It depends on the semantics of fits_long. There are values outside of your min/max range that are representable by 64-bit integers, but if you convert them to a double and back, you may get a different value (off by as much as 4096, since, IIRC, the specification allows any rounding strategy).

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);
    }

That's not how floating-point works. The IEEE 754 standard is quite clear that each floating-point datum represents exactly one real number (or infinity or NaN). Floating-point numbers are not intervals.

Analogously, `float x = 2.25; int a = x;` assigns the value two to a, but this does not imply that the integer two also represents 2.25. Two is just two.