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

[David Herrmann]

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