[systemd-devel] Help with making socket activation work.

[Andrei Borzenkov]

27.06.2025 14:59, Stef Bon wrote:
> Hi,
> 
> I've got a system daemon, and I want to make use of socket activation. It has a
> socket /run/osns/system.sock, where clients belonging to user sessions
> can connect to.
> 
> What I've tried I do not get it to work.
> It used to fork, and the forked process created the socket. I've
> changed this using the code on
> http://0pointer.de/blog/projects/socket-activation.html
> 
> the osns-system.socket file:
> 
> [Socket]
> ListenStream=/run/osns/system.sock
> 
> [Install]
> WantedBy=socket.target
> 
> and osns-system.service file
> 
> [Unit]
> Description=OSNS System Service
> After=syslog.target
> 
> [Service]
> Type=forking
> KillMode=process
> ExecStart=/usr/libexec/osns/osnssystem
> Restart=no
> 
> [Install]
> WantedBy=multi-user.target
> 
> I've used sd-listen_fds to get the fd when the socket
> /run/osns/system.sock is created, but there is none. The environment
> var LISTEN_PID is the pid of the process before forking. So the
> session manager is not aware of the new main process. So I've added
> 
> PIDFile=/run/osns/osns-system.pid
> 
> to the [Service], but no effect.
> I've changed the Type to notify, and make sd aware of the new pid
> using sd_notifyf(0, "READY=1\n" "MAINPID=%lu", (unsigned long) pid)
> where the pid is of the new forked process.
> I've used this call in the starting process, and when that did not
> work, used it in the new forked process.
> That also did not work.

The famous "it does not work". What you did, what results you got, what 
result did you expect?

> The environment var LISTEN_PID remains the pid
> of the starting process.

How do you expect this environment variable to change? It was set when 
systemd invoked your program. How systemd is supposed to change the 
value of this variable inside already started process? How systemd is 
supposed to predict the value of PID before starting your command?

> What is wrong?
> 

It is still unclear what you did. But I venture to guess you should not 
use Type=forking or process passed file descriptors before forking.