"Keywords" are words that have special meaning to the C compiler. In translation phases 7 and 8, an identifier cannot have the same spelling and case as a C keyword. (See a description of translation phases in the Preprocessor Reference; for information on identifiers, see Identifiers.) The C language uses the following keywords:

:::row::: :::column::: auto
double
int
struct
break
else
long
switch :::column-end::: :::column::: case
enum
register
typedef
char
extern
return
union :::column-end::: :::column::: const
float
short
unsigned
continue
for
signed
void :::column-end::: :::column::: default
goto
sizeof
volatile
do
if
static
while :::column-end::: :::row-end:::

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

Microsoft Specific

The ANSI C standard allows identifiers with two leading underscores to be reserved for compiler implementations. Therefore, the Microsoft convention is to precede Microsoft-specific keyword names with double underscores. These words cannot be used as identifier names. For a description of the ANSI 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:

:::row::: :::column::: __asm³
dllimport²
__int8³
naked²
__based^{1, 3} :::column-end::: :::column::: __except³
__int16³
__stdcall³
__cdecl³
__fastcall :::column-end::: :::column::: __int32³
thread²
__declspec³
__finally³
__int64³ :::column-end::: :::column::: __try³
dllexport²
__inline³
__leave³ :::column-end::: :::row-end:::

¹ The __based keyword has limited uses for 32-bit and 64-bit target compilations.

² These are special identifiers when used with __declspec; their use in other contexts is not restricted.

³ For compatibility with previous versions, these keywords are available both with two leading underscores and a single leading underscore when Microsoft extensions are enabled.

Microsoft extensions are enabled by default. To ensure that your programs are fully portable, you can disable Microsoft extensions by specifying the /Za (Disable language extensions) option during compilation. When you do this, some Microsoft-specific keywords are disabled.

When Microsoft extensions are enabled, you can use the keywords listed above in your programs. For ANSI compliance, most of these keywords are prefaced by a double underscore. The four exceptions, dllexport, dllimport, naked, and thread, are used only with __declspec and therefore do not require a leading double underscore. For backward compatibility, single-underscore versions of the rest of the keywords are supported.

END Microsoft Specific

Elements of C