only to that single identifier. For example, in
@code{__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...), d2
(void)}, the @code{noreturn} attribute applies to all the functions
declared; the @code{format} attribute should only apply to @code{d1},
but at present applies to @code{d2} as well (and so causes an error).
An attribute specifier list may appear immediately before the comma,
@code{=} or semicolon terminating the declaration of an identifier other
than a function definition. At present, such attribute specifiers apply
to the declared object or function, but in future they may attach to the
outermost adjacent declarator. In simple cases there is no difference,
but, for example, in @code{void (****f)(void)
__attribute__((noreturn));}, at present the @code{noreturn} attribute
applies to @code{f}, which causes a warning since @code{f} is not a
function, but in future it may apply to the function @code{****f}. The
precise semantics of what attributes in such cases will apply to are not
yet specified. Where an assembler name for an object or function is
specified (@pxref{Asm Labels}), at present the attribute must follow the
@code{asm} specification; in future, attributes before the @code{asm}
specification may apply to the adjacent declarator, and those after it
to the declared object or function.
An attribute specifier list may, in future, be permitted to appear after
the declarator in a function definition (before any old-style parameter
declarations or the function body).
An attribute specifier list may appear at the start of a nested
declarator. At present, there are some limitations in this usage: the
attributes apply to the identifer declared, and to all subsequent
identifiers declared in that declaration (if it includes a
comma-separated list of declarators), rather than to a specific
declarator. When attribute specifiers follow the @code{*} of a pointer
declarator, they must presently follow any type qualifiers present, and
cannot be mixed with them. The following describes intended future
semantics which make this syntax more useful only. It will make the
most sense if you are familiar with the formal specification of
declarators in the ISO C standard.
Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration @code{T
D1}, where @code{T} contains declaration specifiers that specify a type
@var{Type} (such as @code{int}) and @code{D1} is a declarator that
contains an identifier @var{ident}. The type specified for @var{ident}
for derived declarators whose type does not include an attribute
specifier is as in the ISO C standard.
If @code{D1} has the form @code{( @var{attribute-specifier-list} D )},
and the declaration @code{T D} specifies the type
``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
@code{T D1} specifies the type ``@var{derived-declarator-type-list}
@var{attribute-specifier-list} @var{Type}'' for @var{ident}.
If @code{D1} has the form @code{*
@var{type-qualifier-and-attribute-specifier-list} D}, and the
declaration @code{T D} specifies the type
``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
@code{T D1} specifies the type ``@var{derived-declarator-type-list}
@var{type-qualifier-and-attribute-specifier-list} @var{Type}'' for
@var{ident}.
For example, @code{void (__attribute__((noreturn)) ****f)();} specifies
the type ``pointer to pointer to pointer to pointer to non-returning
function returning @code{void}''. As another example, @code{char
*__attribute__((aligned(8))) *f;} specifies the type ``pointer to
8-byte-aligned pointer to @code{char}''. Note again that this describes
intended future semantics, not current implementation.
@node Function Prototypes
@section Prototypes and Old-Style Function Definitions
@cindex function prototype declarations
@ -2129,6 +2309,9 @@ each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use @code{__aligned__} instead of @code{aligned}.
@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.
@table @code
@cindex @code{aligned} attribute
@item aligned (@var{alignment})
@ -2369,6 +2552,9 @@ brace of a definition.
You may also specify attributes between the enum, struct or union
tag and the name of the type rather than after the closing brace.
@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.
@table @code
@cindex @code{aligned} attribute
@item aligned (@var{alignment})