Guardian Programmer's Guide

Debugging, Trap Handling, and Signal Handling
Guardian Programmer’s Guide — 421922-014
25 - 18
Writing a Trap Handler: Examples
! Open home terminal:
CALL PROCESS_GETINFO_(!process^handle!,
!file^name!,
!file^name^len!,
!priority!,
!moms^processhandle!,
TERM^NAME:MAXLEN,
LEN);
CALL FILE_OPEN_(TERM^NAME:LEN,TERM^NUM);
IF <> THEN CALL DEBUG;
! Set up an arithmetic overflow condition to cause the
! trap:
J := 0;
I := I/J;
END;
In the following example, a trap causes the current code sequence to be abandoned
and an alternate code sequence executed instead. Several points are illustrated:
•
The procedure TRAP_GUARD sets its caller as the destination (the code address
and stack environment) to be used if a trap occurs; it also arms the trap handler.
•
When a trap occurs, the information about the trap is saved globally (for possible
display or analysis).
•
The trap handler exits, and control is returned to the designated destination.
Consistent values are supplied for space ID, S, P, ENV, and L. ENV.RP is set to 0
and RP[0] is set to a specific value so that returning from the trap handler effects a
return from the TRAP_GUARD function with the result equal to TRUE (-1).
•
Incidentally, the trap handler stack is allocated among the globals (below the
normal stack) instead of at the top of the stack.