That would allow a double with a fractional part to return true though, which is not the intention. If you do this before the casted comparison, the cast shouldn't produce an undefined value.

I think this is a good solution without using the math library and having access to the FP status registers. Otherwise the lrint solution seems better.

    int fits_long(double value)
    {
        double max = (double) (1L << 52)
        double min = (double) -max;

        if( min <= value && value <= max)
            return( (double) (long) value == value);
        return( 0);
    }