ioctl VIDIOC_QBUF,DQBUF和驱动交换一个buffer

Video for Linux Two API Specification: Revision 0.24

ioctl VIDIOC_QBUF, VIDIOC_DQBUF

NameVIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver

Synopsis

int ioctl(int fd, int request, struct v4l2_buffer *argp);

Arguments

fd

File descriptor returned by .

request

VIDIOC_QBUF, VIDIOC_DQBUF

argp

Description

应用程序的获取队列和驱动程序的输出队列，是对接的。

QBUF：

应用程序用来入队一个空的获取队列，或者是填充一个驱动的输入队列中的输出buffer，说法依赖于I/O方法。

Applications call the VIDIOC_QBUF ioctl to enqueue an empty (capturing) or filled (output) buffer in the driver's incoming queue. The semantics语义 depend on the selected I/O method.

要是想入队一个内存mapped的buffer，应用程序设置一个v4l_buffer结构体的type域（与先前的v4l_format的type 和 v4l_requestbuffers的type 一样 ），设置memory域为

V4L2_MEMORY_MMAP  以及index域。可用的index的number从0到被分配的buffers的数字。

To enqueue a  buffer applications set the type field of a struct  to the same buffer type as previously struct  type and struct  type, the memory field to V4L2_MEMORY_MMAP and the index field. Valid index numbers range from zero to the number of buffers allocated

 with (struct  count) minus one. The contents of the struct v4l2_buffer returned by a  ioctl will do as well. When the buffer is intended for output (type is V4L2_BUF_TYPE_VIDEO_OUTPUT or V4L2_BUF_TYPE_VBI_OUTPUT) applications must also initialize the bytesused, field and timestamp fields. See for details. When VIDIOC_QBUF is called with a pointer to this structure the driver sets

 the V4L2_BUF_FLAG_MAPPED and V4L2_BUF_FLAG_QUEUED flags and clears the V4L2_BUF_FLAG_DONE flag in the flags field, or it returns an EINVAL error code.

To enqueue a  buffer applications set the type field of a struct  to the same buffer type as previously struct  type and struct  type, the memory field to V4L2_MEMORY_USERPTR and the m.userptr field to the address of the buffer and length to its size. When the buffer is intended for output additional fields must be set as above. When VIDIOC_QBUF is called with a pointer to this structure the driver sets the V4L2_BUF_FLAG_QUEUED flag and clears

 the V4L2_BUF_FLAG_MAPPED and V4L2_BUF_FLAG_DONE flags in the flags field, or it returns an error code. This ioctl locks the memory pages of the buffer in physical memory, they cannot be swapped out to disk. Buffers remain locked until dequeued, until the  or  ioctl are called, or until the device is closed.

应用程序调用DQBUF 出队一个被填满的视频获取buffer或者是一个来自驱动的输出队列的显示用的（输出）buffer。

应用程序，只需要设置一个v4l2_buffer结构的type和memory域。

当使用一个指向这个结构体的指针来调用DQBUF时，驱动会填充这个结构体其他的域，或者发回一个EAGAIN错误码表示缓冲不可用。

Applications call the VIDIOC_DQBUF ioctl to dequeue a filled (capturing) or displayed (output) buffer from the driver's outgoing queue. They just set the type and memoryfields of a struct  as above, when VIDIOC_DQBUF is called with a pointer to this structure the driver fills the remaining fields or returns an error code.

By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the  function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available.

The v4l2_buffer structure is specified in .

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately适当的:

EAGAIN 表示没有阻塞的IO在使用O_NONBLOCK时被选中，并且输出队列中没有buffer。

Non-blocking I/O has been selected using O_NONBLOCK and no buffer was in the outgoing queue.

EINVAL

The buffer type is not supported, or the index is out of bounds, or no buffers have been allocated yet, or the userptr or length are invalid.

ENOMEM

Not enough physical or virtual memory was available to enqueue a user pointer buffer.

EIO

VIDIOC_DQBUF failed due to an internal error. Can also indicate temporary problems like signal loss. Note the driver might dequeue an (empty) buffer despite returning an error, or even stop capturing.

ioctl VIDIOC_OVERLAY		ioctl VIDIOC_QUERYBUF