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(clarifying the status of deprecated keywords)
 
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{{title|C keywords}}
 
{{title|C keywords}}
−
{{c/keyword/sidebar}}
+
{{c/keyword/navbar}}
  
−
This is a list of reserved keywords in C. Since they are used by the language, these keywords are not available for re-definition.  
+
This is a list of reserved keywords in C. Since they are used by the language, these keywords are not available for re-definition. {{rev inl|since=c23|As an exception, they are not considered reserved in [[c/language/attributes|{{spar|attribute-token}}]]s}}
  
−
{| class="wikitable"
+
{|class="wikitable"
 
|-
 
|-
 
|
 
|
 +
{{rlt|alignas}} {{mark c23}}<br>
 +
{{rlt|alignof}} {{mark c23}}<br>
 
{{rlt|auto}}<br>
 
{{rlt|auto}}<br>
 +
{{rlt|bool}} {{mark c23}}<br>
 
{{rlt|break}}<br>
 
{{rlt|break}}<br>
 
{{rlt|case}}<br>
 
{{rlt|case}}<br>
 
{{rlt|char}}<br>
 
{{rlt|char}}<br>
 
{{rlt|const}}<br>
 
{{rlt|const}}<br>
 +
{{rlt|constexpr}} {{mark c23}}<br>
 
{{rlt|continue}}<br>
 
{{rlt|continue}}<br>
 
{{rlt|default}}<br>
 
{{rlt|default}}<br>
Line 18: Line 22:
 
{{rlt|else}}<br>
 
{{rlt|else}}<br>
 
{{rlt|enum}}<br>
 
{{rlt|enum}}<br>
−
{{rlt|extern}}<br>
 
−
{{rlt|float}}
 
 
|
 
|
 +
{{rlt|extern}}<br>
 +
{{rlt|false}} {{mark c23}}<br>
 +
{{rlt|float}}<br>
 
{{rlt|for}}<br>
 
{{rlt|for}}<br>
 
{{rlt|goto}}<br>
 
{{rlt|goto}}<br>
 
{{rlt|if}}<br>
 
{{rlt|if}}<br>
−
{{rlt|inline}} {{mark since c99}}<br>
+
{{rlt|inline}} {{mark c99}}<br>
 
{{rlt|int}}<br>
 
{{rlt|int}}<br>
 
{{rlt|long}}<br>
 
{{rlt|long}}<br>
 +
{{rlt|nullptr}} {{mark c23}}<br>
 
{{rlt|register}}<br>
 
{{rlt|register}}<br>
−
{{rlt|restrict}} {{mark since c99}}<br>
+
{{rlt|restrict}} {{mark c99}}<br>
 
{{rlt|return}}<br>
 
{{rlt|return}}<br>
 
{{rlt|short}}<br>
 
{{rlt|short}}<br>
 
{{rlt|signed}}<br>
 
{{rlt|signed}}<br>
−
{{rlt|sizeof}}
 
 
|
 
|
 +
{{rlt|sizeof}}<br>
 
{{rlt|static}}<br>
 
{{rlt|static}}<br>
 +
{{rlt|static_assert}} {{mark c23}}<br>
 
{{rlt|struct}}<br>
 
{{rlt|struct}}<br>
 
{{rlt|switch}}<br>
 
{{rlt|switch}}<br>
 +
{{rlt|thread_local}} {{mark c23}}<br>
 +
{{rlt|true}} {{mark c23}}<br>
 
{{rlt|typedef}}<br>
 
{{rlt|typedef}}<br>
 +
{{rlt|typeof}} {{mark c23}}<br>
 +
{{rlt|typeof_unqual}} {{mark c23}}<br>
 
{{rlt|union}}<br>
 
{{rlt|union}}<br>
 
{{rlt|unsigned}}<br>
 
{{rlt|unsigned}}<br>
Line 43: Line 54:
 
{{rlt|volatile}}<br>
 
{{rlt|volatile}}<br>
 
{{rlt|while}}<br>
 
{{rlt|while}}<br>
−
{{rlt|_Bool}} {{mark since c99}}<br>
+
|
−
{{rlt|_Complex}} {{mark since c99}}<br>
+
{{rlt|_Alignas}} {{mark c11}} {{mark deprecated c23}}<br>
−
{{rlt|_Imaginary}} {{mark since c99}}
+
{{rlt|_Alignof}} {{mark c11}} {{mark deprecated c23}}<br>
 +
{{rlt|_Atomic}} {{mark c11}}<br>
 +
{{rlt|_BitInt}} {{mark c23}}<br>
 +
{{rlt|_Bool}} {{mark c99}} {{mark deprecated c23}}<br>
 +
{{rlt|_Complex}} {{mark c99}}<br>
 +
{{rlt|_Decimal128}} {{mark c23}}<br>
 +
{{rlt|_Decimal32}} {{mark c23}}<br>
 +
{{rlt|_Decimal64}} {{mark c23}}<br>
 +
{{rlt|_Generic}} {{mark c11}}<br>
 +
{{rlt|_Imaginary}} {{mark c99}}<br>
 +
{{rlt|_Noreturn}} {{mark c11}} {{mark deprecated c23}}<br>
 +
{{rlt|_Static_assert}} {{mark c11}} {{mark deprecated c23}}<br>
 +
{{rlt|_Thread_local}} {{mark c11}} {{mark deprecated c23}}<br>
 
