To query the current video output applications call the
VIDIOC_G_OUTPUT ioctl with a pointer to an integer
where the driver stores the number of the output, as in the
struct v4l2_output index field. This ioctl
will fail only when there are no video outputs, returning the
EINVAL error code.
To select a video output applications store the number of the
desired output in an integer and call the
VIDIOC_S_OUTPUT ioctl with a pointer to this integer.
Side effects are possible. For example outputs may support different
video standards, so the driver may implicitly switch the current
standard. It is good practice to select an output before querying or
negotiating any other parameters.
Information about video outputs is available using the
VIDIOC_ENUMOUTPUT ioctl.