windows - Pipe output(stdout) from running process Win32Api

Question

I need to get (or pipe) the output from a process that is already running, using the windows api.

Basically my application should allow the user to select a window to pipe the input from, and all input will be displayed in a console. I would also be looking on how to get a pipe on stderr later on.

Important: I did not start the process using CreateProcess() or otherwise. The process is already running, and all I have is the handle to the process (returned from GetWindowThreadProcessId()).

Answer 1

The cleanest way of doing this without causing any ill effects, such that may occur if you used the method Adam implied of swapping the existing stdout handle with your own, is to use hooking.

If you inject a thread into the existing application and swap calls to WriteFile with an intercepted version that will first give you a copy of what's being written (filtered by handle, source, whatever) then pass it along to the real ::WriteFile with no harm done. Or you can intercept the call higher up by only swapping out printf or whichever call it is that the software is using (some experimentation needed, obviously).

HOWEVER, Adam is spot-on when he says this isn't what you want to do. This is a last resort, so think very, very carefully before going down this line!