[Python-Dev] Audio devices

[Greg Ward]

On 11 March 2003, Guido van Rossum said:
> Great!  I wonder if you have any thoughts on why running
> test_ossaudiodev hangs when run on Linux Red Hat 7.3?  I'm currently
> using a 2.4.18-24.7.x kernel.  I have no idea what other info would be
> useful to debug this.

The most obvious cause is that some other process has the audio device
open, and your audio {hardware, device driver} only allows one at a
time.

If you're running one of those newfangled GUI environments like KDE or
GNOME, it's quite likely that the esound or aRTSd (however you spell it)
daemon started when you logged in, and is thus blocking all access to
your /dev/dsp.  This sucks, but IMHO it's not ossaudiodev's job to know
about esound and similar.

One way to test this is to take your system down to single-user (or at
least a console-only, no-X11 runlevel) and then try running
test_ossaudiodev.

Hmmm, it looks like calling open() with O_NONBLOCK helps.  I know this
does *not* affect later read()/write() -- there's a special ioctl() for
non-blocking read/write -- but it *does* appear to fix blocking open().
At least for me it turned a second open() attempt on the same device
from "hang" to "IOError: [Errno 16] Device or resource busy:
'/dev/dsp2'".

Try this patch; if it works I'll check it in:

--- Modules/ossaudiodev.c       10 Mar 2003 03:17:06 -0000      1.24
+++ Modules/ossaudiodev.c       11 Mar 2003 15:56:24 -0000
@@ -131,7 +131,7 @@
           basedev = "/dev/dsp";
     }
 
-    if ((fd = open(basedev, imode)) == -1) {
+    if ((fd = open(basedev, imode|O_NONBLOCK)) == -1) {
         PyErr_SetFromErrnoWithFilename(PyExc_IOError, basedev);
         return NULL;
     }

test_ossaudiodev.py will still need fixing to handle the EBUSY error,
but at least this should prevent hanging on open().

        Greg
-- 
Greg Ward <gward@python.net>                         http://www.gerg.ca/
Hand me a pair of leather pants and a CASIO keyboard -- I'm living for today!