docs/84639: pcm.4, handbook - note that vchan settings can't be changed while device in use

Sam Lawrance lawrance at
Sun Aug 7 06:50:21 UTC 2005

>Number:         84639
>Category:       docs
>Synopsis:       pcm.4, handbook - note that vchan settings can't be changed while device in use
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    freebsd-doc
>State:          open
>Class:          doc-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sun Aug 07 06:50:19 GMT 2005
>Originator:     Sam Lawrance
>Release:        FreeBSD 5.4-STABLE i386
System: FreeBSD 5.4-STABLE FreeBSD 5.4-STABLE #0: Sat Jun 11 21:44:19 EST 2005 sam at i386

	I was following the handbook in setting up virtual sound channels.
When the hw.snd.pcm0.vchans sysctl did not appear in my list,  I checked into
it a little.  It seems that that this sysctl is not available when the sound
device is in use.

I think a note should be made of this, particularly for new users who are
setting up and playing around with sound devices.

1. Run xmms and play "Groove Armada - Lovebox" :-)
2. Run sysctl hw.snd.pcm0.vchans


Patch for pcm manpage:

Index: pcm.4
RCS file: /home/ncvs/src/share/man/man4/pcm.4,v
retrieving revision 1.39
diff -u -r1.39 pcm.4
--- pcm.4	22 May 2005 15:25:14 -0000	1.39
+++ pcm.4	7 Aug 2005 06:03:22 -0000
@@ -236,7 +236,8 @@
 .Dq 0
 will disable
-for this device.
+for this device.  This setting is not available while the specified
+device is in use.
 .Ss Recording Channels
 On devices that have more than one recording source (ie: mic and line),

Patch for handbook multimedia section:

Index: chapter.sgml
RCS file: /home/ncvs/FreeBSD/doc/en_US.ISO8859-1/books/handbook/multimedia/chapter.sgml,v
retrieving revision 1.107
diff -u -r1.107 chapter.sgml
--- chapter.sgml	6 Jun 2005 10:04:50 -0000	1.107
+++ chapter.sgml	7 Aug 2005 05:53:44 -0000
@@ -436,6 +436,12 @@
       virtual channels any devices which are attached later will be
+    <note>
+      <para>You cannot change the number of virtual channels for a
+      device while it is in use.  First close any programs using the
+      device, such as music players or sound daemons.</para>
+    </note>
     <para>If you are not using &man.devfs.5;, you will have to point
       your applications at <filename>/dev/dsp0</filename>.<replaceable>x</replaceable>, where
       <replaceable>x</replaceable> is 0 to 3 if <varname>hw.snd.pcm.0.vchans</varname> is set

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