Man page 9 : snd_ctl_add_slave

snd_ctl_add_slave - Add a virtual slave control

int snd_ctl_add_slave(struct snd_kcontrol *  master , struct snd_kcontrol *  slave );

master
    vmaster element

slave
    slave element to add

Add a virtual slave control to the given master element created via snd_ctl_create_virtual_master beforehand.

All slaves must be the same type (returning the same information via info callback). The function doesnAqt check it, so itAqs your responsibility.

Also, some additional limitations: at most two channels, logarithmic volume control (dB level) thus no linear volume, master can only attenuate the volume without gain

Zero if successful or a negative error code.