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. 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) `_Alignof` (C11) `_Atomic` (C11) `_BitInt` (C23) `_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` (removed in C23)	`stdalign.h`
`_Alignof` (C11)	`alignof` (removed in C23)	`stdalign.h`
`_Atomic` (C11)	`atomic_bool, atomic_int, ...`	`stdatomic.h`
`_BitInt` (C23)	(no macro)
`_Bool` (C99)	`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)	`noreturn`	`stdnoreturn.h`
`_Static_assert` (C11)	`static_assert` (removed in C23)	`assert.h`
`_Thread_local` (C11)	`thread_local` (removed in C23)	`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` `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 9899:2023):

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

C keywords

References

See also