Definition of Controls
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## Definition of Controls

To create a new control, you need to define the following three callbacks: `info`, `get` and `put`. Then, define a struct snd_kcontrol_new record, such as:

Example 6.1. Definition of a Control

      static struct snd_kcontrol_new my_control = {
              .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
              .name = "PCM Playback Switch",
              .index = 0,
              .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
              .private_value = 0xffff,
              .info = my_control_info,
              .get = my_control_get,
              .put = my_control_put
      };

The `iface` field specifies the control type, `SNDRV_CTL_ELEM_IFACE_XXX`, which is usually `MIXER`. Use `CARD` for global controls that are not logically part of the mixer. If the control is closely associated with some specific device on the sound card, use `HWDEP`, `PCM`, `RAWMIDI`, `TIMER`, or `SEQUENCER`, and specify the device number with the `device` and `subdevice` fields.

The `name` is the name identifier string. Since ALSA 0.9.x, the control name is very important, because its role is classified from its name. There are pre-defined standard control names. The details are described in the Control Names subsection.

The `index` field holds the index number of this control. If there are several different controls with the same name, they can be distinguished by the index number. This is the case when several codecs exist on the card. If the index is zero, you can omit the definition above.

The `access` field contains the access type of this control. Give the combination of bit masks, `SNDRV_CTL_ELEM_ACCESS_XXX`, there. The details will be explained in the Access Flags subsection.

The `private_value` field contains an arbitrary long integer value for this record. When using the generic `info`, `get` and `put` callbacks, you can pass a value through this field. If several small numbers are necessary, you can combine them in bitwise. Or, it's possible to give a pointer (casted to unsigned long) of some record to this field, too.

The `tlv` field can be used to provide metadata about the control; see the Metadata subsection.

The other three are callback functions.

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