ioctl VIDIOC_QBUF, VIDIOC_DQBUF &manvol; VIDIOC_QBUF VIDIOC_DQBUF Exchange a buffer with the driver int ioctl int fd int request struct v4l2_buffer *argp fd &fd; request VIDIOC_QBUF, VIDIOC_DQBUF argp 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 &v4l2-buffer; to the same buffer type as was previously used with &v4l2-format; type and &v4l2-requestbuffers; type. Applications must also set the index field. Valid index numbers range from zero to the number of buffers allocated with &VIDIOC-REQBUFS; (&v4l2-requestbuffers; count) minus one. The contents of the struct v4l2_buffer returned by a &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 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 &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;. 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 &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 &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 &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 does not have to be passed; the m.planes member must be set to NULL in that case. By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the &func-open; function, VIDIOC_DQBUF returns immediately with an &EAGAIN; when no buffer is available. The v4l2_buffer structure is specified in . &return-value; EAGAIN 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. 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. 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.