C Keywords

Keywords are words that have special meaning to the C compiler. In translation phases 7 and 8, an identifier can't have the same spelling and case as a C keyword. For more information, see translation phases in the Preprocessor Reference. For more information on identifiers, see Identifiers.

Standard C keywords

The C language uses the following keywords:

auto
break
case
char
const
continue
default
do
double
else
enum

extern
float
for
goto
if
inline ^{1, a}
int
long
register
restrict ^{1, a}
return

short
signed
sizeof
static
struct
switch
typedef
typeof
typeof_unqual
union
unsigned
void
volatile

while
_Alignas ^{2, a}
_Alignof ^{2, a}
_Atomic ^{2, b}
_Bool ^{1, a}
_Complex ^{1, b}
_Generic ^{2, a}
_Imaginary ^{1, b}
_Noreturn ^{2, a}
_Static_assert ^{2, a}
_Thread_local ^{2, b}

¹ Keywords introduced in ISO C99.
² Keywords introduced in ISO C11.
^a Starting in Visual Studio 2019 version 16.8, these keywords are supported in code compiled as C when the /std:c11 or /std:c17 compiler options are specified.
^b Starting in Visual Studio 2019 version 16.8, these keywords are recognized but not supported by the compiler in code compiled as C when the /std:c11 or /std:c17 compiler options are specified.

You can't redefine keywords. However, you can specify text to replace keywords before compilation by using C preprocessor directives.

Microsoft-specific C keywords

The ANSI and ISO C standards allow identifiers with two leading underscores to be reserved for compiler implementations. The Microsoft convention is to precede Microsoft-specific keyword names with double underscores. These words can't be used as identifier names. For a description of the rules for naming identifiers, including the use of double underscores, see Identifiers.

The following keywords and special identifiers are recognized by the Microsoft C compiler:

__asm⁵
__based^{3, 5}
__cdecl⁵
__declspec⁵
__except⁵
__fastcall
__finally⁵

__inline⁵
__int16⁵
__int32⁵
__int64⁵
__int8⁵
__leave⁵
__restrict

__stdcall⁵
__try⁵
__typeof__
__typeof_unqual__
dllexport⁴
dllimport⁴
naked⁴
static_assert⁶
thread⁴

³ The __based keyword has limited uses for 32-bit and 64-bit target compilations.
⁴ These are special identifiers when used with __declspec; their use in other contexts is unrestricted.
⁵ For compatibility with previous versions, these keywords are available both with two leading underscores and a single leading underscore when Microsoft extensions are enabled.
⁶ If you don't include <assert.h>, the Microsoft Visual C compiler maps static_assert to the C11 _Static_assert keyword.

Microsoft extensions are enabled by default. To help create portable code, you can disable Microsoft extensions by specifying the /Za (Disable language extensions) option during compilation. When you use this option, some Microsoft-specific keywords are disabled.

When Microsoft extensions are enabled, you can use the keywords listed above in your programs. To conform to the language standard, most of these keywords have a leading double underscore. The four exceptions, dllexport, dllimport, naked, and thread, are used only with __declspec and don't require a leading double underscore. For backward compatibility, single-underscore versions of the rest of the keywords are supported.

See also

Elements of C