C keywords

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

auto break case char const continue default do double else enum extern.

float for goto if inline (C99) int long register restrict (C99) return short.

signed sizeof static struct switch typedef union unsigned void volatile while.

_Alignas (C11) _Alignof (C11) _Atomic (C11) _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	stdalign.h
_Alignof (C11)	alignof	stdalign.h
_Atomic (C11)	atomic_bool, atomic_int, ...	stdatomic.h
_Bool (C99)	bool	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	assert.h
_Thread_local (C11)	thread_local	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 line error warning (C23) pragma .

defined __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