Download AMOS AlphaPascal

SYSTEMS FUNCTtONS AND PROCEDURES
Page 14—15
exception of a Control—C
followed by a command to resume,
Thus the
ERRORTRAP procedures themselves have aborted to AMOS In most circumstances.
In addition1 STDERRORTRAP resets INFOP .XEQERR to zero before returning if
execution is to be resumed. This is because, errors MUST NOT occur in the
error handler itself for obvious reasons.
AtphaPascal assumes it is
executing an error handler whenever XEQERR is nonzero. If an occur does
occur within an error handler, the message
?Attempt to call ERRORTRAP while in ERRORTRAP
is displayed, a direct abort to AMOS is made without closing any open files.
Thus, by reseting XEQERR to zero, STDERRORTRAP signals to AlphaPascal that
error handling is finished and further errors are again acceptable.
Should you decide not to call STDERRORTRAP at all, please keep in mind the
following points:
1.
2.
The only errors from which you may safely resume execution are B (a
Control—C) and 10 (I/O error). An attempt to resume execution by
returning
from ERRORTRAP with any other errors will probably crash
the system.
It is acceptable to use EXIT to abort some function or procedure,
your program, when any error occurs. Of course you can only
EXIT to leave a function or procedure which is currently active, so
you will probably want to have around some BOOLEAN variables to
keep track of whether or not you are currently within routines
which you might wish to EXIT from ERRORTRAP.
or
3.
14.10.3
Remember to set XEQERR back
otherwise your next error
ERRORTRAP.
to zero before
leaving ERRORTRAP,
AMOS
without calling
abort
to
wilt
XERRORTRAP
possible to
When STDERRORTRAP is called by entering a Control—C,
it
is
request
a
backtrace of suspended functions and procedures. This backtrace
begins with the caller of the caller of STDERRORTRAP, which is usually the
Thus,
caller of ERRORTRAP, and hence the routine which was suspended.
should STDERRORTRAP be called by a function or procedure local to your
This can
ERRORTRAP procedure, the backtrace wilt begin in the wrong place.
INFOREC
system
be corrected by using XERRORTRAP, which takes a copy of the
as its argument.
It is used as follows: