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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: DTV.dmx
+
+.. _DMX_QBUF:
+
+*************************
+ioctl DMX_QBUF, DMX_DQBUF
+*************************
+
+Name
+====
+
+DMX_QBUF - DMX_DQBUF - Exchange a buffer with the driver
+
+.. warning:: this API is still experimental
+
+Synopsis
+========
+
+.. c:macro:: DMX_QBUF
+
+``int ioctl(int fd, DMX_QBUF, struct dmx_buffer *argp)``
+
+.. c:macro:: DMX_DQBUF
+
+``int ioctl(int fd, DMX_DQBUF, struct dmx_buffer *argp)``
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :c:func:`open()`.
+
+``argp``
+    Pointer to struct :c:type:`dmx_buffer`.
+
+Description
+===========
+
+Applications call the ``DMX_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 ``index`` field. Valid index
+numbers range from zero to the number of buffers allocated with
+:ref:`DMX_REQBUFS` (struct :c:type:`dmx_requestbuffers` ``count``) minus
+one. The contents of the struct :c:type:`dmx_buffer` returned
+by a :ref:`DMX_QUERYBUF` ioctl will do as well.
+
+When ``DMX_QBUF`` is called with a pointer to this structure, it locks the
+memory pages of the buffer in physical memory, so they cannot be swapped
+out to disk. Buffers remain locked until dequeued, until the
+device is closed.
+
+Applications call the ``DMX_DQBUF`` ioctl to dequeue a filled
+(capturing) buffer from the driver's outgoing queue.
+They just set the ``index`` field with the buffer ID to be queued.
+When ``DMX_DQBUF`` is called with a pointer to struct :c:type:`dmx_buffer`,
+the driver fills the remaining fields or returns an error code.
+
+By default ``DMX_DQBUF`` blocks when no buffer is in the outgoing
+queue. When the ``O_NONBLOCK`` flag was given to the
+:c:func:`open()` function, ``DMX_DQBUF`` returns
+immediately with an ``EAGAIN`` error code when no buffer is available.
+
+The struct :c:type:`dmx_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.
+
+EAGAIN
+    Non-blocking I/O has been selected using ``O_NONBLOCK`` and no
+    buffer was in the outgoing queue.
+
+EINVAL
+    The ``index`` is out of bounds, or no buffers have been allocated yet.
+
+EIO
+    ``DMX_DQBUF`` failed due to an internal error. Can also indicate
+    temporary problems like signal loss or CRC errors.