The VIDIOC_STREAMON and
VIDIOC_STREAMOFF ioctl start and stop the capture
or output process during streaming (memory
mapping or user pointer) I/O.
Specifically the capture hardware is disabled and no input
buffers are filled (if there are any empty buffers in the incoming
queue) until VIDIOC_STREAMON has been called.
Accordingly the output hardware is disabled, no video signal is
produced until VIDIOC_STREAMON has been called.
The ioctl will succeed only when at least one output buffer is in the
incoming queue.
The VIDIOC_STREAMOFF ioctl, apart of
aborting or finishing any DMA in progress, unlocks any user pointer
buffers locked in physical memory, and it removes all buffers from the
incoming and outgoing queues. That means all images captured but not
dequeued yet will be lost, likewise all images enqueued for output but
not transmitted yet. I/O returns to the same state as after calling
VIDIOC_REQBUFS and can be restarted accordingly.
Both ioctls take a pointer to an integer, the desired buffer or
stream type. This is the same as struct v4l2_requestbuffers
type.
Note applications can be preempted for unknown periods right
before or after the VIDIOC_STREAMON or
VIDIOC_STREAMOFF calls, there is no notion of
starting or stopping "now". Buffer timestamps can be used to
synchronize with other events.