USB Anchors

What is anchor?

A USB driver needs to support some callbacks requiring a driver to cease all IO to an interface. To do so, a driver has to keep track of the URBs it has submitted to know they’ve all completed or to call usb_kill_urb for them. The anchor is a data structure takes care of keeping track of URBs and provides methods to deal with multiple URBs.

Allocation and Initialisation

There’s no API to allocate an anchor. It is simply declared as struct usb_anchor. init_usb_anchor() must be called to initialise the data structure.


Once it has no more URBs associated with it, the anchor can be freed with normal memory management operations.

Association and disassociation of URBs with anchors

An association of URBs to an anchor is made by an explicit call to usb_anchor_urb(). The association is maintained until an URB is finished by (successful) completion. Thus disassociation is automatic. A function is provided to forcibly finish (kill) all URBs associated with an anchor. Furthermore, disassociation can be made with usb_unanchor_urb()

Operations on multitudes of URBs


This function kills all URBs associated with an anchor. The URBs are called in the reverse temporal order they were submitted. This way no data can be reordered.


All URBs of an anchor are unanchored en masse.


This function waits for all URBs associated with an anchor to finish or a timeout, whichever comes first. Its return value will tell you whether the timeout was reached.


Returns true if no URBs are associated with an anchor. Locking is the caller’s responsibility.


Returns the oldest anchored URB of an anchor. The URB is unanchored and returned with a reference. As you may mix URBs to several destinations in one anchor you have no guarantee the chronologically first submitted URB is returned.