nptr

    A pointer to the character string to be converted to an unsigned
    long.

 endptr

    The address of an object where the function can store a pointer
    to a pointer to the first unrecognized character encountered in
    the conversion process (that is, the character that follows the
    last character in the string being converted). If endptr is a
    NULL pointer, the address of the first unrecognized character is
    not retained.

 base

    The value, 2 through 36, to use as the base for the conversion.
    Leading zeros after the optional sign are ignored, and 0x or 0X
    is ignored if the base is 16.

    If the base is 0, the sequence of characters is interpreted by
    the same rules used to interpret an integer constant: after
    the optional sign, a leading 0 indicates octal conversion, a
    leading 0x or 0X indicates hexadecimal conversion, and any other
    combination of leading characters indicates decimal conversion.