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C keywords

From cppreference.net
< c
C
C language
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. As an exception, they are not considered reserved in attribute-token s . (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: 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

See also

C++ documentation for keywords