[C] catch stderr of a process

[flowjob]

Ok,I'm quite new to C,and for fun and training I want to make something that catches stderr and prints it to the screen,but I don't know how I can attach to a process to be able to read it's stdin/stderr...

So my question is,how can I attach to a process and read its stderr in C?

[ca0s]

Look for pipes.
I have not read it all, but I suppose you could do it like this
http://jineshkj.wordpress.com/2006/12/22/how-to-capture-stdin-stdout-and-stderr-of-child-program/

[flowjob]

It's quite helpful about the howto in general ,but doesn't really solves my problem,because...

1. I only want to catch stderr
2. I want to attach to a existing process instead of creating a new one...

[ca0s]

Attaching is more complicated. Look for ptrace. Try to hook write() syscalls which have '2' as file descriptor.
http://www.linuxjournal.com/article/6100
You could also code a dynamic library and override write calls, using the LD_PRELOAD env. variable.
http://scaryreasoner.wordpress.com/2007/11/17/using-ld_preload-libraries-and-glibc-backtrace-function-for-debugging/

[flowjob]

So I've to do something like debugging the process I want to attach to?