(gcc.info.gz) Attribute Syntax

 
 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
           constant.
 
  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
 implemented.
 
  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 *, ...),
           d2 (void)
 
 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
 ignored.
 
  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
 standard.
 
  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.
 
  For example,
 
      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
 supported.
 
  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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