[Matroska-devel] State of the libebml / libmatroska API and codebase

Arnavion arnavion at gmail.com
Sun Sep 16 11:54:38 CEST 2012


Hi Moritz,

Thank you, and I'll check out the alternatives you mentioned. It'd be nice
if it was mentioned somewhere that these libraries are unmaintained.

Regards,
Arnav Singh

On Sun, Sep 16, 2012 at 5:14 PM, Moritz Bunkus <moritz at bunkus.org> wrote:

> Hey,
>
> note: I'm not the original author, but MKVToolNix relies heavily on
> both libraries and I've done bug fixes and other things on the
> libraries.
>
> On Sun, Sep 16, 2012 at 10:52 AM, Arnavion <arnavion at gmail.com> wrote:
>
> > Apart from the technical specs on the website about how EBML and Matroska
> > files are laid out, and the documentation in the headers, is there no
> other
> > documentation?
>
> Nope.
>
> > The code seems to be... severely broken in places.
>
> That is correct. I only correct stuff that I stumble upon during the
> development of MKVToolNix. The only other application outside of the
> Matroska project itself that uses libebml & libmatroska that I know of
> is VLC. That's also the reason no one is working on those libraries:
> almost no one uses them.
>
> > Is my understanding correct? If so, then what exactly is the
> > user of this API expected to do w.r.t. the argument he passes in and the
> > element he gets back?
>
> Quite possible.
>
> > I understand that the API is deliberately low-level, but are there no
> > abstractions for this? No GetTracks method, for example?
>
> You're correct, it is low-level, and no, there are no higher-level
> functions.
>
> > (Coding standards nitpick) libmatroska's headers have "using namespace
> > libebml;" in them, which means any file which includes a libmatroska
> header
> > can use libebml classes without prefixing the namespace. Headers should
> not
> > have "using namespace" declarations.
>
> Absolutely correct, but uncorrectable without breaking backwards
> compatibility.
>
> > None of the functions have remotely correct documentation.
>
> True.
>
> > If one is to take functions such as FindNextID and FindNextElement at
> face value, what
> > then is the purpose of SkipData?
>
> SkipData skips the element's data (meaning it seeks in the file) after
> the header of an element has been read. For example, FindNextID only
> reads the EBML ID and the length field but not the data (otherwise you
> would suck the whole segment into memory if you were to read the
> segment's ID). SkipData can then be called if you're not interested in
> an ID that was found.
>
> > What is the state of these two projects?
>
> Pretty much unmaintained. I only fix issues that I come upon, and
> libmatroska receives new classes for newly added Matroska elements.
> That's about all.
>
> Two weeks ago I added some more convenience functions in libebml
> (GetValue(), SetValue(), SetValueUTF8() for EbmlUnicodeString) so that
> you don't have to use the "operator ...()" functions provided by the
> classes for storing values. Those changes are in SVN only so far, and
> I'm still in the process of transforming MKVToolNix accordingly.
>
> > These TODOs and broken behavior have survived for a long time; will they
> ever be fixed,
>
> Highly unlikely. Especially things that would break API compatibility
> will never get fixed or applications would get broken.
>
> > or are users expected to work around them as it seems they have been
> doing so far?
>
> Yes.
>
> > My apologies that this sounds so inflamatory. I'm just bewildered that
> these
> > libraries are in this state.
>
> No problem at all, you're completely right.
>
> However, there is light at the end of the tunnel. In this case this
> means that there are alternatives:
>
> - Steve Lhomme has re-written libebml and libmatroska as pure C
> libraries called libebml2 and libmatroska2. His intention was to
> maintain those actively, though I don't know what their state is at
> the moment.
> - There's also Haali's Matroska access library that he uses in his
> splitter. His library (written in C if I'm not mistaken) is available
> to people who email him; see http://haali.su/mkv/
> - You could also decide to use another more high-level framework like
> gstreamer or ffmpeg. Both contain Matroska readers.
>
> Kind regards,
> mosu
> _______________________________________________
> Matroska-devel mailing list
> Matroska-devel at lists.matroska.org
> http://lists.matroska.org/cgi-bin/mailman/listinfo/matroska-devel
> Read Matroska-Devel on GMane:
> http://dir.gmane.org/gmane.comp.multimedia.matroska.devel
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.matroska.org/pipermail/matroska-devel/attachments/20120916/6ca49161/attachment.html>


More information about the Matroska-devel mailing list