The fcvt function converts value to a null-terminated string and
    returns a pointer to it. The resulting low-order digit is rounded
    to the correct digit for outputting ndigits digits in C F-format.
    The decpt argument is assigned the position of the decimal point
    relative to the first character in the string.

    In C F-format, ndigits is the number of digits desired after the
    decimal point. Very large numbers produce a very long string of
    digits before the decimal point, and ndigit of digits after the
    decimal point. For large numbers, it is preferable to use the
    gcvt or ecvt function so that E-format is used.

    Repeated calls to the fcvt function overwrite any existing
    string.

    The ecvt, fcvt, and gcvt functions represent the following
    special values specified in the IEEE Standard for floating-point
    arithmetic:

    Value         Representation

    Quiet NaN     NaNQ
    Signalling    NaNS
    NaN
    +Infinity     Infinity
    -Infinity     -Infinity

    The sign associated with each of these values is stored into the
    sign argument. In IEEE floating-point representation, a value
    of 0 (zero) can be positive or negative, as set by the sign
    argument.

    See also gcvt and ecvt.