To query the attributes of an audio output applications
initialize the index field and zero out the
reserved array of a struct v4l2_audioout and
call the VIDIOC_G_AUDOUT ioctl with a pointer
to this structure. Drivers fill the rest of the structure or return an
EINVAL error code when the index is out of bounds. To enumerate all audio
outputs applications shall begin at index zero, incrementing by one
until the driver returns EINVAL.
Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.
See ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT(2) for a description of struct v4l2_audioout.