[Jack-Devel] List of jackd startup and named instance questions

[Ethan Funk]

Hello all.

I have embarked on a long journey of porting my radio automation and
mixing system from OSX (making extensive use of Apple's CoreAudio API)
to a Linux/JACK/gstreamer system and potentially, making it a cross
platform application.  The application is actually a collection of
programs, ranging from a studio UI, library management UI, database
back-end, and an faceless audio-engine/mix system.  I am currently
working on the audio-engine port, since potentially I could port that
and continue to use the old UIs on Macs until I can get those ported as
well.

In order to make this manageable, I decided to pull some audio features
out of my existing audio-engine, notably AudioUnit effects hosting, IAX
phone support, and multiple audio-device support.  I should be able to
add VL2 effects hosting and IAX back in at some point.  But for the
short term, I can use jack to host effects in another program for now,
and just give up IAX/asterisk integration. The multiple audio-device
support, via my own clock drift measurement and re-sampling scheme, is
a loss for now, but it looks like alas-in and alas-out programs could
be stop-gap approaches to that, and appears to use a similar underlying
approach.

So far, the porting is going much faster than I had expected.  Jack2 is
a very clean and well though out API.  It is not unlike CoreAudio, only
it's MUCH easier to use.  Leveraging JACKs inter-application audio
routing, I have further broken my audio-engine down into a mixing
system and control session host,  with separate programs executed for
playing and recording/streaming audio content.  This a very nice, clean
way to break thing up, made possible by JACK.  Thanks.

That is the big picture.  I have come across some questions that I need
a little help with, all regarding the start-up process of the jackd
server:

1) At first, JACK seems to be designed for a single jackd server
running on a system, with the server control API lacking, as far as I
can tell, a method to control multiple servers by name.  But then, with
the name command line option, I can actually run multiple servers tied
to different audio devices.  Does the server control API simple not
support names at this point?  Maybe this is just how JACK evolved,
originally one jackd per machine, then named server support added, but
not fully implemented across all the API?

2)  Again, with the named server:  I can start a named jackd server.  I
can connect to it using QjackCtl by setting the name property in
the advanced tab of QjackCtl settings. But when I try to connect to the
named server from my audio-engine application, via a jack_client_open()
call, passing the name char string, my application instead tries to
start up a jackd server instance, using the command noted in the jackd
man page (first line of  $HOME/.jackdrc).  Is this a bug, or am I
missing some detail?

3) Regarding the jack_client_open() behavior when no server is yet
running: the function call does seem to execute the first line of
 $HOME/.jackdrc and start the jackd server running.  However, it
appears to inherit all the file descriptors from my application. This
is a problem because my application is designed to self-restart on a
crash.  With jackd holding my application's TCP control socket open, my
application can't restart (bind again to the desired TCP port) until
after I kill the jackd process.  I assume  the auto-jackd startup code
is forking and execing, and the code simply isn't closing the parent's
file descriptors.  Is this a bug or intentional?  Is there a way I can
detect if a jackd server is running ahead of time, so I can start the
server myself using my own fork/exec which would closing my descriptors
on the child, then, once I know jackd is running, call
jack_client_open() in my app?

4) because my audio-engine is a faceless application, and can be run
without a desktop session, I need it to be able to connect to a jackd
server run by other users, or to start a jackd server that can be used
by other users.  I can verify that with Ubuntu 18.10, I can not start
jackd from a user account, then connect to it from my application
running as a different user, even if I run my app as root, not that I
intend to do that as a real world workaround.  Is there some approach,
group permissions possibly, to allowing other users to access a jackd
server?

5) Wishful thinking:  Give jackd the ability to read QjackCtl config
files, so I could configure things from the GUI, then stop jackd and be
able to restart it from the server-control API or command line with a
command line option pointing to a config file.  Better yet, make a
persistent JACK-aware place to store such file in the file hierarchy.

Thanks,
Ethan Funk

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