Original Message:
I need to multiply two 64 bit numbers, but Lua is losing precision with big numbers. (for example 99999999999999999 is shown as 100000000000000000) After multiplying I need a 64 bit solution, so I need a way to limit the solution to 64 bits. (I know, if the solution would be precise, I could just use % 0x10000000000000000, so that would work too)
EDIT: With Lua 5.3 and the new 64 bit integer support, this problem doesn't exist anymore. Neat.
Lua uses double-precision floating points for all math, including integer arithmetic (see http://lua-users.org/wiki/FloatingPoint). This gives you about 53 bits of precision, which (as you've noticed) is less than you need.
There are a couple of different ways to get better precision in Lua. Your best bet is to find the most active such effort and piggy-back off it. In that case, your question has already been answered; check out What is the standard (or best supported) big number (arbitrary precision) library for Lua?
If your Lua distribution has packages for it, the easy answer is lmapm.