jackd doesn't work with some alsa pcm devices

[fps]

E.g.:

fps@mango 07:45:46 ~/Downloads/ $ jackd --version
jackd version 0.122.0 tmpdir /dev/shm protocol 24
fps@mango 11:04:35 ~/Downloads/ $ jackd -d alsa -d plug:default
jackd 0.122.0
Copyright 2001-2009 Paul Davis, Stephane Letz, Jack O'Quinn, Torben Hohn and others.
jackd comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details

JACK compiled with System V SHM support.
loading driver ..
creating alsa driver ... plug:default|plug:default|1024|2|48000|0|0|nomon|swmeter|-|32bit
ALSA lib control.c:953:(snd_ctl_open_noupdate) Invalid CTL plug:default
control open "plug:default" (No such file or directory)
configuring for 48000Hz, period = 1024 frames (21.3 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 32bit float little-endian
You appear to be using the ALSA software "plug" layer, probably
a result of using the "default" ALSA device. This is less
efficient than it could be. Consider using a hardware device
instead rather than using the plug layer. Usually the name of the
hardware device that corresponds to the first soun
ALSA: use 3 periods for capture
ALSA: final selected sample format for playback: 32bit float little-endian
You appear to be using the ALSA software "plug" layer, probably
a result of using the "default" ALSA device. This is less
efficient than it could be. Consider using a hardware device
instead rather than using the plug layer. Usually the name of the
hardware device that corresponds to the first soun
ALSA: use 3 periods for playback
jackd watchdog: timeout - killing jackd
Aborted (core dumped)

The default device is eventually routed to pulseaudio in this case. Might be a side effect of the watchdog in this version?

[fps]

With version 124.1 shipped with archlinux jackd doesn't get killed by the watchdog anymore, but it's still non-functional.

jack_rec -d 2 -f foo system:capture_1

only manages to write a 44 bytes header and never returns.

[zamaudio]

Don't you mean 'plughw:default' ?

[fps]

No.

[fps]

To be more precise "plug:default" is a perfectly valid PCM device name in ALSA. Try this for example:

aplay -D plug:default *.wav

in a folder containing .wav files.