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 G开发者_运维知识库CC 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. */

_Bool is a keyword in C99: it specifies a type, just like int or double.

6.5.2

2 An object declared as type _Bool is large enough to store the values 0 and 1.

C99 defines bool, true and false in stdbool.h.

stdbool.h defines macros true and false, but remember they are defined to be 1 and 0.

That is why sizeof(true) equals sizeof(int), which is 4 for 32 bit architectures.

stdbool.h was introduced in c99

C99 added a bool type whose semantics are fundamentally different from those of just about all integer types that had existed before in C, including user-defined and compiler-extension types intended for such purposes, and which some programs may have "type-def"ed to bool.

For example, given bool a = 0.1, b=2, c=255, d=256;, the C99 bool type would set all four objects to 1. If a C89 program used typedef unsigned char bool, the objects would receive 0, 1, 255, and 0, respectively. If it used char, the values might be as above, or c might be -1. If it had used a compiler-extension bit or __bit type, the results would likely be 0, 0, 1, 0 (treating bit in a way equivalent to an unsigned bit-field of size 1, or an unsigned integer type with one value bit).

Since C23, bool, true and false are C keywords and don't require any #includes.

bool becomes one of the fundamental builtin data types.

_Bool remains valid and is treated as "Alternative Spelling".

The header <stdbool.h> provides only the obsolescent macro __bool_true_false_are_defined which expands to the integer constant 1.

You can find the latest draft here: https://open-std.org/JTC1/SC22/WG14/www/docs/n2912.pdf

No such thing, probably just a macro for int