C keywords - cppreference.com
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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)
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:
The following tokens are recognized by the preprocessor when they are used outside the context of a preprocessor directive:
The following additional keywords are classified as extensions and conditionally-supported:
References
- C23 standard (ISO/IEC 9899:2024):
- 6.4.1 Keywords (p: 53)
- J.5.9 The fortran keyword (p: 601)
- J.5.10 The asm keyword (p: 601)
- 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