Because V4L2 covers a wide variety of devices not all aspects of the API are equally applicable to all types of devices. Furthermore devices of the same type have different capabilities and this specification permits the omission of a few complicated and less important parts of the API.
The VIDIOC_QUERYCAP ioctl is available to check if the kernel
device is compatible with this specification, and to query the functions and I/O
methods supported by the device. Other features can be queried
by calling the respective ioctl, for example VIDIOC_ENUMINPUT
to learn about the number, types and names of video connectors on the
device. Although abstraction is a major objective of this API, the
ioctl also allows driver specific applications to reliable identify
the driver.
All V4L2 drivers must support
VIDIOC_QUERYCAP. Applications should always call
this ioctl after opening the device.