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HOTPLUG(4) Device Drivers Manual HOTPLUG(4)

hotplugdevices hot plugging

pseudo-device hotplug 1

#include <sys/types.h>
#include <sys/device.h>
#include <sys/hotplug.h>

The hotplug pseudo-device passes device attachment and detachment events to userland. When a device attaches or detaches, the corresponding event is queued. The events can then be obtained from the queue through the read(2) call on the /dev/hotplug device file. Once an event has been read, it's deleted from the queue. The event queue has a limited size and if it's full all new events will be dropped. Each event is described with the following structure declared in the <sys/hotplug.h> header file:

struct hotplug_event {
	int		he_type;	/* event type		*/
	enum devclass	he_devclass;	/* device class		*/
	char		he_devname[16];	/* device name		*/

The he_type field describes the event type and can be either HOTPLUG_DEVAT for device attachment or HOTPLUG_DEVDT for detachment. The he_devclass field describes the device class. All device classes can be found in the <sys/device.h> header file:
enum devclass {
	DV_DULL,	/* generic, no special info */
	DV_CPU,		/* CPU (carries resource utilization) */
	DV_DISK,	/* disk drive (label, etc) */
	DV_IFNET,	/* network interface */
	DV_TAPE,	/* tape device */
	DV_TTY		/* serial line interface */

The he_devname is a device name including unit number, e.g. sd1.

Only one structure can be read per call. If there are no events in the queue, the read(2) call will block until an event appears.

hotplug: event lost, queue full
New events will be dropped until all pending events have been read.

read(2), hotplugd(8)

The hotplug device first appeared in OpenBSD 3.6.

The hotplug driver was written by Alexander Yurchenko <>.

September 14, 2015 OpenBSD-6.0