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

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.

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.