[systemd-devel] How to get argument list for methods

[Mantas Mikulėnas]

GLib's `gdbus introspect` can show you the parameter names if they're
included in the introspection XML:

    gdbus introspect -y -d org.freedesktop.systemd1 -o
/org/freedesktop/systemd1/unit/cups_2eservice

But introspection is mainly for use by language bindings (e.g. perl-dbus) –
it is not a substitute for documentation.

For anything beyond that, systemd has manual pages for each bus service it
implements, e.g.:

    man org.freedesktop.systemd1

https://www.freedesktop.org/software/systemd/man/latest/org.freedesktop.systemd1.html

On Tue, Jun 17, 2025, 06:33 Chris Kottaridis <chris at quietwind.net> wrote:

> I am beginning to use the sd-bus library. I have been able to get a couple of properties, MainPID and SubState.
>
> I would like to use the Stop method. I am using cups as an example. This commands shows that in thee Unit interface there is a Start and Stop method:
>
> $ busctl introspect org.freedesktop.systemd1 /org/freedesktop/systemd1/unit/cups_2eservice
>
> NAME                                TYPE      SIGNATURE RESULT/VALUE FLAGS
>
> org.freedesktop.systemd1.Unit       interface -               -                                        -
>
> .Start                              method    s               o                                        -
> .Stop                               method    s               o                                        -
>
> Each has a signature of "s" I assume it takes one parameter of type string.
>
> I have tried the sample on this page. Pasting it into a file and compiling and running it works
> great.
>
> https://0pointer.net/blog/the-new-sd-bus-api-of-systemd.html
>
> This sample starts the cups service by acccessing the systemd Manager and calling StartUnit.
>
>         /* Issue the method call and store the respons message in m */        r = sd_bus_call_method(bus,                               "org.freedesktop.systemd1",           /* service to contact */                               "/org/freedesktop/systemd1",          /* object path */                               "org.freedesktop.systemd1.Manager",   /* interface name */                               "StartUnit",                          /* method name */                               &error,                               /* object to return error in */                               &m,                                   /* return message on success */                               "ss",                                 /* input signature */                               "cups.service",                       /* first argument */                               "replace");                           /* second argument */
>
> It has a signature of "ss" which I assume means the method takes 2 arguments each of a type string.
>
> My question is for any given method how do I get what the arguments for it are ?
>
> Like if I use Start what are valid values to pass for the one argument?
>
> In StartUnit the first argument seems obvious with an example to be the unit you want to start. I
> am not sure what the "replace" means or what other strings would be valid.
>
> Again I am interested in the Stop method specifically, but in general would like to know where I can
> get a list of the arguments and possible values for any method.
>
> Thanks
> Chris
>
>
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