Traditionally, Vorbis provided a high-level API for decoding and seeking
in Ogg Vorbis streams, called libvorbisfile, which was packaged
alongside libvorbis. This was very useful for simple players and
applications which just wanted to decode a file, without building an
entire media framework.
The basic libopus is meant to be used in many contexts that do not
require Ogg (such as RTP), and so it does not include any Ogg stream
manipulation functions. I have just written a new library, libopusfile,
that serves the same purpose as libvorbisfile, but for Ogg Opus files.
Like libvorbisfile, it fully supports chaining and sample-accurate
seeking. It also includes a few low-level Ogg-related functions
corresponding to the ones traditionally found in libvorbis (header
parsing and manipulation, etc.).
There are no current releases (or even autotools build support). There
is, however, code in git:
http://git.xiph.org/?p=opusfile.git;a=summary
Also, API documentation:
https://mf4.xiph.org/jenkins/job/opusfile/ws/doc/html/index.html
Although this code was heavily based on libvorbisfile, it should still
be treated as new code and is likely full of bugs. Please try it out and
let me know what explodes.