.. Permission is granted to copy, distribute and/or modify this
.. document under the terms of the GNU Free Documentation License,
.. Version 1.1 or any later version published by the Free Software
.. Foundation, with no Invariant Sections, no Front-Cover Texts
.. and no Back-Cover Texts. A copy of the license is included at
.. Documentation/media/uapi/fdl-appendix.rst.
..
.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections

.. _VIDIOC_QUERYBUF:

*********************
ioctl VIDIOC_QUERYBUF
*********************

Name
====

VIDIOC_QUERYBUF - Query the status of a buffer


Synopsis
========

.. c:function:: int ioctl( int fd, VIDIOC_QUERYBUF, struct v4l2_buffer *argp )
    :name: VIDIOC_QUERYBUF


Arguments
=========

``fd``
    File descriptor returned by :ref:`open() <func-open>`.

``argp``
    Pointer to struct :c:type:`v4l2_buffer`.


Description
===========

This ioctl is part of the :ref:`streaming <mmap>` I/O method. It can
be used to query the status of a buffer at any time after buffers have
been allocated with the :ref:`VIDIOC_REQBUFS` ioctl.

Applications set the ``type`` field of a struct
:c:type:`v4l2_buffer` to the same buffer type as was
previously used with struct :c:type:`v4l2_format` ``type``
and struct :c:type:`v4l2_requestbuffers` ``type``,
and the ``index`` field. Valid index numbers range from zero to the
number of buffers allocated with
:ref:`VIDIOC_REQBUFS` (struct
:c:type:`v4l2_requestbuffers` ``count``) minus
one. The ``reserved`` and ``reserved2`` fields must be set to 0. When
using the :ref:`multi-planar API <planar-apis>`, the ``m.planes``
field must contain a userspace pointer to an array of struct
:c:type:`v4l2_plane` and the ``length`` field has to be set
to the number of elements in that array. After calling
:ref:`VIDIOC_QUERYBUF` with a pointer to this structure drivers return an
error code or fill the rest of the structure.

In the ``flags`` field the ``V4L2_BUF_FLAG_MAPPED``,
``V4L2_BUF_FLAG_PREPARED``, ``V4L2_BUF_FLAG_QUEUED`` and
``V4L2_BUF_FLAG_DONE`` flags will be valid. The ``memory`` field will be
set to the current I/O method. For the single-planar API, the
``m.offset`` contains the offset of the buffer from the start of the
device memory, the ``length`` field its size. For the multi-planar API,
fields ``m.mem_offset`` and ``length`` in the ``m.planes`` array
elements will be used instead and the ``length`` field of struct
:c:type:`v4l2_buffer` is set to the number of filled-in
array elements. The driver may or may not set the remaining fields and
flags, they are meaningless in this context.

The struct :c:type:`v4l2_buffer` structure is specified in
:ref:`buffer`.


Return Value
============

On success 0 is returned, on error -1 and the ``errno`` variable is set
appropriately. The generic error codes are described at the
:ref:`Generic Error Codes <gen-errors>` chapter.

EINVAL
    The buffer ``type`` is not supported, or the ``index`` is out of
    bounds.