Function isdigit 

#[unsafe(no_mangle)]
pub extern "C" fn isdigit(c: c_int) -> c_int

See https://pubs.opengroup.org/onlinepubs/9799919799/functions/isdigit.html.

Tests whether c is a character of class digit in the current locale.

Returns a non-zero value if true, 0 if false.

The list of character classes defined by POSIX is specified here: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap07.html