Description

   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.)