The strtod function recognizes an optional sequence of white-
    space characters (as defined by isspace), then an optional plus
    or minus sign, then a sequence of digits optionally containing a
    radix character, then an optional letter (e or E) followed by an
    optionally signed integer. The first unrecognized character ends
    the conversion.

    The string is interpreted by the same rules used to interpret
    floating constants.

    The radix character is defined the program's current locale
    (category LC_NUMERIC).

    This function returns the converted value. For strtod, overflows
    are accounted for in the following manner:

    o  If the correct value causes an overflow, HUGE_VAL (with a plus
       or minus sign according to the sign of the value) is returned
       and errno is set to ERANGE.

    o  If the correct value causes an underflow, 0 is returned and
       errno is set to ERANGE.

    If the string starts with an unrecognized character, then the
    conversion is not performed, *endptr is set to nptr, a 0 value is
    returned, and errno is set to EINVAL.)