Do I need to cast to unsigned char before calling toupper(), tolower(), et al.?

Question

A while ago, someone with high reputation here on Stack Overflow wrote in a comment that it is necessary to cast a char-argument to unsigned char b

Answer 1

The reference is referring to the value being representable as an unsigned char, not to it being an unsigned char. That is, the behavior is undefined if the actual value is not between 0 and UCHAR_MAX (typically 255). (Or EOF, which is basically the reason it takes an int instead of a char.)