Mixer event notification

By default, all mixer applications are required to keep up-to-date with all mixer changes.

Keeping up-to-date with all mixer changes is done by enqueuing a mixer-change event on all applications other than the application making a change. The driver enqueues these events on all applications that have an open mixer handle, unless the application uses the snd_mixer_set_filter() API call to mask out events it's not interested in.

Applications use the snd_mixer_read() function to read the enqueued mixer events. The arguments to this functions are the mixer handle and a structure of callback functions to call based on the event type.

You can use the select() function to determine when to call snd_mixer_read(). To get the file descriptor to pass to select(), call snd_mixer_file_descriptor() .

Here's a short example:

static void mixer_callback_group (void *private_data,
                                  int cmd,
                                  snd_mixer_gid_t * gid)
{
    switch (cmd)
    {
    case SND_MIXER_READ_GROUP_VALUE:
        printf ("Mixer group %s %d changed value \n",
                gid->name, gid->index);
        break;

    case SND_MIXER_READ_GROUP_ADD:
        break;

    case SND_MIXER_READ_GROUP_REMOVE:
        break;
    }
}

int mixer_update (int fd, void *data, unsigned mode)
{
    snd_mixer_callbacks_t callbacks = { 0, 0, 0, 0 };

    callbacks.group = mixer_callback_group;
    snd_mixer_read (mixer_handle, &callbacks);
    return (Pt_CONTINUE);
}

int main (void)
{
    snd_mixer_t *mixer_handle;
    int ret;

    if ((ret = snd_mixer_open (&mixer_handle, 0, 0) < 0))
        printf ("Unable to open/read mixer - %s",
                snd_strerror (ret));

    PtAppAddFd (NULL,
                snd_mixer_file_descriptor (mixer_handle),
                Pt_FD_READ, mixer_update, NULL);
    ...
}