(This FAQ may be outdated. Please refer to official help).
I'm sure we won't need this post after Codeforces comes out of beta, but meanwhile it could be useful :) Feel free to suggest what should be added here.
Competitions: generalQ. What are the rules of these contests?
Q. How do I hack other's solutions?
A. First lock your own solution in the main ('problems') tab. Then in your room double click or Ctrl+click the solution you want to hack. Check this post for complete rules concerning hacking.
Q. How is rating calculated?
A. The rating system is described in these two posts. Note that even for a joined contest the rating updates are calculated separately for the first and the second divisions. Therefore, a situation is possible, when a div2-coder performed worse than a div1-coder in the same contest, but gained more rating points.
Competitions: writing code
Q. How do I read/write 64bit integers in C/C++?
A. If you submit under "GNU C++" or "GNU C" option, use this:
but NOT this:
If you submit under "MS C++" option, both variants should work.
And of course, with C++ you can always write
std::cout << n << "\n";
Q. How can my program determine whether it runs in Codeforces server?
A. For most programming languages Codeforces environment defines the symbol ONLINE_JUDGE. Again, see this post for details.
Using the site
Q. How do I read other people's code?
A. After the contest is over, go to its page (for example, http://codeforces.com/contest/15 ) and click on this picture: . You will see a page with several submits (currently 100, I think). You can sort and filter the solutions there. To see the fastest solutions, you need to manually paste a URL like this: http://codeforces.com/contest/15/status/A?order=BY_ARRIVED_ASC
(currently there's no way to do this via UI)
Q. How can I find some useful information on this site?
Q. Why are my comments / post empty?
Unable to parse markup [type=CF_TEX]formulas which are enabled by this symbol. Try to avoid it.
Q. Can I post a highlighted code sample?
A. Currently the best way is to submit the code to a service like codepad.org and post only links to the submit.
Q. I've found a bug! When I upvote a post, its rating increases by 2 points, but when I downvote a post, its rating decreases only by 1 point.
A. It's not a bug, it's a feature.