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):
autobreakcasecharconstcontinuedefaultdodoubleelseenumexternfloatforgotoifintlongregisterreturnshortsignedstaticstructswitchtypedefunionunsignedvoidvolatilewhile
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. */