Comment by ekimekim

I think the most correct way would be to explicitly set the rounding mode to down or towards-zero, then calculate largest_double_that_fits = (double) LONG_MAX, then check value <= largest_double_that_fits. Since largest_double_that_fits is guarenteed to be <= LONG_MAX, and value <= largest_double_that_fits, it follows that value <= LONG_MAX.