ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT
&manvol;

# VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT

Subscribe or unsubscribe event

    	int ioctl
    	int fd
    	int request
    	struct v4l2_event_subscription
    *argp

## Arguments

`fd`

  &fd;

`request`

  VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT

`argp`

## Description

Subscribe or unsubscribe V4L2 event. Subscribed events are dequeued by using the &VIDIOC-DQEVENT; ioctl.

### struct v4l2_event_subscription

&cs-str;
__u32 | type | Type of the event.
__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 .
__u32 | reserved[5] | Reserved for future extensions. Drivers and applications must set the array to zero.

### Event Types

&cs-def;
`V4L2_EVENT_ALL` | 0 | All events. V4L2_EVENT_ALL is valid only for VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
`V4L2_EVENT_VSYNC` | 1 | This event is triggered on the vertical sync. This event has a &v4l2-event-vsync; associated with it.
`V4L2_EVENT_EOS` | 2 | This event is triggered when the end of a stream is reached. This is typically used with MPEG decoders to report to the application when the last of the MPEG stream has been decoded.

`V4L2_EVENT_CTRL`

3

This event requires that the id matches the control ID from which you want to receive events. This event is triggered if the control's value changes, if a button control is pressed or if the control's flags change. This event has a &v4l2-event-ctrl; associated with it. This struct contains much of the same information as &v4l2-queryctrl; and &v4l2-control;.

If the event is generated due to a call to &VIDIOC-S-CTRL; or &VIDIOC-S-EXT-CTRLS;, then the event will not be sent to the file handle that called the ioctl function. This prevents nasty feedback loops. If you do want to get the event, then set the `V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK` flag.

This event type will ensure that no information is lost when more events are raised than there is room internally. In that case the &v4l2-event-ctrl; of the second-oldest event is kept, but the changes field of the second-oldest event is ORed with the changes field of the oldest event.

`V4L2_EVENT_FRAME_SYNC`

4

Triggered immediately when the reception of a frame has begun. This event has a &v4l2-event-frame-sync; associated with it.

If the hardware needs to be stopped in the case of a buffer underrun it might not be able to generate this event. In such cases the frame_sequence field in &v4l2-event-frame-sync; will not be incremented. This causes two consecutive frame sequence numbers to have n times frame interval in between them.

`V4L2_EVENT_PRIVATE_START` | 0x08000000 | Base event number for driver-private events.

### Event Flags

&cs-def;
`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 &VIDIOC-S-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 &VIDIOC-S-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;