STRTOUL(3) Linux Programmer's Manual STRTOUL(3)
strtoul, strtoull, strtouq - convert a string to an unsigned long integer
unsigned long int strtoul(const char *nptr, char **endptr, int base);
unsigned long long int strtoull(const char *nptr, char **endptr,
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
XOPEN_SOURCE >= 600 || _BSD_SOURCE || _SVID_SOURCE || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99
The strtoul() function converts the initial part of the string in nptr to an unsigned long int value according to the given base, which
must be between 2 and 36 inclusive, or be the special value 0.
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 int value in the obvious manner, stopping at the first character which is not
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, strtoul()
stores the original value of nptr in *endptr (and returns 0). In particular, if *nptr is not '