.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _VIDIOC_SUBSCRIBE_EVENT:
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.. _VIDIOC_UNSUBSCRIBE_EVENT:
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******************************************************
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ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT
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******************************************************
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Name
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====
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VIDIOC_SUBSCRIBE_EVENT - VIDIOC_UNSUBSCRIBE_EVENT - Subscribe or unsubscribe event
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Synopsis
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========
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.. c:macro:: VIDIOC_SUBSCRIBE_EVENT
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``int ioctl(int fd, VIDIOC_SUBSCRIBE_EVENT, struct v4l2_event_subscription *argp)``
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.. c:macro:: VIDIOC_UNSUBSCRIBE_EVENT
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``int ioctl(int fd, VIDIOC_UNSUBSCRIBE_EVENT, struct v4l2_event_subscription *argp)``
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Arguments
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=========
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``fd``
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File descriptor returned by :c:func:`open()`.
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``argp``
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Pointer to struct :c:type:`v4l2_event_subscription`.
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Description
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===========
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Subscribe or unsubscribe V4L2 event. Subscribed events are dequeued by
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using the :ref:`VIDIOC_DQEVENT` ioctl.
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.. tabularcolumns:: |p{4.6cm}|p{4.4cm}|p{8.7cm}|
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.. c:type:: v4l2_event_subscription
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.. flat-table:: struct v4l2_event_subscription
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:header-rows: 0
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:stub-columns: 0
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:widths: 1 1 2
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* - __u32
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- ``type``
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- Type of the event, see :ref:`event-type`.
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.. note::
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``V4L2_EVENT_ALL`` can be used with
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:ref:`VIDIOC_UNSUBSCRIBE_EVENT <VIDIOC_SUBSCRIBE_EVENT>` for
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unsubscribing all events at once.
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* - __u32
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- ``id``
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- ID of the event source. If there is no ID associated with the
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event source, then set this to 0. Whether or not an event needs an
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ID depends on the event type.
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* - __u32
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- ``flags``
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- Event flags, see :ref:`event-flags`.
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* - __u32
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- ``reserved``\ [5]
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- Reserved for future extensions. Drivers and applications must set
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the array to zero.
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.. tabularcolumns:: |p{6.8cm}|p{2.2cm}|p{8.5cm}|
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.. _event-flags:
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.. flat-table:: Event Flags
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:header-rows: 0
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:stub-columns: 0
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:widths: 3 1 4
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* - ``V4L2_EVENT_SUB_FL_SEND_INITIAL``
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- 0x0001
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- When this event is subscribed an initial event will be sent
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containing the current status. This only makes sense for events
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that are triggered by a status change such as ``V4L2_EVENT_CTRL``.
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Other events will ignore this flag.
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* - ``V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK``
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- 0x0002
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- If set, then events directly caused by an ioctl will also be sent
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to the filehandle that called that ioctl. For example, changing a
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control using :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` will cause
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a V4L2_EVENT_CTRL to be sent back to that same filehandle.
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Normally such events are suppressed to prevent feedback loops
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where an application changes a control to a one value and then
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another, and then receives an event telling it that that control
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has changed to the first value.
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Since it can't tell whether that event was caused by another
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application or by the :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>`
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call it is hard to decide whether to set the control to the value
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in the event, or ignore it.
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Think carefully when you set this flag so you won't get into
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situations like that.
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Return Value
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============
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On success 0 is returned, on error -1 and the ``errno`` variable is set
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appropriately. The generic error codes are described at the
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:ref:`Generic Error Codes <gen-errors>` chapter.
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