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.