Subject: Trapping runtime errors

25. *****
 Q: How can I trap a runtime error?

 A: What you are probably asking for is a method writing a program
termination routine of your own. To do this, you have to replace
Turbo Pascal's ExitProc with your own customized exec procedure.
Several Turbo Pascal text books show ho to do this. See e.g. Tom
Swan (1989), Mastering Turbo Pascal 5.5, Third edition, Hayden
Books, pp. 440-454; Michael Yester (1989), Using Turbo Pascal, Que,
pp. 376-382; Stephen O'Brien (1988), Turbo Pascal, Advanced
Programmer's Guide, pp. 28-30; Tom Rugg & Phil Feldman (1989), Turbo
Pascal Programmer's Toolkit, Que, pp. 510-515. Here is an example
  var OldExitProcAddress : Pointer;
      x : real;
  {$F+} procedure MyExitProcedure; {$F-}
  begin
    if ErrorAddr <> nil then
      begin
        writeln ('Runtime error number ', ExitCode, ' has occurred');
        writeln ('The error address in decimal is ',
                  Seg(ErrorAddr^):5,':',Ofs(ErrorAddr^):5);
        writeln ('That''s all folks, bye bye');
        ErrorAddr := nil;
        ExitCode  := 0;
      end;
    {... Restore the pointer to the original exit procedure ...}
    ExitProc := OldExitProcAddress;
  end;  (* MyExitProcedure *)
  (* Main *)
  begin
    OldExitProcAddress := ExitProc;
    ExitProc := @MyExitProcedure;
    x := 7.0; writeln (1.0/x);
    x := 0.0; writeln (1.0/x);   {The trap}
    x := 7.0; writeln (4.0/x);   {We won't get this far}
  end.
:
Actually, I utilize this idea in my /pc/ts/tspa3470.zip Turbo Pascal
units collection, which includes a TSERR.TPU. If you put TSERR in
your program's uses statement, all the run time errors will be given
verbally besides the usual, cryptic error number. That's all there
is to it. That is, the inclusion to the uses statement to your main
program (if you have the program in several units) is all you have
to do to enable this handy feature.
:
Hans.Siemons@f149.n512.z2.fidonet.org notes "This line:
    ExitProc := OldExitProcAddress;
should IMHO never be placed at the end of your exit handler. If for
one reason or another your own handler should cause a runtime error,
it would go in an endless loop. If the first statement restores the
exit chain, this can never happen. I do agree that is not very
likely that your exit handler produces any runtime error, but it
performs I/O, and since it is located in a FAQ, people are bound to
use, and maybe extend it with more tricky stuff."
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