I'm 100% human, but also 100% amateur. Until 5 minutes ago, I understood <ctype h> to be necessary for me to use typedef to enable me to refer to structs by an alias, instead of writing "struct"! The perils of being self-taught. Now I can see the only errors thrown up when I remove the include are isdigit and isalnum! This is why I posted on HN!

EDIT: I think I can fix the isalnum bug risk that you identified by checking that the value of the ch variable in the get_string_input function is non-negative before allowing it to be appended to str.