How do we perform long arithmetic calculations in c++? Is there any builtin library with g++? I'd googled for it only to find all external(out of standard g++) libraries. If anyone has written a class for this purpose, please share the code here or mail it to me. Thanks.

No, such library doesn't exist. You've got three options:
1. Write it yourself.
2. Use Java, if you know it well.
3. Say «Well, it is definitely not my problem».

I would recommend option number two ;)