(gcc.info.gz) Attribute Syntax
(gcc.info.gz) Function Attributes
(gcc.info.gz) C Extensions
(gcc.info.gz) Function Prototypes
5.28 Attribute Syntax
This section describes the syntax with which `__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.
There are some problems with the semantics of attributes in C++. For
example, there are no manglings for attributes, although they may affect
code generation, so problems may arise when attributed types are used in
conjunction with templates or overloading. Similarly, `typeid' does
not distinguish between types with different attributes. Support for
attributes in C++ may be restricted in future to attributes on
declarations only, but not on nested declarators.
Function Attributes, for details of the semantics of attributes
applying to functions. Variable Attributes, for details of the
semantics of attributes applying to variables. Type Attributes,
for details of the semantics of attributes applying to structure, union
and enumerated types.
An "attribute specifier" is of the form `__attribute__
((ATTRIBUTE-LIST))'. An "attribute list" is a possibly empty
comma-separated sequence of "attributes", where each attribute is one
of the following:
* Empty. Empty attributes are ignored.
* A word (which may be an identifier such as `unused', or a reserved
word such as `const').
* A word, followed by, in parentheses, parameters for the attribute.
These parameters take one of the following forms:
* An identifier. For example, `mode' attributes use this form.
* An identifier followed by a comma and a non-empty
comma-separated list of expressions. For example, `format'
attributes use this form.
* A possibly empty comma-separated list of expressions. For
example, `format_arg' attributes use this form with the list
being a single integer constant expression, and `alias'
attributes use this form with the list being a single string
An "attribute specifier list" is a sequence of one or more attribute
specifiers, not separated by any other tokens.
In GNU C, an attribute specifier list may appear after the colon
following a label, other than a `case' or `default' label. The only
attribute it makes sense to use after a label is `unused'. This
feature is intended for code generated by programs which contains labels
that may be unused but which is compiled with `-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 `#ifdef' conditional. GNU C++ does not permit such
placement of attribute lists, as it is permissible for a declaration,
which could begin with an attribute list, to be labelled in C++.
Declarations cannot be labelled in C90 or C99, so the ambiguity does
not arise there.
An attribute specifier list may appear as part of a `struct', `union'
or `enum' specifier. It may go either immediately after the `struct',
`union' or `enum' keyword, or after the closing brace. The former
syntax is preferred. 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.
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), or to a particular
declarator within a declaration. Where an attribute specifier is
applied to a parameter declared as a function or an array, it should
apply to the function or array rather than the pointer to which the
parameter is implicitly converted, but this is not yet correctly
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,
`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 obsolescent usage where a type of `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.
At present, the first parameter in a function prototype must have some
type specifier which is not an attribute specifier; this resolves an
ambiguity in the interpretation of `void f(int (__attribute__((foo))
x))', but is subject to change. At present, if the parentheses of a
function declarator contain only attributes then those attributes are
ignored, rather than yielding an error or warning or implying a single
parameter of type int, but this is subject to change.
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. Such attribute specifiers apply only to the
identifier before whose declarator they appear. For example, in
__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...),
the `noreturn' attribute applies to all the functions declared; the
`format' attribute only applies to `d1'.
An attribute specifier list may appear immediately before the comma,
`=' or semicolon terminating the declaration of an identifier other
than a function definition. Such attribute specifiers apply to the
declared object or function. Where an assembler name for an object or
function is specified ( Asm Labels), the attribute must follow
the `asm' specification.
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).
Attribute specifiers may be mixed with type qualifiers appearing inside
the `' of a parameter array declarator, in the C99 construct by which
such qualifiers are applied to the pointer to which the array is
implicitly converted. Such attribute specifiers apply to the pointer,
not to the array, but at present this is not implemented and they are
An attribute specifier list may appear at the start of a nested
declarator. At present, there are some limitations in this usage: the
attributes correctly apply to the declarator, but for most individual
attributes the semantics this implies are not implemented. When
attribute specifiers follow the `*' of a pointer declarator, they may
be mixed with any type qualifiers present. The following describes the
formal semantics of this syntax. It will make the most sense if you
are familiar with the formal specification of declarators in the ISO C
Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration `T D1',
where `T' contains declaration specifiers that specify a type TYPE
(such as `int') and `D1' is a declarator that contains an identifier
IDENT. The type specified for IDENT for derived declarators whose type
does not include an attribute specifier is as in the ISO C standard.
If `D1' has the form `( ATTRIBUTE-SPECIFIER-LIST D )', and the
declaration `T D' specifies the type "DERIVED-DECLARATOR-TYPE-LIST
TYPE" for IDENT, then `T D1' specifies the type
"DERIVED-DECLARATOR-TYPE-LIST ATTRIBUTE-SPECIFIER-LIST TYPE" for IDENT.
If `D1' has the form `* TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST
D', and the declaration `T D' specifies the type
"DERIVED-DECLARATOR-TYPE-LIST TYPE" for IDENT, then `T D1' specifies
the type "DERIVED-DECLARATOR-TYPE-LIST
TYPE-QUALIFIER-AND-ATTRIBUTE-SPECIFIER-LIST TYPE" for IDENT.
void (__attribute__((noreturn)) ****f) (void);
specifies the type "pointer to pointer to pointer to pointer to
non-returning function returning `void'". As another example,
char *__attribute__((aligned(8))) *f;
specifies the type "pointer to 8-byte-aligned pointer to `char'". Note
again that this does not work with most attributes; for example, the
usage of `aligned' and `noreturn' attributes given above is not yet
For compatibility with existing code written for compiler versions that
did not implement attributes on nested declarators, some laxity is
allowed in the placing of attributes. If an attribute that only applies
to types is applied to a declaration, it will be treated as applying to
the type of that declaration. If an attribute that only applies to
declarations is applied to the type of a declaration, it will be treated
as applying to that declaration; and, for compatibility with code
placing the attributes immediately before the identifier declared, such
an attribute applied to a function return type will be treated as
applying to the function type, and such an attribute applied to an array
element type will be treated as applying to the array type. If an
attribute that only applies to function types is applied to a
pointer-to-function type, it will be treated as applying to the pointer
target type; if such an attribute is applied to a function return type
that is not a pointer-to-function type, it will be treated as applying
to the function type.
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