[libvirt-users] Obtaining the PID of a domain's QEMU process from C

[Erik Skultety]

On Sat, Mar 09, 2019 at 04:32:00PM +0100, Michal Prívozník wrote:
> On 3/1/19 2:31 AM, Shawn Anastasio wrote:
> > Hello all,
> >
> > I'm currently writing a C program that uses the libvirt API and I need a
> > way to obtain the pid of a given domain's QEMU process.
> >
> > Specifically, I'm writing an ivshmem server that uses SO_PEERCRED to get
> > the pid of clients that connect to it, and I would like to use that pid
> > to look up the domain in libvirt to determine the proper domain ID to
> > return to the client.
> >
> > As far as I can tell, libvirt doesn't expose this information in an easy
> > to access manner. Of course it is possible to call `ps` and grep for the
> > information I'm looking for, but I was hoping for a cleaner solution.
> >
> > If anybody knows how to do this, advice would be greatly appreciated.
>
> There isn't an API for that as we don't want users to fiddle with qemu

That's right, and it should stay that way. On the other hand, the same way we
can't prevent anyone from editing /etc/libvirt/qemu/<domain>.xml or
/var/run/libvirt/qemu/<domain>.xml or run the 'ps' command or whatever we might
as well report the PID as part of virConnectListAllDomains data. I don't have
a problem with reporting PIDs in principle, provided it's used for informatory
purposes.
Having said that, there's the question of why libvirt should report something
that it doesn't need to consume, IOW we report machine ID which we can use to
control the machine, we also report UUID which we can consume, but we'd do
absolutely nothing with the PID besides reporting it.
Another thing is that reporting PIDs of machines running on a remote host is
quite useless for locally running clients.

Erik

> behid its back. Anyway, apart from ps there might be a solution. Libvirt
> of course know the PID and stores it - in the status XML. This is a file
> that libvirt uses to store runtime information for the domain in case
> the daemon restarts. It's located under
> /var/run/libvirt/qemu/$domain.xml. But you'll need root priviledges to
> read it.
>
> Also, it is very wise to follow the comment at the beginning of the file
> and not modify it ;-) And just like with any libvirt internals, we make
> no guarantees they won't change in the long run.
>
> Michal
>
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