|}
 
|}
  
−
Also, each name that contains a double underscore __ or begins with an underscore followed by an uppercase letter is always reserved to the implementation and should not be used as an identifier. Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace; such names may be used as identifiers in user-defined namespaces, as names of class members, etc.
+
The most common keywords that begin with an underscore are generally used through their convenience macros:
 +
{|class="wikitable"
 +
|-
 +
|keyword
 +
|used as
 +
|defined in
 +
|-
 +
|{{rlt|_Alignas}} {{mark c11}} {{mark deprecated c23}}
 +
|{{ltt|c/types|alignas}} {{mark until c23|removed=yes}}
 +
|{{tt|stdalign.h}}
 +
|-
 +
|{{rlt|_Alignof}} {{mark c11}} {{mark deprecated c23}}
 +
|{{ltt|c/types|alignof}} {{mark until c23|removed=yes}}
 +
|{{tt|stdalign.h}}
 +
|-
 +
|{{rlt|_Atomic}} {{mark c11}}
 +
|{{ltt|c/thread|atomic_bool, atomic_int, ...}}
 +
|{{tt|stdatomic.h}}
 +
|-
 +
|{{rlt|_BitInt}} {{mark c23}}
 +
|(no macro)
 +
|
 +
|-
 +
|{{rlt|_Bool}} {{mark c99}} {{mark deprecated c23}}
 +
|{{ltt|c/types|bool}} {{mark until c23|removed=yes}}
 +
|{{tt|stdbool.h}}
 +
|-
 +
|{{rlt|_Complex}} {{mark c99}}
 +
|{{ltt|c/numeric/complex/complex}}
 +
|{{tt|complex.h}}
 +
|-
 +
|{{rlt|_Decimal128}} {{mark c23}}
 +
|(no macro)
 +
|
 +
|-
 +
|{{rlt|_Decimal32}} {{mark c23}}
 +
|(no macro)
 +
|
 +
|-
 +
|{{rlt|_Decimal64}} {{mark c23}}
 +
|(no macro)
 +
|
 +
|-
 +
|{{rlt|_Generic}} {{mark c11}}
 +
|(no macro)
 +
|
 +
|-
 +
|{{rlt|_Imaginary}} {{mark c99}}
 +
|{{ltt|c/numeric/complex/imaginary}}
 +
|{{tt|complex.h}}
 +
|-
 +
|{{rlt|_Noreturn}} {{mark c11}} {{mark deprecated c23}}
 +
|{{ltt|c/types|noreturn}}
 +
|{{tt|stdnoreturn.h}}
 +
|-
 +
|{{rlt|_Static_assert}} {{mark c11}} {{mark deprecated c23}}
 +
|{{ltt|c/error/static_assert}} {{mark until c23|removed=yes}}
 +
|{{tt|assert.h}}
 +
|-
 +
|{{rlt|_Thread_local}} {{mark c11}} {{mark deprecated c23}}
 +
|{{ltt|c/thread/thread_local}} {{mark until c23|removed=yes}}
 +
|{{tt|threads.h}}
 +
|}
 +
 
 +
<!-- begin c23 -->
 +
Some keywords are deprecated and retained as alternative spellings for compatibility purposes. These can be used wherever the keyword can.
 +
{|class="wikitable"
 +
|-
 +
|keyword
 +
|alternative spelling
 +
|-
 +
|{{tt|alignas}} {{mark c23}}
 +
|{{tt|_Alignas}} {{mark c11}} {{mark deprecated c23}}
 +
|-
 +
|{{tt|alignof}} {{mark c23}}
 +
|{{tt|_Alignof}} {{mark c11}} {{mark deprecated c23}}
 +
|-
 +
|{{tt|bool}} {{mark c23}}
 +
|{{tt|_Bool}} {{mark c99}} {{mark deprecated c23}}
 +
|-
 +
|{{tt|static_assert}} {{mark c23}}
 +
|{{tt|_Static_assert}} {{mark c11}} {{mark deprecated c23}}
 +
|-
 +
|{{tt|thread_local}} {{mark c23}}
 +
|{{tt|_Thread_local}} {{mark c11}} {{mark deprecated c23}}
 +
|}
 +
 
 +
It is unspecified whether any of the spellings of these keywords, their alternate forms, or {{tt|true}} and {{tt|false}} is implemented as a predefined macro.
 +
<!-- end c23 -->
 +
 
 +
Each name that begins with a double underscore {{ttb|__}} or an underscore {{ttb|_}} followed by an uppercase letter is reserved: see [[c/language/identifier#Reserved_identifiers|identifier]] for details.
  
