Is bool a native C type?

Question

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.

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.

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) { ... }

No, there is no bool in ISO C90.

Here's a list of keywords in standard C (not C99):

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

Here's an article discussing some other differences with C as used in the kernel and the standard: http://www.ibm.com/developerworks/linux/library/l-gcc-hacks/index.html