Description

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

   Repeated calls to the ecvt 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 fcvt.