C keywords

 

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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
asm	fortran

 

This is a list of reserved keywords in C. Since they are used by the language, these keywords are not available for re-definition. As an exception, they are not considered reserved in attribute-tokens(since C23)

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

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

keyword	used as	defined in
_Alignas (C11) (deprecated in C23)	alignas (removed in C23)	stdalign.h
_Alignof (C11) (deprecated in C23)	alignof (removed in C23)	stdalign.h
_Atomic (C11)	atomic_bool, atomic_int, ...	stdatomic.h
_BitInt (C23)	(no macro)
_Bool (C99) (deprecated in C23)	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) (deprecated in C23)	noreturn	stdnoreturn.h
_Static_assert (C11) (deprecated in C23)	static_assert (removed in C23)	assert.h
_Thread_local (C11) (deprecated in C23)	thread_local (removed in C23)	threads.h

Some keywords are deprecated and retained as alternative spellings for compatibility purposes. These can be used wherever the keyword can.

keyword	alternative spelling
alignas (C23)	_Alignas (C11) (deprecated in C23)
alignof (C23)	_Alignof (C11) (deprecated in C23)
bool (C23)	_Bool (C99) (deprecated in C23)
static_assert (C23)	_Static_assert (C11) (deprecated in C23)
thread_local (C23)	_Thread_local (C11) (deprecated in C23)

It is unspecified whether any of the spellings of these keywords, their alternate forms, or true and false is implemented as a predefined macro.

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

• C23 standard (ISO/IEC 9899:2024):

• 6.4.1 Keywords (p: TBD)

• J.5.9 The fortran keyword (p: TBD)

• J.5.10 The asm keyword (p: TBD)

• 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