Re: why do elements disappear?

[Takashi Iwai <tiwai@suse.de>]

At Tue, 15 Feb 2005 22:12:53 +0100,
Ronald S. Bultje wrote:
> 
> Hi,
> 
> a mixer app of mine (you know, one of those controlling volumes) is
> updating itself (if other apps change settings) by continuously running
> get-volume (et al) on all mixer elements every once in a while (10Hz).

Well, this isn't a good idea.  You should do update passively via
poll().  The exception is a control with VOLATILE flag like VU meter,
which won't send change notification.  But normal controls send
notification if the value is changed, or the control is
created/deleted, so that the app can react.

> Right, so I could use signals or whatever it is that libalsa provides,
> but this is what OSS and such require, so I'm using this for now. This
> all works really well, except if new applications are started accessing
> the soundcard and the soundcard is a sb!live (which is true for quite a
> bunch of people). It will then sometimes crash. Backtraces look like
> this:
> 
> #3  <signal handler called>
> #4  0x001ef7a2 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2
> #5  0x023cb955 in raise () from /lib/tls/libc.so.6
> #6  0x023cd319 in abort () from /lib/tls/libc.so.6
> #7  0x023c4f41 in __assert_fail () from /lib/tls/libc.so.6
> #8  0x00b5e237 in snd_mixer_selem_get_playback_volume ()
>    from /lib/libasound.so.2
> 
> Here's some more debug on the track we're accessing (this is GStreamer
> information, but you'll get the idea):
> 
> (gdb) print *alsa_track
> $1 = {parent = {parent = {g_type_instance = {g_class = 0x8a37870},
> ref_count = 3, qdata = 0x8ab1528}, label = 0x8a63488 "EMU10K1 PCM",
> flags = GST_MIXER_TRACK_OUTPUT, num_channels = 3, min_volume = 0,
> max_volume = 65535}, element = 0x8a7bb00, track_num = 40, alsa_flags =
> 2, volumes = {65535, 65535, 65535, 0 <repeats 29 times>}}
> 
> OK, so this is specific to the EMU10K1 PCM tracks (there's 32 of them).
> Those are *not* the first elements, definately, I don't know what they
> do exactly though though... But it's something specific to them.
> 
> I figured I could be an idiot, so here's valgrind:
> 
> ==20529== Invalid read of size 4
> ==20529==    at 0x1BB870B0: snd_mixer_selem_has_playback_switch (in
> /lib/libasound.so.2.0.0)
> ==20529==    by 0x1D01AB2B: gst_alsa_mixer_update (gstalsamixer.c:412)
> ==20529==    by 0x1D01AC4A: gst_alsa_mixer_get_volume
> (gstalsamixer.c:439)
> ==20529==    by 0x1C416998: gst_mixer_get_volume (mixer.c:185)
> ==20529==  Address 0x1CE2B678 is 0 bytes inside a block of size 44
> free'd
> ==20529==    at 0x1B904FB1: free (vg_replace_malloc.c:153)
> ==20529==    by 0x1BB8390C: snd_mixer_elem_remove (in
> /lib/libasound.so.2.0.0)
> ==20529==    by 0x1BB86630: (within /lib/libasound.so.2.0.0)
> ==20529==    by 0x1BB8668C: (within /lib/libasound.so.2.0.0)
> 
> It appears that alsa removes the EMU10K1 PCM elements when I call
> snd_mixer_handle_events(). Why does it do that, how do I make it stop
> doing it and/or how do I handle this?

EMU10K1 PCM control is dynamically added/removed when the
corresponding PCM is opened/closed (more precisely, INACTIVE flag is
removed/added at each open/close).

If the app is programmed in a passive manner, it will receive the ADD
and REMOVE events when new controls are created/deleted.  Then the app
has to take the corresponding action.


Takashi


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