C keywords

 

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Strings library

Algorithms

Numerics

Date and time utilities

Input/output support

Localization support

Concurrency support (C11)

Technical Specifications

Symbol index

 

C language

Basic concepts

Keywords

Preprocessor

Statements

Expressions

Initialization

Declarations

Functions

Miscellaneous

History of C

Technical Specifications

 

Keywords

alignas (C23) alignof (C23) auto bool (C23) break case char const constexpr (C23) continue default do double else enum	extern false (C23) float for goto if inline (C99) int long nullptr (C23) register restrict (C99) return short signed	sizeof static static_assert (C23) struct switch thread_local (C23) true (C23) typedef typeof (C23) typeof_unqual (C23) union unsigned void volatile while	_Alignas (C11) _Alignof (C11) _Atomic (C11) _BigInt (C23) _Bool (C99) _Complex (C99) _Decimal128 (C23) _Decimal32 (C23) _Decimal64 (C23) _Generic (C11) _Imaginary (C99) _Noreturn (C11) _Static_assert (C11) _Thread_local (C11)
conditionally-supported
fortran
asm

 

This is a list of reserved keywords in C. Since they are used by the language, these keywords are not available for re-definition.

alignas (C23) alignof (C23) auto bool (C23) break case char const constexpr (C23) continue default do double else enum

extern false (C23) float for goto if inline (C99) int long nullptr (C23) register restrict (C99) return short signed

sizeof static static_assert (C23) struct switch thread_local (C23) true (C23) typedef typeof (C23) typeof_unqual (C23) union unsigned void volatile while

_Alignas (C11) _Alignof (C11) _Atomic (C11) _BitInt (C23) _Bool (C99) _Complex (C99) _Decimal128 (C23) _Decimal32 (C23) _Decimal64 (C23) _Generic (C11) _Imaginary (C99) _Noreturn (C11) _Static_assert (C11) _Thread_local (C11)

The most common keywords that begin with an underscore are generally used through their convenience macros:

keyword	used as	defined in
_Alignas (C11)	alignas (removed in C23)	stdalign.h
_Alignof (C11)	alignof (removed in C23)	stdalign.h
_Atomic (C11)	atomic_bool, atomic_int, ...	stdatomic.h
_BitInt (C23)	(no macro)
_Bool (C99)	bool (removed in C23)	stdbool.h
_Complex (C99)	complex	complex.h
_Decimal128 (C23)	(no macro)
_Decimal32 (C23)	(no macro)
_Decimal64 (C23)	(no macro)
_Generic (C11)	(no macro)
_Imaginary (C99)	imaginary	complex.h
_Noreturn (C11)	noreturn	stdnoreturn.h
_Static_assert (C11)	static_assert (removed in C23)	assert.h
_Thread_local (C11)	thread_local (removed in C23)	threads.h

Also, each name that begins with a double underscore __ or an underscore _ followed by an uppercase letter is reserved: see identifier for details.

Note that digraphs <%, %>, <:, :>, %:, and %:%: provide an alternative way to represent standard tokens.

The following tokens are recognized by the preprocessor when they are used within the context of a preprocessor directive:

if elif else endif

ifdef ifndef elifdef (C23) elifndef (C23) define undef

include embed (C23) line error warning (C23) pragma

defined __has_include (C23) __has_embed (C23) __has_c_attribute (C23)

The following tokens are recognized by the preprocessor when they are used outside the context of a preprocessor directive:

_Pragma (C99)

The following additional keywords are classified as extensions and conditionally-supported:

asm fortran

References

• C17 standard (ISO/IEC 9899:2018):

• 6.4.1 Keywords (p: 42-43)

• J.5.9 The fortran keyword (p: 422)

• J.5.10 The asm keyword (p: 422)

• C11 standard (ISO/IEC 9899:2011):

• 6.4.1 Keywords (p: 58-59)

• J.5.9 The fortran keyword (p: 580)

• J.5.10 The asm keyword (p: 580)

• C99 standard (ISO/IEC 9899:1999):

• 6.4.1 Keywords (p: 50)

• J.5.9 The fortran keyword (p: 514)

• J.5.10 The asm keyword (p: 514)

• C89/C90 standard (ISO/IEC 9899:1990):

• 3.1.1 Keywords

• G.5.9 The fortran keyword

• G.5.10 The asm keyword

See also

C++ documentation for C++ keywords