Can isdigit legitimately be locale dependent in C

Question

In the section covering setlocale, the ANSI C standard states in a footnote that the only ctype.h functions whose behaviour is not affected by the current locale are isdigit

Answer 1

The required character set is defined in section 2.2.1. Section 2.2.1.2 then goes on to describe the behavior of extension characters:

• The single-byte characters defined in $2.2.1 shall be present.
• The presence, meaning, and representation of any additional members is locale-specific.