Opening a PCM device in shared mode

Opening a PCM device in shared mode

[Sayem Ahmed]

Hello all,

I am a beginner in ALSA API. I am developing a sound application where I 
need to open a PCM device for recording/playback audio data. I want to 
open this device in shared mode so that other applications can use the 
same device. I have downloaded the example ALSA program from here 
<http://www.alsa-project.org/alsa-doc/alsa-lib/_2test_2pcm_8c-example.html> 
and run it on my PC. It works fine when it runs alone. But when I start 
a song on a media player and then run this example, it shows an error 
saying -

|Playback  open error:  Device  or resource busy

|

Looking at the source code of this example I could say that 
thesnd_pcm_open 
<http://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html#gb7eaaec6f27e4ca48cada9795cd2479b>function 
at line 882 is throwing this error. It finds the device busy since 
another process is currently using it. I also tried the reverse way - 
first started the example then tried to start a song. In this case the 
media player stays idle, showing "idle" just beside the progress bar (I 
am using Banshee). I am assuming that snd_pcm_open gains exclusive right 
of the device resource so that no other process can use it.

But I don't want that. I want to play sound to a audio device without 
requiring any exclusive rights so that other processes in the PC can 
share the same device for outputting audio data.

How can I do that? How can I open a PCM device so that other processes 
can also share the same device?

P.S. : I have two sound cards on my system, one is built-in in 
motherboard, another one is a USB Card. I tried opening the USB (by 
specifying "plughw:1,0" as argument in the snd_pcm_open) card and then 
got this error.


Thank you in advance.



Regards,
Sayem Ahmed

Re: Opening a PCM device in shared mode

[Sayem Ahmed]

Hi Anders,

Thanks for your reply. I have created a /*.asoundrc*/ file in my home 
directory as you suggested and added the following configuration there 
(as you can see, these are basically the exact default configuration 
that the dmix plugin has) -

     pcm.lol
    {
         type dmix
         ipc_key 1024
         ipc_key_add_uid true
         slave {
             pcm "hw:0,0"
             period_time 125000
             rate 48000
             format S32_LE
             channels 2
         }
     }

Now when I start my program while a media player is running, it doesn't 
show any /*Device or Resource busy */error. It successfully opens the 
device and starts to write the sine wave into it. But the problem is I 
can't hear its output. I should have heard two sounds simultaneously but 
I can only hear the song in the media player, not the output sound from 
the example program. Similar things happens when I start the example 
first and the start a media player, I can only hear sounds from the 
first one.

Also, when I try to run the example program on different rate (i.e., 
rate 8000, channel 1) it throws an error saying format doesn't match. I 
guess it's because of the configuration rate that I specified in the 
file. I tried to supply multiple values for rate using array notations ( 
rate [8000, 48000] ) but it threw an error telling that the /*.asoundrc 
*/file is corrupted or old.

Actually the whole point of my exploration is this - I want to display a 
list of available audio cards/devices in my system to my application 
user who will then select the input and output audio card to 
record/playback audio data accordingly.

Thank you.


Regards,
Sayem Ahmed


On 11/21/2011 01:41 PM, Anders Gnistrup wrote:
> Hi Sayem
>
> It's because you open the device plughw:0,0.
> It's actually the device hw:0,0 with a additional "plug" pluggin. The plug pluggin takes care of resampling.
> Devices is not mixing device by design. This is handled by plugins (the dmix plugin)
>
> What you want is a device that can mix (and possible resample). For that you need a new virtual device.
> I have had the same problem myself, and the solution I have used is a .asoundrc configuration.
>
> pcm.mydmix
> {
>      type dmix
>      ipc_key 1024
>      slave {
>          pcm "hw:0,0"
>          period_time 0
>          period_size 1024
>          buffer_size 8192
>          rate 44100
> 	format "S32_LE"
>      }
> }
>
> Instead of the slave definition you could make your own. Now the device mydmix can be used as a mixing device.
>

Re: Opening a PCM device in shared mode

[Clemens Ladisch]

Sayem Ahmed wrote:
>Also, when I try to run the example program on different rate (i.e., 
>rate 8000, channel 1) it throws an error saying format doesn't match.

The "default" device for your hardware should have
the plug plugin on top of the dmix plugin.

>Actually the whole point of my exploration is this - I want to display
>a 
>list of available audio cards/devices in my system to my application 
>user who will then select the input and output audio card to 
>record/playback audio data accordingly.

This is the job of the desktop environment.
Is there a reason why your application needs to
duplicate this?


Regards,
Clemens

Re: Opening a PCM device in shared mode

[Sayem Ahmed]

On 11/24/2011 11:31 PM, Clemens Ladisch wrote:
> Is there a reason why your application needs to
> duplicate this?

Yes. Clients would like to install multiple sound cards into their 
system so that they can perform multiple VoIP calls at the same time. 
For each call a different sound card will be used. This is why I need 
this functionality in my application.

Re: Opening a PCM device in shared mode

[Clemens Ladisch]

Sayem Ahmed wrote:
> On 11/24/2011 11:31 PM, Clemens Ladisch wrote:
> > Is there a reason why your application needs to
> > duplicate this?
>
> Yes. Clients would like to install multiple sound cards into
> their system so that they can perform multiple VoIP calls at
> the same time.

Use snd_card_next() to enumerate cards.
To open the default device of a specific card, try device name
"default:CARD=x", but this won't work if the user has redefined
the "default" device definition.


Regards,
Clemens