V4L2命令之 VIDIOC_STREAMON, VIDIOC_STREAMOFF

http://linuxtv.org/downloads/v4l-dvb-apis/vidioc-streamon.html

Name

VIDIOC_STREAMON, VIDIOC_STREAMOFF — Start or stop streaming I/O

Synopsis

intioctl(int fd,
int request,
const int *argp);

 

Arguments

fd

File descriptor returned byopen().

request

VIDIOC_STREAMON, VIDIOC_STREAMOFF

argp

Description

The VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl start and stop the captureor output process during streaming (memory mapping oruser 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) untilVIDIOC_STREAMON has been called.Accordingly the output hardware is disabled, no video signal is produced untilVIDIOC_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. such as V4L2_BUF_TYPE_VIDEO_CAPTURE.

If VIDIOC_STREAMON is called when streaming is already in progress, or if VIDIOC_STREAMOFF is called when streaming is already stopped, then the ioctl does nothing and 0 is returned.

Note that applications can be preempted for unknown periods right before or after theVIDIOC_STREAMON orVIDIOC_STREAMOFF calls, there is no notion of starting or stopping "now". Buffer timestamps can be used to synchronize with other events.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at theGeneric Error Codes chapter.

EINVAL

The buffertype is not supported, or no buffers have been allocated (memory mapping) or enqueued (output) yet.

EPIPE

The driver implementspad-level format configuration and the pipeline configuration is invalid.