HowTo access a mixer control

From the ALSA wiki

I couldn't find any documentation anywhere about this. After lots of trial and error I got just enough from the amixer source code to work out what I needed. All error handling is omitted for clarity.

So here is the minimum code needed to duplicate the following command line:

amixer -c  1 cset name='Line Playback Volume',0 66 

The coloured parameters are the same colour in the code.

We will use the high-level control API. First we need to open an hctl and load its data:

snd_hctl_t *hctl;
err = snd_hctl_open(&hctl, "hw:1", 0));
err = snd_hctl_load(hctl);

Now we need to initialise a snd_ctl_elem_id_t structure which alsa can use to find the control we want. We must specify the interface and either the numeric id or the name of the control.

snd_ctl_elem_id_t *id;
snd_ctl_elem_id_alloca(&id);
snd_ctl_elem_id_set_interface(id, SND_CTL_ELEM_IFACE_MIXER);
snd_ctl_elem_id_set_name(id, "Line Playback Volume");

With this initialised we can use it to get a snd_hctl_elem_t object and a snd_ctl_elem_value_t object. The elem is retrieved as follows:

snd_hctl_elem_t *elem = snd_hctl_find_elem(hctl, id);

The elem_value is initialised with the id of the element and the value we want to set like this:

snd_ctl_elem_value_t *control;
snd_ctl_elem_value_alloca(&control);
snd_ctl_elem_value_set_id(control, id);    
snd_ctl_elem_value_set_integer(control, 0, 66);

Finally we are ready to write our value to the mixer control:

err = snd_hctl_elem_write(elem, control);

And if we don't want to do anything else we can close the high level control handle:

snd_hctl_close(hctl);