NetBSD Problem Report #34647

From  Thu Sep 28 10:05:03 2006
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Date: Thu, 28 Sep 2006 11:54:09 +0300 (EEST)
From: (Andreas Gustafsson)
Reply-To: (Andreas Gustafsson)
Subject: audio(4) silence insertion is broken
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>Number:         34647
>Category:       kern
>Synopsis:       audio(4) silence insertion is broken
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    nat
>State:          closed
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Sep 28 10:10:00 +0000 2006
>Closed-Date:    Sun Jun 18 14:59:51 +0000 2017
>Last-Modified:  Sun Jun 18 14:59:51 +0000 2017
>Originator:     Andreas Gustafsson
>Release:        NetBSD 3.99.24
System: NetBSD 3.99.24 NetBSD 3.99.24 (GENERIC) #0: Fri Aug 4 13:14:16 EEST 2006 i386
Architecture: i386
Machine: i386

The audio(4) man page states:

   If a writing process does not call write(2) frequently enough to
   provide samples at the pace the hardware consumes them silence is

In actuality, random noises appear in the audio output in this
situation.  I have observed this behavior on two different machines,
one having an Ensoniq AudioPCI (eap(4)) and the other having an Acer
Labs M5451 (autri(4)), which leads me to believe that the problem is
in the generic audio(4) framework rather than in any one specific
hardware driver.


Compile and run the following program, for example by saving it as
test.c and running "cc test.c -lm && ./a.out":

#include <fcntl.h>
#include <memory.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv) {
        int fd;
        char buf[4000];

        fd = open("/dev/audio", O_WRONLY);
        if (fd == -1) {
                fprintf(stderr, "could not open /dev/audio\n");

        memset(buf, 0, sizeof buf);
        write(fd, buf, sizeof buf);


This should play half a second of silence followed by another ten
seconds of silence inserted by the audio driver due to lack of data;
any sound appearing appearing on the sound card output means you
are suffering from the bug.


None provided.


From: (Andreas Gustafsson)
Subject: Re: kern/34647: audio(4) silence insertion is broken
Date: Tue, 3 Jun 2008 19:52:40 +0300

 The original test program included in kern/34647 has a slight bug;
 it should use bytes 0xff rather than 0x00 for silence since the
 default /dev/audio encoding is mu-law.  As a result, running it
 will result in a pair of clicks even when running it on a correctly
 working system.

 Here is an updated test program, which uses the correct silence
 value and does not produce the clicks.  It has also been changed
 to repeat the test indefinitely rather than just doing it once.

 #include <fcntl.h>
 #include <memory.h>
 #include <stdio.h>
 #include <stdlib.h>

 int main(int argc, char **argv) {
         int fd;
         char buf[4000];
 	fd = open("/dev/audio", O_WRONLY);
 	if (fd == -1) {
 		fprintf(stderr, "could not open /dev/audio\n");
 	memset(buf, 0xff, sizeof buf);
 	for (;;) {
 		write(fd, buf, sizeof buf);
 	return 0;

 If I play some audio and then run the test program on a machine with
 an eap sound card and a kernel built from -current source as of
 20080516-1010 EET, I still get noise sounding like fragments of the
 previously played audio (not just clicks) for a few seconds.

 If the test program is run multiple times, you need to play back some
 non-silent audio between the runs to ensure that the audio buffers
 do not already contain silence, for example by running

   dd if=/dev/urandom of=/dev/audio bs=32k count=1

 Andreas Gustafsson,

Responsible-Changed-From-To: kern-bug-people->nat
Responsible-Changed-When: Wed, 12 Apr 2017 14:02:35 +0000

From: "Nathanial Sloss" <>
Subject: PR/34647 CVS commit: src/sys/dev
Date: Wed, 12 Apr 2017 14:15:51 +0000

 Module Name:	src
 Committed By:	nat
 Date:		Wed Apr 12 14:15:51 UTC 2017

 Modified Files:
 	src/sys/dev: audio.c

 Log Message:
 Insert silence into the mix ring if there is no audio (or audio cannot
 keep up) from the channels.

 Addresses PR kern/34647.

 To generate a diff of this commit:
 cvs rdiff -u -r1.321 -r1.322 src/sys/dev/audio.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

State-Changed-From-To: open->feedback
State-Changed-When: Wed, 12 Apr 2017 14:24:46 +0000
A change has been committed to address this.

From: Andreas Gustafsson <>
Subject: Re: kern/34647 (audio(4) silence insertion is broken)
Date: Sun, 18 Jun 2017 17:51:17 +0300

 On April 12, wrote:
 > A change has been committed to address this.

 I have now tried the test program from the PR on a -current system with

   hdafg0 at hdaudio0: vendor 11d4 product 184a

 and verified that there are no audible clicks or other noises.
 Thank you!
 Andreas Gustafsson,

State-Changed-From-To: feedback->closed
State-Changed-When: Sun, 18 Jun 2017 14:59:51 +0000
Confirmed fixed.


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