What about giving isdigit and friends defined behavior for any argument value that's within the range of any of char, signed char, or unsigned char?

The background (I know Doug knows this): isdigit() takes an argument of type int, which is required to be either within the range of unsigned char, or have the value EOF (required to be negative, typically -1).

The problem: plain char is often signed, typically with a range of -128..+127. You might have a negative char value in a string -- but passing any negative value other than EOF to isdigit() has undefined behavior. Thus to use isdigit() safely on arbitrary data, you have to cast the argument to unsigned char:

    if (isdigit((unsigned char)s[i])) ...

A lot of C programmers aren't aware of this and will pass arbitrary char values to isdigit() and friends -- which works fine most of the time, but risks going kaboom.

Changing this could raise issues if -1 is a valid character value and also the value of EOF, but practically speaking -1 or 0xff will almost never be a digit in any real-world character set. (It's ÿ in Unicode and Latin-1, which might cause problems for islower and isalnum.)