Attribute specifier sequence(since C23)

Introduces implementation-defined attributes for types, objects, expressions, etc.

[[attr]] [[attr1, attr2, attr3(args)]] [[attribute-prefix::attr(args)]]

Formally, the syntax is


`[[` attribute-list `]]`		(since C23)

where attribute-list is a comma-separated sequence of zero or more attribute-tokens


standard-attribute	(1)

attribute-prefix `::` identifier	(2)

standard-attribute `(` argument-list `)`	(3)

attribute-prefix `::` identifier `(` argument-list `)`	(4)

1) standard attribute, such as [[fallthrough]]

2) attribute with a namespace, such as [[gnu::unused]]

3) standard attribute with arguments, such as [[deprecated("reason")]]

4) attribute with both a namespace and an argument list, such as [[gnu::nonnull(1)]]

Explanation

Attributes provide the unified standard syntax for implementation-defined language extensions, such as the GNU and IBM language extensions __attribute__((...)), Microsoft extension __declspec(), etc.

An attribute can be used almost everywhere in the C program, and can be applied to almost everything: to types, to variables, to functions, to names, to code blocks, to entire translation units, although each particular attribute is only valid where it is permitted by the implementation: [[expect_true]] could be an attribute that can only be used with an if, and not with a class declaration. [[omp::parallel()]] could be an attribute that applies to a code block or to a for loop, but not to the type int, etc. (note these two attributes are fictional examples, see below for the standard and some non-standard attributes)

In declarations, attributes may appear both before the whole declaration and directly after the name of the entity that is declared, in which case they are combined. In most other situations, attributes apply to the directly preceding entity.

Unlike alignas and [[noreturn]] in C++, the _Alignas and _Noreturn specifiers are not a part of the attribute specifier sequence.

Two consecutive left square bracket tokens ([[) may only appear when introducing an attribute-specifier or inside an attribute argument.

Besides the standard attributes listed below, implementations may support arbitrary non-standard attributes with implementation-defined behavior. All attributes unknown to an implementation are ignored without causing an error.

Every standard-attribute is reserved for standardization. That is, every non-standard attribute is in the attribute-prefix provided by the implementation, e.g. [[gnu::may_alias]] and [[clang::no_sanitize]].

Standard attributes

Only the following attributes are defined by the C standard. Every standard attribute whose name is of form attr can be also spelled as __attr__ and its meaning is not changed.

`[[deprecated]]`(C23) `[[deprecated("reason")]]`(C23)	indicates that the use of the name or entity declared with this attribute is allowed, but discouraged for some reason
`[[fallthrough]]`(C23)	indicates that the fall through from the previous case label is intentional and should not be diagnosed by a compiler that warns on fall-through
`[[nodiscard]]`(C23) `[[nodiscard("reason")]]`(C23)	encourages the compiler to issue a warning if the return value is discarded
`[[maybe_unused]]`(C23)	suppresses compiler warnings on unused entities, if any

Attribute testing


`__has_c_attribute(` attribute-token `)`

Checks for the presence of an attribute token named by attribute-token.

For standard attributes, it will expand to the year and month in which the attribute was added to the working draft (see table below), the presence of vendor-specific attributes is determined by a non-zero integer constant.

__has_c_attribute can be expanded in the expression of #if and #elif. It is treated as a defined macro by #ifdef, #ifndef and defined but cannot be used anywhere else.

attribute-token	Attribute	Value	Standard
`deprecated`	`[[deprecated]]`	201904L	(C23)
`fallthrough`	`[[fallthrough]]`	201904L	(C23)
`maybe_unused`	`[[maybe_unused]]`	201904L	(C23)
`nodiscard`	`[[nodiscard]]`	202003L	(C23)

Example

Run this code

[[gnu::always_inline]] [[gnu::hot]] [[gnu::const]] [[nodiscard]]
int f(void); // declare f with four attributes
 
[[gnu::always_inline, gnu::const, gnu::hot, nodiscard]]
int f(void); // same as above, but uses a single attr specifier that contains four attributes
 
int f(void) { return 0; }
 
int main(void)
{
}

External links

Attributes in Clang

Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Atomic operations (C11)
Thread support (C11)
Technical Specifications

Basic concepts
Keywords
Preprocessor
Statements
Expressions
Initialization
Declarations
Functions
Miscellaneous
History of C
Technical Specifications

pointer
array
enum
struct
union
bit field
atomic types (C11)
const
volatile
restrict (C99)
_Alignas (C11)
storage duration and linkage
external and tentative definitions
typedef
_Static_assert (C11)
attributes (C23)

deprecated (C23)
fallthrough (C23)
nodiscard (C23)
maybe_unused (C23)