Is bool a native C type?

C99 has it in stdbool.h, but in C90 it must be defined as a typedef or enum:

typedef int bool;
#define TRUE  1
#define FALSE 0

bool f = FALSE;
if (f) { ... }

Alternatively:

typedef enum { FALSE, TRUE } boolean;

boolean b = FALSE;
if (b) { ... }

C99 added a builtin _Bool data type (see Wikipedia for details), and if you #include <stdbool.h>, it provides bool as a macro to _Bool.

You asked about the Linux kernel in particular. It assumes the presence of _Bool and provides a bool typedef itself in include/linux/types.h.

bool exists in the current C - C99, but not in C89/90.

In C99 the native type is actually called _Bool, while bool is a standard library macro defined in stdbool.h (which expectedly resolves to _Bool). Objects of type _Bool hold either 0 or 1, while true and false are also macros from stdbool.h.

Note, BTW, that this implies that C preprocessor will interpret #if true as #if 0 unless stdbool.h is included. Meanwhile, C++ preprocessor is required to natively recognize true as a language literal.