Manual Pages  — STRTOUL

The string may begin with an arbitrary amount of white space (as determined by isspace(3)) followed by a single optional ‘+’ or ‘-’ sign. If base is zero or 16, the string may then include a "0x" prefix, and the number will be read in base 16; otherwise, a zero base is taken as 10 (decimal) unless the next character is ‘0’, in which case it is taken as 8 (octal).

The remainder of the string is converted to an unsigned long value in the obvious manner, stopping at the end of the string or at the first character that does not produce a valid digit in the given base. (In bases above 10, the letter ‘A’ in either upper or lower case represents 10, ‘B’ represents 11, and so forth, with ‘Z’ representing 35.)

If endptr is not NULL, strtoul() stores the address of the first invalid character in *endptr. If there were no digits at all, however, strtoul() stores the original value of nptr in *endptr. (Thus, if *nptr is not ‘\0’ but **endptr is ‘\0’ on return, the entire string was valid.)