On 14/11/2017 04:26, aleivag wrote:
Hi all:
hope you guys are doing great!. So i have a few questions, hope this
is the best place for them.
I've been doing a lot of work with `sd-bus.h` (basically i've been
trying to bind it to other languages to then interact with systemd
natively).
I've been reading the man pages/blog post/general docs, but mostly the
src code. and i stumble across
https://github.com/systemd/systemd/blob/master/src/shared/bus-util.c#L598-L605
and saw that you can connect to systemd using the sockets, for root
would be `/run/systemd/private`, and for users something like
`/run/user/<uid>/systemd/private` and this trigger lots of questions,
that i have not been able to answer, so here they are:
Question 1)
what would be the advantage of connecting through dbus instead of
directly through the socket?
the way i connect to systemd is with `sd_bus_open_system` but i can
also do
```
sd_bus_new(&bus);
sd_bus_set_address(bus, "unix:path=/run/systemd/private");
sd_bus_start(bus);
```
why (or when) would one be better than the other?
My understanding is that you should always connect through the normal
system dbus socket and not use systemd's private socket
The private socket (afaiu) is meant to be used only by systemctl. It
talks directly to systemd (without a dbus daemon) and is here to make
sure systemd and systemctl can communicate even when dbus is broken or
not started yet
question 2);
i also look that you can do the same with the user connections (and
this is mostly true when the --user flag is given, at least on
systemd-run), and you can connect to something like
`/run/user/<uid>/systemd/private`, where `/run/user/<uid>` is
$XDG_RUNTIME_DIR, and i guess this is really the best form to connect
to systemd as a user, but is there any difference between using that
socket or doing `sd_bus_open_user`. ?
question 3)
systemd source code is pretty clear, really easy to learn from, also
sd-bus.h is incredible helpful and easy to use.
But the docs is good, don't get me wrong, but it could definitely use
more love. for instance the usage of the sockets
`/run/{user/uid,}/systemd/private`, is not documented anywhere that i
could find. is this intentional?, is this because this is a
implementation detail that may change in the future?. if so, what
would be the correct way to connect to systemd's socket?
i was particularly surprise that the sockets' path are hardcoded in
the code.
Yesn systemd private sockets are private and systemd does not want to
document private interfaces.
That being said, the system socket location is hardcoded not just in
systemd, but in the dbus specification itself. It won't change, so it 's
ok to hardcode it...
I didn't answer all your questions, but I hope the bits I know helped
Jeremy
if this parts have not just gotten into the docs, i would be more than
happy to submit the PR for the docs.
Thank you guys!
Alvaro Leiva
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