notify user from service with python

Question

In the following text the word "write" refers to the unix write command which writes a message to another user's tty.

I have a service running (in inetd, but that shouldn't matter) which needs to notify an arbitrary user. Until now I tried to call the write command with subprocess. But that doesn't work always and on some machines leads to an error message like:

write: you are uid 65534, but your login is as uid 1000

(please note that the service is running as nobody.) Write permission to the terminal is turned on with mesg. So I looked at inetutils-talkd (the GNU implementation), which does the same. I saw that it uses the ttymsg function and believe that this function does what I want, but I think that this function isn't available to python.

So the main questions are: Is there something like the ttymsg function in python OR how can i get write to do what i want?

Answer 1

The file /var/run/utmp contains information about the active sessions on terminals. It can be parsed with:

import struct

sig = 'hi32s4s32s256shhiii36x'
size = struct.calcsize(sig)
file = open('/var/run/utmp', 'rb')
chunk = file.read(size)
entrys = []
while len(chunk) == size:
    entry = struct.unpack(chunk)
    entrys.append()
    chunk = file.read(size)

With that approach I can gather the needed data to choose a terminal and write to that terminal.