When you try to capture from JACK, this is only possible with the
"oldstyle" jackd-0.x. If you want to use the newer, SMP-capable jack2,
the VLC inputs cannot be connected:

Cannot connect ports owned by inactive clients: "vlc-input-26611" is not
active
Cannot connect ports owned by inactive clients: "vlc-input-26611" is not
active

Unlike jack1, jack2 expects the process callback function to return 0.

The attached patch fixes this problem. It's been already fixed the same
way in the jack output plugin (see modules/audio_output/jack.c:290).

Taken from Debian bug #532339
(http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=532339

)

Signed-off-by: Christophe Mutricy <xtophe@videolan.org>