Converts a given wide-character string to a double-precision
    number.

    Format

      #include  <wchar.h>

      double wcstod  (const wchar_t *nptr, wchar_t **endptr);

1  –  Arguments

 nptr

    A pointer to the wide-character string to be converted to a
    double-precision number.

 endptr

    The address of an object where the function can store the address
    of the first unrecognized wide character that terminates the
    scan. If endptr is a NULL pointer, the address of the first
    unrecognized wide character is not retained.

2  –  Description

    The wcstod function recognizes an optional sequence of white-
    space characters (as defined by iswspace), 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 in the program's current locale
    (category LC_NUMERIC).

    This function returns the converted value. For wcstod, 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 wide character, *endptr
    is set to nptr and a 0 value is returned.

3  –  Return Values

    x                  The converted string.
    0                  Indicates the conversion could not be
                       performed. The function sets errno to one
                       of:

                       o  EINVAL - No conversion could be performed.

                       o  ERANGE - The value would cause an
                          underflow.

                       o  ENOMEM - Not enough memory available for
                          internal conversion buffer.

    HUGE_VAL           Overflow occurred; errno is set to ERANGE.