ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT
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Name

VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT — Subscribe or unsubscribe event

Synopsis

int fsfuncioctl(int fd,
 int request,
 struct v4l2_event_subscription *argp);
 

Arguments

fd

File descriptor returned by open().

request

VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT

argp

Description

Subscribe or unsubscribe V4L2 event. Subscribed events are dequeued by using the VIDIOC_DQEVENT ioctl.

Table A.95. struct v4l2_event_subscription

__u32typeType of the event.
__u32idID 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.
__u32flagsEvent flags, see Table A.97, “Event Flags”.
__u32reserved[5]Reserved for future extensions. Drivers and applications must set the array to zero.

Table A.96. Event Types

V4L2_EVENT_ALL0All events. V4L2_EVENT_ALL is valid only for VIDIOC_UNSUBSCRIBE_EVENT for unsubscribing all events at once.
V4L2_EVENT_VSYNC1This event is triggered on the vertical sync. This event has a struct v4l2_event_vsync associated with it.
V4L2_EVENT_EOS2This 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_CTRL3

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 struct v4l2_event_ctrl associated with it. This struct contains much of the same information as struct v4l2_queryctrl and struct 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 struct 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_SYNC4

Triggered immediately when the reception of a frame has begun. This event has a struct 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 struct 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_START0x08000000Base event number for driver-private events.

Table A.97. Event Flags

V4L2_EVENT_SUB_FL_SEND_INITIAL0x0001When 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_FEEDBACK0x0002

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

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.

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