HP 3000 Manuals

Trap Handling Programmer's Guide

Chapter 4  MPE XL CONTROL-Y Traps 

If you are running a program in an interactive session, you can arm a
trap handling procedure that receives control whenever a subsystem break
signal is entered from the session terminal.  On most terminals, you
transmit the subsystem break signal by pressing the [[CTRL]] [[Y]] key.
To do this, press the [[Y]] key while holding down the [[CTRL]] key.  For
this reason, subsystem break traps are commonly called CONTROL-Y traps.


NOTE  You can change the key that causes such a subsystem break.  Refer
      to the discussion of controlcode 25 of the FCONTROL intrinsic in
      the MPE XL Intrinsics Reference Manual (32650-90028).


There are two MPE XL intrinsics that are used in handling CONTROL-Y
traps:

 *  XCONTRAP

 *  RESETCONTROL

The XCONTRAP intrinsic arms or disarms the user-written CONTROL-Y trap
handling procedure.  When a process is initiated, it has no CONTROL-Y
trap handler.  Only one process in a session can receive a CONTROL-Y trap
at any one time.  The process that called XCONTRAP most recently receives
the next CONTROL-Y trap.  Once a process has received a CONTROL-Y trap,
it cannot receive another until it calls the RESETCONTROL intrinsic.
Only processes running in a session can arm CONTROL-Y traps.  The trap
handler can be any procedure in the program or in the libraries to which
the program is bound.  The CONTROL-Y trap handler has no parameters.

The RESETCONTROL intrinsic resets the CONTROL-Y trap for a process so the
process can accept another subsystem break signal.  The process must have
previously armed a CONTROL-Y trap handler with the XCONTRAP intrinsic.
After your CONTROL-Y trap handler has been invoked, you should call
RESETCONTROL when you are ready to receive another subsystem break
signal.  RESETCONTROL can be called from within the CONTROL-Y trap
handler, or from any other procedure.


NOTE  Once your process has received a subsystem break signal, only a
      call to RESETCONTROL allows it to receive another such signal.
      Calling XCONTRAP again does not have this effect.


Discussions of these intrinsics follow.

MPE/iX 5.0 Documentation