Arms or disarms the user written subsystem break trap handling
procedure.
Use |
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The XCONTRAP intrinsic arms or disarms the user-written subsystem
break ([CTRL]Y) trap handling procedure.
When a session is initiated, the user written [CTRL]Y
trap handler is disarmed. If you are running a program in an interactive
session, you can arm a special trap that transfers control in the
program to a trap handling procedure whenever a subsystem break
signal is entered from the session terminal. (This is called the
[CTRL]Y trap since [CTRL]Y is
the default subsystem break character.)
The subsystem break signal is normally transmitted by pressing
[CTRL]Y. In transparent editing mode, you are allowed
to define a different character to be used to invoke subsystem break
by specifying the new character as part of param
in a call to FCONTROL(41).
You can also use FDEVICECONTROL to alter the subsystem break
character in either standard or transparent mode. See the discussion
of the FDEVICECONTROL intrinsic earlier in this chapter for the
proper parameter settings to use for this purpose.
If enabled, subsystem break allows the terminal user to stop
a "program local" or "subsystem local"
command. Before subsystem break can be enabled, however, you must
call XCONTRAP, specifying the external label of a user written procedure
which contains the steps that will be taken if subsystem break is
entered during execution of your program as the value of plabel.
If it is desirable for your program to do so, subsystem break
can be temporarily disabled through a call to FCONTROL(16). It can
then be reenabled through a call to FCONTROL(17).
Only one process in a session can receive a [CTRL]Y
trap at any one time. The process that called XCONTRAP most recently
receives the next [CTRL]Y trap. Once a process
has received a [CTRL]Y trap, it cannot receive
another until it calls the RESETCONTROL intrinsic. Only processes
running in a session (not in a job) can arm [CTRL]Y
traps. The trap handler can be any procedure in the program or in
the libraries to which the program is bound. The [CTRL]Y
trap handler has no parameters.
The following summarizes subsystem break processing and the
intrinsics involved:
A user written procedure must be provided
to define how the program should act upon receiving a subsystem
break.
A call to XCONTRAP must be included in the program,
specifying the external label of the procedure written in Step 1
as the value of plabel. This arms the [CTRL]Y
Trap.
A call to FCONTROL(17) must be included in the program
to enable the subsystem break function.
If the subsystem break character is received during
execution of the program, the procedure specified in the call to
XCONTRAP is executed.
A call to RESETCONTROL must be executed when the
program is ready to receive another subsystem break.
A new subsystem break character may be defined through
FCONTROL(41), if the terminal is placed in transparent mode or through
FDEVICECONTROL
If desirable at any time, FCONTROL(16) may be called
to disable subsystem break. A call to FCONTROL(17) enables it again.
Additional Discussion |
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See the discussion of enabling/disabling the user written
[CTRL]Y trap handler in Trap Handling
Programmer's Guide. See also the discussion of FCONTROL(16,17),
FCONTROL(41), and RESETCONTROL in this manual. See Chapter 5 “Using Subsystem Break” of this manual
for an example illustrating subsystem break processing.
