You can also use (from the AVR C library):
The strtoul() function converts the string in \c 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 \c '+' or \c '-'
sign. If \c base is zero or 16, the string may then include a
\c "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
\c '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 \c 'A' in either upper or lower case represents 10, \c 'B'
represents 11, and so forth, with \c 'Z' representing 35.)
If \c endptr is not NULL, strtoul() stores the address of the first
invalid character in \c *endptr. If there were no digits at all,
however, strtoul() stores the original value of \c nptr in \c
*endptr. (Thus, if \c *nptr is not \c '\\0' but \c **endptr is \c '\\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 \c
errno is set to \ref avr_errno "ERANGE". If no conversion could
be performed, 0 is returned.
extern unsigned long strtoul(const char *__nptr, char **__endptr, int __base);