We don't really have any ways to do that, unless the bittorrent file uses sparse files. I'll move that to GStreamer for the filesrc to add support for that.

Is there a library available that understands partially downloaded torrent files?

If the torrent downloader is written properly in the first place, it would probably use sparse files. I'm not sure there's any good ways to detect sparse files though...

dragon player uses gstreamer. it automatically skips empty places on sparse files.

*** Bug 571200 has been marked as a duplicate of this bug. ***

dragonplayer uses phonon, which doesn't use the GStreamer sources but kio afaik. Does someone know how to detect and use sparse files now? :) Seems that kio has something for this and Wikipedia talks about detection by comparing the output of du and ls ;) http://en.wikipedia.org/wiki/Sparse_file#Detection

Possibly also of interest:

 man lseek

   Seeking file data and holes
       Since version 3.1, Linux supports the following additional values for whence:

       SEEK_DATA
              Adjust the file offset to the next location in the file greater than or equal to offset containing data.  If offset points to data, then the file offset is set to offset.

       SEEK_HOLE
              Adjust the file offset to the next hole in the file greater than or equal to offset.  If offset points into the middle of a hole, then the file offset is set to offset.  If there is no hole past offset, then the  file
              offset is adjusted to the end of the file (i.e., there is an implicit hole at the end of any file).

       In both of the above cases, lseek() fails if offset points past the end of the file.

       These  operations  allow  applications  to map holes in a sparsely allocated file.  This can be useful for applications such as file backup tools, which can save space when creating backups and preserve holes, if they have a
       mechanism for discovering holes.

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