unsigned long int strtoul(const char *nptr, char **endptr, int base);
strtoul converts the initial portion of the string pointed to by nptr to a long int representation.
First, strtoul decomposes the input string into three parts: an initial, possibly empty, sequence of white-space characters (as specified by isspace), a subject sequence resembling an integer represented in some radix determined by the value of base, and a final string of one or more unrecognized characters, including the terminating null character of the input string. strtoul then attempts to convert the subject sequence to an integer, and return the result.
When converting, no integer suffix (such as U, L, UL, LL, ULL) is allowed.
If the value of base is zero, the expected form of the subject sequence is an optional plus or minus sign followed by an integer constant.
If the value of base is between 2 and 36 (inclusive), the expected form of the subject sequence is an optional plus or minus sign followed by a sequence of letters and digits representing an integer with the radix specified by base. The letters from a (or A) through z (or Z) represent the values 10 through 35; only letters and digits whose ascribed values are less than that of base are permitted.
If the value of base is 16, the characters ‘0x’ or ‘0X’ may optionally precede the sequence of letters and digits, following the optional sign.
The subject sequence is defined as the longest initial subsequence of the input string, starting with the first non-white-space character, that is of the expected form. The subject sequence contains no characters if the input string is empty or consists entirely of white space, or if the first non-white-space character is other than a sign or a permissible letter or digit.
If the subject sequence has the expected form and the value of base is zero, the sequence of characters starting with the first digit is interpreted as an integer constant. If the subject sequence has the expected form and the value of base is between 2 and 36, it is used as the base for conversion.
If the subject sequence begins with a minus sign, the value resulting from the conversion is negated.
A pointer to the final string is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
If the subject sequence is empty or does not have the expected form, no conversion is performed, the value of nptr is stored in the object pointed to by endptr, provided that endptr is not a null pointer.
strtoul returns the converted value, if any. If no conversion could be performed, zero is returned. If the correct value is outside the range of representable values, LONG_MAX or ULONG_MAX is returned according to the sign of the value, if any, and the value of the macro ERANGE is stored in errno.