There is no package that can just be added to a blog so people can try C online and see the results.

You could provide a link to gcc or another compiler so your readers can try it themselves.

If you were really serious, a CGI script could be created on the server side which:

1. Runs in a sandbox (this is critical to get right and is potentially dangerous if done incorrectly)
2. Compiles the source code
3. Runs the compiled code
4. Returns the results

Sorry that I can't give you an easy answer, but there it is.

Originally I userstood your question to be targeting the native platform on which the browser is running:

Consider that Browsers may be running on many different platforms, operatinng systems and processor architectures. Compiling C in the way you describe might be technically doable, but practically infeasible.

I was basing "practically infeasible" on the difficulty of supporting the plethora of widely used browser platforms.

Now I understand that you are thinking more on the lines of targeting a virtual environment. I'll amend practically infeasible to "a large amount of work".

If I understand your intent it is to run a C compiler which emits, shall we say, x86 compiled code and executes it. So to do that we need an emulation of the x86 environment in, say, JavaScript. What's more I think your intent is that the conmpiler itself execute in this environment, so that you can re-use gcc. So you'll need to emulate a file-system too. It's "obvious" that this could be done, but it really is a lot of work. Is it really worth it?

Competition code is small (I guess) even with lots of programmers the number of simultaneous compiles can't be so huge with a decent queued request system, a touch of Ajax, and a bit of back-end scaling how costly is it to support the expected population? What's the ratio of developers to back end systems?

Anyway, if I were to address this problem I'd go for taking the code for an opensource browser and melding in the gcc code. Produce a compiler/browser hybrid. Give that to the developers and tell them "Use this and get zippy compilation speeds, or use your own browser and join the queue."