Is bool a native C type?

I've noticed that the Linux kernel code uses bool, but I thought that bool was a C++ type. Is bool a standard C extension (e.g., ISO C90) or a GCC extension?


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


/* Many years ago, when the earth was still cooling, we used this: */

typedef enum
{
    false = ( 1 == 0 ),
    true = ( ! false )
} bool;

/* It has always worked for me. */