Is bool a native C type?
CGccLinux KernelBooleanC Problem Overview
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?
C Solutions
Solution 1 - C
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.
Solution 2 - C
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.
Solution 3 - C
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) { ... }
Solution 4 - C
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
Solution 5 - C
/* 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. */
Solution 6 - C
_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.
Solution 7 - C
C99 defines bool, true and false in stdbool.h.
Solution 8 - C
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.
Solution 9 - C
http://en.wikipedia.org/wiki/Stdbool.h">stdbool.h</a> was introduced in c99
Solution 10 - C
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).
Solution 11 - C
No such thing, probably just a macro for int