Linux: Capture output of an already running process ( in pure C! )

Question

My situation is the following: I've got a lot of small gizmos ( pretty close to routers, not exactly but anyway that's irrelevant) ; they are running a bare-bones MIPS-based Linux distro.

To control them, one can telnet there ( thru serial port ) and issue commands to an interactive bash-like shell which then writes back some output. The shell's input and output are both attached to /dev/ttyAS0.

Now, I'd like to automate all of this, i.e. write a program that will run inside the gizmo, be a small server listening on some port, and which would pass on any command to the said shell, capture shell's output and relay it back to whoever contacted to server.

I:

1) can install (small, <500KB) programs inside the gizmo 2) can't modify the OS, startup scripts, the shell, anything 3) have root access 4) know how to write a SOAP server 5) know how to get a SOAP message, translate it to a command and inject it into /dev/ttyAS0 6) DONT KNOW how to capture the shell's reply 7) know how to, having shell's reply, translate it back to a SOAP message and reply to the original inquirer.

So basically, the problem is 6) : how to, having injected a string to /dev/ttyAS0 and thus having made the shell execute it, capture the shell's output ?

I am aware of

http://etbe.coker.com.au/2008/02/27/redirecting-output-from-a-running-process/

i.e. I know that I could change the shell's stdout if I had GDB ( or strace ) running inside the box, but I can't install it there - it's too big and anyway this approach seems too much like a hack.

So, summarizing:

How root can capture stdout of an already running process, IN PURE C, without gdb or strace, with no access to the way the process is started?

Or - almost equivalently - how to capture what's being written to a terminal, IN PURE C ?

Answer 1

You might want to take a look at reptyr. It will probably need some adaptation to work for your system though

Answer 2

Have you tried driving the serial port with a kermit script? I would probably forgo trying to insert a more clever proxy on the device and just try and drive the existing interface.

If you really want to get it on the device, you may be able to look at the source to something like screen or kermit to get a sense of how they interact with ttys.