I've dealt with this some. For representing 64-bit numbers to pass to the Win32 API functions that take them, I came up with a couple functions to use 8-byte binary buffers. I can't remember where I have that right now, though.

Another trick is when the actual realistic range is somewhat less. Although the parameter says 64-bit file size or whatever, I know it will really be a lot less. A floating-point value will hold a 56 bit (I think) mantissa without loss, so I use those.

—John