Question

I have a C program which calls fork()

And I have a python script which executes the C program with

child = subprocess.Popen(command, shell=True, stderr=subprocess.PIPE,stdout=subprocess.PIPE, bufsize=0)

Now I can read from stdout and stderr with child.stderr.read(1) or child.communicate(), … But my problem is now, how can I get only the output from the forked process. Is this even possible? Can I get the pid from both, the original C program and the fork?

kind regards,
thank you very much 🙂

Fabian

Answer 1

What you’re asking for is going to be complicated and will not be possible in pure python — you would need some OS-specific mechanisms.

You’re really asking for two things:

• Identify a forked process from the PID of the parent process
• Intercept the standard in/out of an arbitrary process

You could probably do the former by parsing /proc if you were on Linux, the latter is really a debugger-like piece of functionality (e.g. How can a process intercept stdout and stderr of another process on Linux?)

More likely, you will want to change the way your C program works — for example, doing the fork()/daemonization from the python script instead of intermediate C code would let you get the child process directly.