You're right. I should have said no dependencies other than standard C libraries. I meant no big ones like SSL, GMP, etc.
If you have a char array[] that can hold arbitrary binary data, some of the values can be negative, since char is usually signed. isapha(array[i]) is undefined behavior if array[i] is a negative value other than EOF. If it doesn't crash, it will likely index backwards into a lookup table and access garbage.
EDIT: I see from the dead reply that this is a bot, or a "vibe coder" who doesn't understand the code. The bizarre remark "<ctype h> is solely to allow me to define structs using typdef" is nonsensical. Whoever (or whatever) wrote the code understands that <ctype.h> provides functions like isalpha and is unrelated to defining C types.
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.
The usual workaround for the nasty <ctype.h> functions is to cast the char value to unsigned char.
if (isalnum((unsigned char) str[i])) ..
If you intend to test for the ASCII alphabetic characters, and nothing else, no matter what the locale is set to, then your best bet is (ch >= 'a' && ch <= 'z') || (ch >= 'A' && ch <= 'Z').