Why do major compilers use typedef for stdint.h but use #define for stdbool.h?

Question

I just noticed that gcc and clang both appear to use typedefs for stdint.h but #define for stdbool.h.

example: clang's stdint.h

 #ifdef __INT8_TYPE__
 #ifndef __int8_t_defined  /* glibc sys/types.h also defines int8_t*/
 typedef __INT8_TYPE__ int8_t;
 #endif /* __int8_t_defined */
 typedef __UINT8_TYPE__ uint8_t;
 # define __int_least8_t int8_t
 # define __uint_least8_t uint8_t
 #endif /* __INT8_TYPE__ */

clang's stdbool.h

#ifndef __cplusplus
#define bool _Bool
#define true 1
#define false 0
#elif defined(__GNUC__) && !defined(__STRICT_ANSI__)
/* Define _Bool, bool, false, true as a GNU extension. */
#define _Bool bool
#define bool  bool
#define false false
#define true  true
#endif

Why isn't it typedef _Bool bool; ?

(gcc stdint.h and stdbool.h)

Answer 1

stdbool.h defines bool as a macro because the C standard (section 7.18) says bool shall be defined as a macro, and stdint.h defines intN_t etc as typedefs because the C standard (section 7.20) says intN_t etc shall be defined as typedefs.

Okay, why does the C standard say these things? I cannot tell you for sure, but a clue is in section 7.18 paragraph 4:

Notwithstanding the provisions of 7.1.3, a program may undefine and perhaps then redefine the macros bool, true, and false.

If bool were a typedef and true and false were, I don't know, enum constants, they couldn't have allowed you to do that, as there is no way to undo those kinds of declarations.

Okay, why does the C committee want to allow you to do that? This is even more speculative, but probably for the same reason they added stdbool.h and _Bool instead of making bool, true, and false keywords as they are in C++: they wanted to preserve compatibility with old programs that defined bool, true, and false themselves, even if those programs use third-party headers that include stdbool.h...

No such backward compatibility concerns apply to the types defined by stdint.h; some systems provided (some) of them as extensions, but they were always typedefs.

Answer 2

I think this is just part of the standard.

If you go to page 253, under "7.16 Boolean type and values ", it clearly says:

1) The header <stdbool.h> defines four macros.

2) The macro

bool

expands to _Bool.