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- .. -*- coding: utf-8; mode: rst -*-
- .. _VIDIOC_SUBSCRIBE_EVENT:
- .. _VIDIOC_UNSUBSCRIBE_EVENT:
- ******************************************************
- ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT
- ******************************************************
- Name
- ====
- VIDIOC_SUBSCRIBE_EVENT - VIDIOC_UNSUBSCRIBE_EVENT - Subscribe or unsubscribe event
- Synopsis
- ========
- .. c:function:: int ioctl( int fd, VIDIOC_SUBSCRIBE_EVENT, struct v4l2_event_subscription *argp )
- :name: VIDIOC_SUBSCRIBE_EVENT
- .. c:function:: int ioctl( int fd, VIDIOC_UNSUBSCRIBE_EVENT, struct v4l2_event_subscription *argp )
- :name: VIDIOC_UNSUBSCRIBE_EVENT
- Arguments
- =========
- ``fd``
- File descriptor returned by :ref:`open() <func-open>`.
- ``argp``
- Description
- ===========
- Subscribe or unsubscribe V4L2 event. Subscribed events are dequeued by
- using the :ref:`VIDIOC_DQEVENT` ioctl.
- .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
- .. c:type:: v4l2_event_subscription
- .. flat-table:: struct v4l2_event_subscription
- :header-rows: 0
- :stub-columns: 0
- :widths: 1 1 2
- * - __u32
- - ``type``
- - Type of the event, see :ref:`event-type`.
- .. note::
- ``V4L2_EVENT_ALL`` can be used with
- :ref:`VIDIOC_UNSUBSCRIBE_EVENT <VIDIOC_SUBSCRIBE_EVENT>` for
- unsubscribing all events at once.
- * - __u32
- - ``id``
- - ID of the event source. If there is no ID associated with the
- event source, then set this to 0. Whether or not an event needs an
- ID depends on the event type.
- * - __u32
- - ``flags``
- - Event flags, see :ref:`event-flags`.
- * - __u32
- - ``reserved``\ [5]
- - Reserved for future extensions. Drivers and applications must set
- the array to zero.
- .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
- .. _event-flags:
- .. flat-table:: Event Flags
- :header-rows: 0
- :stub-columns: 0
- :widths: 3 1 4
- * - ``V4L2_EVENT_SUB_FL_SEND_INITIAL``
- - 0x0001
- - When this event is subscribed an initial event will be sent
- containing the current status. This only makes sense for events
- that are triggered by a status change such as ``V4L2_EVENT_CTRL``.
- Other events will ignore this flag.
- * - ``V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK``
- - 0x0002
- - If set, then events directly caused by an ioctl will also be sent
- to the filehandle that called that ioctl. For example, changing a
- control using :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>` will cause
- a V4L2_EVENT_CTRL to be sent back to that same filehandle.
- Normally such events are suppressed to prevent feedback loops
- where an application changes a control to a one value and then
- another, and then receives an event telling it that that control
- has changed to the first value.
- Since it can't tell whether that event was caused by another
- application or by the :ref:`VIDIOC_S_CTRL <VIDIOC_G_CTRL>`
- call it is hard to decide whether to set the control to the value
- in the event, or ignore it.
- Think carefully when you set this flag so you won't get into
- situations like that.
- 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.
|
