Vorbisfile's libvorbisfile may be used safely in a threading environment
so long as thread access to individual OggVorbis_File instances is serialized.
Only one thread at a time may enter a function that takes a given OggVorbis_File instance, even if the
functions involved appear to be read-only.
Multiple threads may enter
libvorbisfile at a given time, so long as each thread's function calls
are using different OggVorbis_File
instances.
Any one OggVorbis_File instance may be used safely from multiple threads so long as only one thread at a time is making calls using that instance.