Ask HN: How do sites like ideone, topcode compile code on webserver

Hi,<p>I would like to know how do sites like ideone.com, topcoder.com etc take the code, compile it in the backend, run it and come back with the results.<p>My hunch is that the code is moved into a text file and then its compiled via gcc/javac/other compilers and then execute it and come back with the results.<p>I have two questions.<p>1. Is this scalable, and if not how to scale it up 2. How is the code sandboxed so that some malicious code doesn't get executed<p>Thanks