VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver
Applications call the VIDIOC_QBUF ioctl to enqueue an empty (capturing) or filled (output) buffer in the driver’s incoming queue. The semantics depend on the selected I/O method.
To enqueue a buffer applications set the type field of a struct v4l2_buffer to the same buffer type as was previously used with struct v4l2_format type and struct v4l2_requestbuffers type. Applications must also set the index field. Valid index numbers range from zero to the number of buffers allocated with ioctl VIDIOC_REQBUFS (struct v4l2_requestbuffers count) minus one. The contents of the struct v4l2_buffer returned by a ioctl VIDIOC_QUERYBUF ioctl will do as well. When the buffer is intended for output (type is V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE, or V4L2_BUF_TYPE_VBI_OUTPUT) applications must also initialize the bytesused, field and timestamp fields, see Buffers for details. Applications must also set flags to 0. The reserved2 and reserved fields must be set to 0. When using the multi-planar API, the m.planes field must contain a userspace pointer to a filled-in array of struct v4l2_plane and the length field must be set to the number of elements in that array.
To enqueue a memory mapped buffer applications set the memory field to V4L2_MEMORY_MMAP. 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 user pointer buffer applications set the memory field to V4L2_MEMORY_USERPTR, the m.userptr field to the address of the buffer and length to its size. When the multi-planar API is used, m.userptr and length members of the passed array of struct v4l2_plane have to be used instead. 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 VIDIOC_STREAMOFF or ioctl VIDIOC_REQBUFS ioctl is called, or until the device is closed.
To enqueue a DMABUF buffer applications set the memory field to V4L2_MEMORY_DMABUF and the m.fd field to a file descriptor associated with a DMABUF buffer. When the multi-planar API is used the m.fd fields of the passed array of struct v4l2_plane have to be used instead. 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 buffer. Locking a buffer means passing it to a driver for a hardware access (usually DMA). If an application accesses (reads/writes) a locked buffer then the result is undefined. Buffers remain locked until dequeued, until the VIDIOC_STREAMOFF or ioctl VIDIOC_REQBUFS ioctl is called, or until the device is closed.
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, memory and reserved fields of a struct v4l2_buffer as above, when VIDIOC_DQBUF is called with a pointer to this structure the driver fills the remaining fields or returns an error code. The driver may also set V4L2_BUF_FLAG_ERROR in the flags field. It indicates a non-critical (recoverable) streaming error. In such case the application may continue as normal, but should be aware that data in the dequeued buffer might be corrupted. When using the multi-planar API, the planes array must be passed in as well.
By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available.
The struct v4l2_buffer structure is specified in Buffers.
On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.
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. Reusing such buffer may be unsafe though and its details (e.g. index) may not be returned either. It is recommended that drivers indicate recoverable errors by setting the V4L2_BUF_FLAG_ERROR and returning 0 instead. In that case the application should be able to safely reuse the buffer and continue streaming.