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Python’s daemon PPID not equal to 1

Creating daemons in Linux is quite complex issue, but it’s very well documented in daemon(7) manual. Thankfully there is python-daemon module for Python 2 and 3 that implement PEP3143, so I’m using it.

Here comes the question: when I was playing with python-daemon module I was surprised that daemon’s PPID is not 1. Why?

Simple example:

import daemon
import time
import os

with open('/tmp/test.log', 'w') as logfile:
    c = daemon.DaemonContext(stdout=logfile)
    with c:
        print('In daemon...')
        for i in range(10):
            print('My PID is {}, PPID is {}'.format(os.getpid(), os.getppid()))
            time.sleep(2)

Content of test.log after 20 seconds from starting the above script (I recommend tail -f /tmp/test.log):

In daemon...
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736
My PID is 31397, PPID is 1736

It turned out that process with PID 1736 is /lib/systemd/systemd:

patryk@mycomp:/tmp$ ps -fq 1736
UID        PID  PPID  C STIME TTY          TIME CMD
patryk    1736     1  0 kwi12 ?        00:00:00 /lib/systemd/systemd --user

Recently I was implementing daemons in C (on the same machine with systemd installed) and AFAIR all daemons had PPID = 1. All manuals I came across mention that daemon’s PPID is always 1.

Does systemd changed it? Does systemd process awaits for all processes – including daemons? Is it daemon’s correct behavior?

Related questions:

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Answer

The short answer is: It doesn’t matter.

The important thing is that all daemons have a parent process like init that will reap children when they die, by calling wait(). Otherwise the process will become a zombie when it exits.

There’s nothing particularly special about having a parent PID of 1. The manual page you linked to says this process is PID 1 for old, SysV-style daemons, but doesn’t say so for new SystemD-style daemons. init always runs as PID 1, and traditionally it was the parent of daemons. But it doesn’t need to be so.

systemd --user manages user services. Thus, it makes sense why this process becomes your daemon’s parent process when you run it (as a user).

One must be careful when reading documentation about *nix, a platform that’s been around for decades. Things change and manuals become obsolete, or can be interpreted in the wrong context. SystemD brings major change to a lot of things about the Linux platform.

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