__ctype_b_loc Definition + Documentation

[Andrii Dokhniak]

Hi, I am a noob. Sorry about that.
I started by trying to figure out why `isalpha()` returns 1024 instead of 1
in case of true. I expanded that macro, and it lead me to a call to
`__ctype_b_loc()`.

When reading file `ctype.h` (e64a1e81aadf6c401), line 63, I saw this
```c
/* These are defined in ctype-info.c.
   The declarations here must match those in localeinfo.h.
....
*/
extern const unsigned short int **__ctype_b_loc (void)
     __THROW __attribute__ ((__const__));
```
But when reading file `ctype-info.c` (e64a1e81aadf6c401), or any other `.c`
file in `/ctype` directory I have not found another mention of
`__ctype_b_loc`.

So my question is: Where is `__ctype_b_loc` actually defined, is it some
sort of compiler built in? And if the documentation is wrong, where can I
read more about it?.

I found some old thread on reddit, with some source code of that function,
but I have no idea where they got it