关键字是C语言内部使用的名字，每个关键字具有对应的功能

auto
break
case
char
const
continue
default
do
double
else
enum
extern
float
for
goto
if
int
long
register
return
short
signed
sizeof
static
struct
switch
typedef
union
unsigned
void
volatile
while
inline
restrict
_Bool
_Complex
_Imaginary
_Alignas
_Alignof
_Atomic
_Static_assert
_Noreturn
_Thread_local
_Generic