Dividing large numbers beyond what a long\double can hold

I'm working on a program where I need to calculate large factorials (greater than 170 or whatever the max is that a double can handle). I have found a function that can do the factorial calculation and output it as a string, but with it in string form I cannot perform any calculations on it. Does anyone know of a multiply and\or divide function that can take parameters as strings (including numbers with exponents)? Thanks!