Description

   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.