 
Note that digraphs {{tt|<%}}, {{tt|%>}}, {{tt|<:}}, {{tt|:>}}, {{tt|%:}}, and {{tt|%:%:}} provide an [[c/language/operator_alternative|alternative way to represent standard tokens]].
 
Note that digraphs {{tt|<%}}, {{tt|%>}}, {{tt|<:}}, {{tt|:>}}, {{tt|%:}}, and {{tt|%:%:}} provide an [[c/language/operator_alternative|alternative way to represent standard tokens]].
 +
 +
The following tokens are recognized by the [[c/preprocessor|preprocessor]] when they are used ''within'' the context of a preprocessor directive:
 +
{|class="wikitable"
 +
|-
 +
|
 +
{{ltt|c/preprocessor/conditional|if}}<br>
 +
{{ltt|c/preprocessor/conditional|elif}}<br>
 +
{{ltt|c/preprocessor/conditional|else}}<br>
 +
{{ltt|c/preprocessor/conditional|endif}}<br>
 +
|
 +
{{ltt|c/preprocessor/conditional|ifdef}}<br>
 +
{{ltt|c/preprocessor/conditional|ifndef}}<br>
 +
{{ltt|c/preprocessor/conditional|elifdef}} {{mark c23}}<br>
 +
{{ltt|c/preprocessor/conditional|elifndef}} {{mark c23}}<br>
 +
{{ltt|c/preprocessor/replace|define}}<br>
 +
{{ltt|c/preprocessor/replace|undef}}<br>
 +
|
 +
{{ltt|c/preprocessor/include|include}}<br>
 +
{{ltt|c/preprocessor/embed|embed}} {{mark c23}}<br>
 +
{{ltt|c/preprocessor/line|line}}<br>
 +
{{ltt|c/preprocessor/error|error}}<br>
 +
{{ltt|c/preprocessor/error|warning}} {{mark c23}}<br>
 +
{{ltt|c/preprocessor/impl|pragma}}<br>
 +
|
 +
{{ltt|c/preprocessor/conditional|defined}}<br>
 +
{{ltt|c/preprocessor/include|__has_include}} {{mark c23}}<br>
 +
{{ltt|c/preprocessor/embed|__has_embed}} {{mark c23}}<br>
 +
{{ltt|c/language/attributes#Attribute_testing|__has_c_attribute}} {{mark c23}}
 +
|}
 +
 +
The following tokens are recognized by the preprocessor when they are used ''outside'' the context of a preprocessor directive:
 +
{|class="wikitable"
 +
|-
 +
|
 +
{{ltt|c/preprocessor/impl|_Pragma}} {{mark c99}}
 +
|}
 +
 +
The following additional keywords are classified as extensions and conditionally-supported:
 +
{|class="wikitable"
 +
|-
 +
|
 +
{{ltt|c/language/asm|asm}}<br>
 +
{{rlt|fortran}}
 +
|}
 +
 +
===References===
 +
{{ref std c23}}
 +
{{ref std|section=6.4.1|title=Keywords|p=TBD}}
 +
{{ref std|section=J.5.9|title=The fortran keyword|p=TBD}}
 +
{{ref std|section=J.5.10|title=The asm keyword|p=TBD}}
 +
{{ref std end}}
 +
{{ref std c17}}
 +
{{ref std|section=6.4.1|title=Keywords|p=42-43}}
 +
{{ref std|section=J.5.9|title=The fortran keyword|p=422}}
 +
{{ref std|section=J.5.10|title=The asm keyword|p=422}}
 +
{{ref std end}}
 +
{{ref std c11}}
 +
{{ref std|section=6.4.1|title=Keywords|p=58-59}}
 +
{{ref std|section=J.5.9|title=The fortran keyword|p=580}}
 +
{{ref std|section=J.5.10|title=The asm keyword|p=580}}
 +
{{ref std end}}
 +
{{ref std c99}}
 +
{{ref std|section=6.4.1|title=Keywords|p=50}}
 +
{{ref std|section=J.5.9|title=The fortran keyword|p=514}}
 +
{{ref std|section=J.5.10|title=The asm keyword|p=514}}
 +
{{ref std end}}
 +
{{ref std c89}}
 +
{{ref std|section=3.1.1|title=Keywords}}
 +
{{ref std|section=G.5.9|title=The fortran keyword}}
 +
{{ref std|section=G.5.10|title=The asm keyword}}
 +
{{ref std end}}
 +
 +
===See also===
 +
{{dsc begin}}
 +
{{dsc see cpp|cpp/keyword|C++ keywords|nomono=true}}
 +
{{dsc end}}
 +
 +
{{langlinks|ar|cs|de|es|fr|it|ja|ko|pl|pt|ru|tr|zh}}

Latest revision as of 00:37, 8 November 2024

 
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-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

[edit] 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

[edit] See also

C++ documentation for C++ keywords
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=c/keyword&oldid=177573"