Specifications

Device Driver Entry Points

Cancel-I/O Routine

Prevents further device-speciﬁc processing of the I/O request currently being

processed on a device.

Speciﬁed in

Supply the address of the cancel-I/O routine in the cancel argument of the

DDTAB macro. The macro places this address into DDT$L_CANCEL. Many

drivers specify the system routine IOC$CANCELIO as their cancel-I/O routine.

Called by

System routines call a driver’s cancel-I/O routine under the following

circumstances:

• When a process issues a Cancel-I/O-on-Channel system service ($CANCEL)

• When a process deallocates a device, causing the device’s reference count

(UCB$W_REFC) to become zero (that is, no process I/O channels are assigned

to the device)

• When a process deassigns a channel from a device, using the $DASSGN

system service

• When the command interpreter performs cleanup operations as part of image

termination by canceling all pending I/O requests for the image and closing

all image-related ﬁles open on process I/O channels

Synchronization

A cancel-I/O routine begins execution at fork IPL, holding the corresponding fork

lock in a multiprocessing environment. It must return control to its caller in this

context.

Context

A cancel-I/O routine executes in kernel mode in process context.

A cancel-I/O routine must preserve the contents of all registers except R4 and R5.

Input

Location Contents

R2 Channel index number

R3 Contents of UCB$L_IRP (address of current IRP, if

any, for device)

R4 Address of PCB of the process for which the I/O

request is being canceled

R5 Address of UCB

4–4