[linux-audio-user] Jack and SHM

Jack O'Quin joq at io.com
Thu Jan 22 21:56:43 EST 2004

Christophe Vescovi <vescovi at lpsc.in2p3.fr> writes:

> Well in fact I use jack with POSIX shm for a few month now and every
> thing is fine. I can run all jack audio app I have without any
> problem with my cheap SBLive. I have no xrun with -p1024 and a few
> when switching desktop or starting apps with -p512 (which seems to
> be the normal behaviour with a SBLive).
> But it seems that there is a preference for SysV shm (this is the
> default in configure !!) so I was wondering why.

I sometimes run POSIX shm on my Linux system for testing purposes to
make sure it continues working.  As far as I can tell, it works fine.
The MacOSX port uses POSIX shm exclusively.

An earlier POSIX shm implementation worked, but not reliably.  This
was blamed at the time on bugs in the Linux kernel.  I have not
observed these problems on my system, but they were the reason why
Paul decided to make System V shm the default on Linux.  There were
also install problems due to the requirement that users create an
/etc/fstab entry of type tmpfs for /dev/shm.

It may be that newer kernels have a more robust POSIX shm support.
Or, perhaps the present JACK shm implementation not longer stresses
whatever component was failing before.  Or, maybe I just haven't run
it enough to notice, yet.  :-)

The problems that you mention are serious and should be reported in
the JACK bug database, <http://jackit.sourceforge.net/mantis>.

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