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+.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
+.. c:namespace:: DTV.dmx
+
+.. _DMX_QUERYBUF:
+
+******************
+ioctl DMX_QUERYBUF
+******************
+
+Name
+====
+
+DMX_QUERYBUF - Query the status of a buffer
+
+.. warning:: this API is still experimental
+
+Synopsis
+========
+
+.. c:macro:: DMX_QUERYBUF
+
+``int ioctl(int fd, DMX_QUERYBUF, struct dvb_buffer *argp)``
+
+Arguments
+=========
+
+``fd``
+    File descriptor returned by :c:func:`open()`.
+
+``argp``
+    Pointer to struct :c:type:`dvb_buffer`.
+
+Description
+===========
+
+This ioctl is part of the mmap streaming 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:`DMX_REQBUFS` ioctl.
+
+Applications set the ``index`` field. Valid index numbers range from zero
+to the number of buffers allocated with :ref:`DMX_REQBUFS`
+(struct :c:type:`dvb_requestbuffers` ``count``) minus one.
+
+After calling :ref:`DMX_QUERYBUF` with a pointer to this structure,
+drivers return an error code or fill the rest of the structure.
+
+On success, the ``offset`` will contain the offset of the buffer from the
+start of the device memory, the ``length`` field its size, and the
+``bytesused`` the number of bytes occupied by data in the buffer (payload).
+
+Return Value
+============
+
+On success 0 is returned, the ``offset`` will contain the offset of the
+buffer from the start of the device memory, the ``length`` field its size,
+and the ``bytesused`` the number of bytes occupied by data in the buffer
+(payload).
+
+On error it returns -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 ``index`` is out of bounds.