[alsa-devel] Retrieving the number of input channels on a sound card

[André Prado]

I Found it out.. hope it helps someone.

    snd_pcm_t *capture_handle;
    snd_pcm_hw_params_t *hw_params;

    snd_pcm_open( &capture_handle, str, SND_PCM_STREAM_PLAYBACK, 0 );
    snd_pcm_hw_params_malloc (&hw_params);

    snd_pcm_hw_params_any(capture_handle, hw_params);

    unsigned int num_canais = 0;
    int ret;

    ret = snd_pcm_hw_params_get_channels(hw_params, &num_canais);

    printf("Number of channels: %i\n",num_canais);



2011/6/8 André Prado <andreprado88 at gmail.com>:
> Hello. I am new to Alsa programming and i want to get the number of
> input channels on an audio device.
> I am reading a lot of tutorials and documentations but i am struggling
> to do this.
>
> Here is a code i've saw to  Lists the hardware names of MIDI output
> device/sub-devices. I've understood it. But i want to get the number
> of input channels.
>
> Any help is appreciated. THanks
>
> // listmidi.c
> // Lists the hardware names of MIDI output device/sub-devices
> // upon each ALSA sound card in the system.
> //
> // Compile as:
> // gcc -o listmidi listmidi.c -lasound
>
> #include <stdio.h>
> #include <string.h>
> #include <alsa/asoundlib.h>
>
> int main(int argc, char **argv)
> {
>   register int  err;
>   int           cardNum;
>
>   // Start with first card
>   cardNum = -1;
>
>   for (;;)
>   {
>      snd_ctl_t *cardHandle;
>
>      // Get next sound card's card number. When "cardNum" == -1, then ALSA
>      // fetches the first card
>      if ((err = snd_card_next(&cardNum)) < 0)
>      {
>         printf("Can't get the next card number: %s\n", snd_strerror(err));
>         break;
>      }
>
>      // No more cards? ALSA sets "cardNum" to -1 if so
>      if (cardNum < 0) break;
>
>      // Open this card's control interface. We specify only the card
> number -- not
>      // any device nor sub-device too
>      {
>      char   str[64];
>
>      sprintf(str, "hw:%i", cardNum);
>      if ((err = snd_ctl_open(&cardHandle, str, 0)) < 0)
>      {
>         printf("Can't open card %i: %s\n", cardNum, snd_strerror(err));
>         continue;
>      }
>      }
>
>      {
>      int      devNum;
>
>      // Start with the first MIDI device on this card
>      devNum = -1;
>
>      for (;;)
>      {
>         snd_rawmidi_info_t  *rawMidiInfo;
>         register int        subDevCount, i;
>
>         // Get the number of the next MIDI device on this card
>         if ((err = snd_ctl_rawmidi_next_device(cardHandle, &devNum)) < 0)
>         {
>            printf("Can't get next MIDI device number: %s\n",
> snd_strerror(err));
>            break;
>         }
>
>         // No more MIDI devices on this card? ALSA sets "devNum" to -1 if so.
>         // NOTE: It's possible that this sound card may have no MIDI
> devices on it
>         // at all, for example if it's only a digital audio card
>         if (devNum < 0) break;
>
>         // To get some info about the subdevices of this MIDI device
> (on the card), we need a
>         // snd_rawmidi_info_t, so let's allocate one on the stack
>         snd_rawmidi_info_alloca(&rawMidiInfo);
>         memset(rawMidiInfo, 0, snd_rawmidi_info_sizeof());
>
>         // Tell ALSA which device (number) we want info about
>         snd_rawmidi_info_set_device(rawMidiInfo, devNum);
>
>         // Get info on the MIDI outs of this device
>         snd_rawmidi_info_set_stream(rawMidiInfo, SND_RAWMIDI_STREAM_OUTPUT);
>
>         i = -1;
>         subDevCount = 1;
>
>         // More subdevices?
>         while (++i < subDevCount)
>         {
>            // Tell ALSA to fill in our snd_rawmidi_info_t with info
> on this subdevice
>            snd_rawmidi_info_set_subdevice(rawMidiInfo, i);
>            if ((err = snd_ctl_rawmidi_info(cardHandle, rawMidiInfo)) < 0)
>            {
>               printf("Can't get info for MIDI output subdevice
> hw:%i,%i,%i: %s\n", cardNum, devNum, i, snd_strerror(err));
>               continue;
>            }
>
>            // Print out how many subdevices (once only)
>            if (!i)
>            {
>               subDevCount = snd_rawmidi_info_get_subdevices_count(rawMidiInfo);
>               printf("\nFound %i MIDI output subdevices on card
> %i\n", subDevCount, cardNum);
>            }
>
>            // NOTE: If there's only one subdevice, then the subdevice
> number is immaterial,
>            // and can be omitted when you specify the hardware name
>            printf((subDevCount > 1 ? "    hw:%i,%i,%i\n" : "
> hw:%i,%i\n"), cardNum, devNum, i);
>         }
>      }
>      }
>
>      // Close the card's control interface after we're done with it
>      snd_ctl_close(cardHandle);
>   }
>
>   snd_config_update_free_global();
> }
>
> --
> Att
> André
>



-- 
Att
André