Yes, the user is the person who runs the program that they didn't write. They don't know that they called strtonum() testing for errno == 0 is not actually use

Yes, the user is the person who runs the program that they didn't write. They don't know that they called strtonum() testing for errno == 0 is not actually useful, since errno is not set except on error (that is by design of errno, it is always an indication of what the last error was, not that there was an error) and errstrp doesn't give the Klingon user you mentioned any information at all, since they can't read English. fafner said it better than I in https://news.ycombinator.com/item?id=9184734 https://news.ycombinator.com/item?id=9184734 and yes it does matter, for a standard libc function, because if you don't take care about setting standards, then your standards will be a mess.