> I have a requirement to pass information from the kernel to user space.
> The information is passed fairly rarely, but over time extra parameters
> are added. At the moment we just use a struct, but that means that the
> kernel and the userspace app have to both keep in step. If something
> like XML was used, we could implement new parameters in the kernel, and
> the user space could just ignore them, until the user space is upgraded.
> XML would initially seem a good idea for this, but are there any methods
> currently used in the kernel that could handle these parameter changes
> over time.
okay.
storing data in binary format is efficient space-wise, clearly
optimal, but is difficult to interpret unless you play a lot
of chess and music.
XML is inefficient memory and processing wise for storage of
data, but it is structured, is easy for normal humans to read and
understand as long as microsoft doesn't get their hands on it.
the guy who wrote XMLRPC more than 5 or so years ago did so as
an experiment in order to get himself onto the SOAP committee.
now we have .NET, based on SOAP, and everyone hates it. them.
.NET _and_ SOAP. even if SOAP _can_ be implemented in 6,000 lines
of python.
ramming structs in home-grown marshalled format across
processes is fine (i do it all the time) ...
... until you get large complex data structures, see this url
for an example of horrendously complex multi-linked-listed,
multi-unioned interlinked and _recursive_ data structures
with forward references, that would be a complete nightmare
without automated tools:
http://cvs.sourceforge.net/viewcvs.py/oser/exchange5.5/exploration/test/emsabp/emsabp.idl?rev=1.1&view=markup
if it wasn't for freedce, the creation of, management and
use of such data structures would be a complete nightmare.
no i do _not_ recommend that freedce be used inside the linux kernel.
however what i _would_ advocate for special and specific
circumstances would be to seek out the "pickling" side of
freedce, run the IDL compiler and end up with some c-code
that can then be used as a form of structured storage:
http://www.opengroup.org/tech/rfc/mirror-rfc/rfc2.1.txt
it's a bit of a pain that the code generated stuffs 20 bytes
of garbage (the uuid and version number that you specified
in the .idl file) but that can always be stripped off after
encoding and re-added manually at decode time (or the code in
freedce just chiselled away) - all you lose by following that
trick is the unpickling code's ability to tell you "yes, you
handed me the wrong blob there, bob". as a result, faking
up the version number and uuid on the wrong blob will trash
your function call stack - just in the same way as if you
got a printf with the wrong number of arguments.
what OSF call "pickling" is also known as persistent or structured
storage.
it's a friggin sight better than contemplating XML, but it also comes
with a price tag: in this case, nearly 70,000 lines of library code
that "encapsulates" and "extracts" the data structures for you, at
runtime [yes, you can supply your own malloc and free routines]
in the cases where data structures are _that_ complex, or where time is
of the essence, or you have a significant amount of regular restructuring
anticipated before settling on your final data structures and APIs, the
use of an automated "pickler" - one that does binary NOT some stupid
XMsmelly format - is a good idea.
where it's NOT a good idea is where you have _really_ simple data
structures, or your data structures simply ain't gonna change much
ever, or you have exceptionally careful programmers who have lots of
time on their hands to go over each line of the hand-marshalling and
unmarshalling code, line-by-line.
and yes, as i understand it, the GNU/Hurd has so much structured
storage passing going on that yes, they wrote an IDL compiler and a
mini RPC system (oh, and they have microkernel to make use of it,
too. nyer :)
one of these fine decades i will experiment with using
freedce instead of their home-grown IDL compiler and see if
it's possible to utilise freedce networking to move entire
processes, uninterrupted, from one machine to another.
basically, these tools allow the specialist job of turning complex data
structures into data streams and back again to be separated from the
specialist job of writing a decent kernel.
if your data structures aren't that complex, you really _don't_ need
specialist tools - XML or binary.
l.
p.s. hurrah! first post on LVML :)
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