[systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)

[Anne Mulhern]

----- Original Message -----
> From: "David Herrmann" <dh.herrmann at gmail.com>
> To: "Anne Mulhern" <amulhern at redhat.com>
> Cc: "systemd" <systemd-devel at lists.freedesktop.org>
> Sent: Tuesday, November 24, 2015 10:15:05 AM
> Subject: Re: [systemd-devel] libudev: subdirectories in sysfs (what does "available" mean?)
> 
> Hi
> 
> On Tue, Nov 24, 2015 at 3:54 PM, Anne Mulhern <amulhern at redhat.com> wrote:
> >> From: "David Herrmann" <dh.herrmann at gmail.com>
> >> On Tue, Nov 17, 2015 at 11:57 PM, Anne Mulhern <amulhern at redhat.com>
> >> wrote:
> >> >> From: "David Herrmann" <dh.herrmann at gmail.com>
> >> >> On Mon, Nov 16, 2015 at 5:35 PM, Anne Mulhern <amulhern at redhat.com>
> >> >> wrote:
> >> >> > libudev has some cooperating procedures that return the keys for a
> >> >> > bunch
> >> >> > of
> >> >> > sysfs attributes for a given device.
> >> >> >
> >> >> > These attributes all correspond to files that are stored in the sysfs
> >> >> > device directory.
> >> >> >
> >> >> > In the same directory there are sometimes subdirectories, that
> >> >> > themselves
> >> >> > contain files
> >> >> > with information about their corresponding attribute. The dm
> >> >> > directory
> >> >> > is
> >> >> > one obvious
> >> >> > example.
> >> >> >
> >> >> > Are their any plans for libudev to add an ability to get the values
> >> >> > from
> >> >> > these subdirectories
> >> >> > as some kind of attributes?
> >> >> >
> >> >> > If no, why?
> >> >>
> >> >> sd_device_get_sysattr_value(device, "foo/bar/baz", &value);
> >> >>
> >> >> This should work fine (or its udev_device_* equivalent).
> >> >>
> >> >> Btw., I recommend just using readdir(), open(), read(), and write().
> >> >> sysfs is a filesystem, no reason to wrap all those commands.
> >> >
> >> > Thanks, I'm asking this more as the pyudev maintainer than as someone
> >> > who actually wants these values.
> >> >
> >> > The funny thing is, I recently found out that the list obtained by
> >> > udev_device_get_sysattr_list_entry () and friends contains so
> >> > called "available" keys, but when those get passed to
> >> > udev_device_get_sysattr_value () the result might be NULL.
> >> > That makes sense in the sense that they might represent files
> >> > that are unreadable.
> >> >
> >> > Now I find out that I can make up keys not in the results of
> >> > udev_device_get_sysattr_list_entry () and pass those to
> >> > udev_device_get_sysattr_value() and get a non-null result.
> >> >
> >> > So, what does "available" mean? Do these sysattr_list_entry()
> >> > methods give any useful information?
> >>
> >> "available" probably means attributes which are direct descendants of
> >> the device. That is, sysattr_list_entry() only lists such direct
> >> descendants, while sysattr_value() allows you to query anything (you
> >> probably can even pass "foo/../../bar/baz").
> >>
> >> Thanks
> >> David
> >>
> >
> > I think it's yet more complicated than that. For example,
> > 'dm' (for device mapper) is not in the list of available
> > attributes, but 'dm/name' is certainly an attribute that
> > can be read by sysattr_value().
> 
> 'dm' is not an attribute, so it will never be listed as available
> attribute. Directories are never treated as attributes.
> 
> Thanks
> David
> 

But "bdi" is listed as an attribute, and is a directory.

- mulhern