The strtoul function converts the string in nptr to an
.Vt "unsigned long" value. The strtoull function converts the string in nptr to an
.Vt "unsigned long long" value. The strtoumax function converts the string in nptr to an
.Vt uintmax_t value. The strtouq function converts the string in nptr to a
.Vt u_quad_t value. The conversion is done 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
.Vt "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.)
The strtoul, strtoull, strtoumax and strtouq functions return either the result of the conversion or, if there was a leading minus sign, the negation of the result of the conversion, unless the original (non-negated) value would overflow; in the latter case, strtoul returns ULONG_MAX, strtoull returns ULLONG_MAX, strtoumax returns UINTMAX_MAX, and strtouq returns ULLONG_MAX. In all cases, errno is set to ERANGE. If no conversion could be performed, 0 is returned and the global variable errno is set to EINVAL (the last feature is not portable across all platforms).