unsigned long strtoul(const char *__nptr, char **__endptr, int __base)
The strtoul() function converts the string in
nptr to an unsigned long 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()) 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 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, 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() function 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, and errno is set
to ERANGE. If no conversion
could be performed, 0 is returned.