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

Sadly Stroustrup was careless :-(
And yes, latin letters codes should be non-negative (and no cast are required)...
Some implementations correctly works without casting to unsigned char...
By the some experience, it may cost a several hours to find the cause of segfault of a such toupper (when it is known that a segfault are there)...
And there are also isupper, islower etc