Next: , Previous: TFDD, Up: GPC Units

6.15.19 Trap runtime errors

The following listing contains the interface of the Trap unit.

This unit allows you to trap runtime errors, so a runtime error will not abort the program, but pass the control back to a point within the program. Use with care, and read the notes in the interface, please.

     { Trapping runtime errors
       The Trap unit allows you to trap runtime errors, so a runtime
       error will not abort the program, but pass the control back to a
       point within the program.
       The usage is simple. The TrapExec procedure can be called with a
       function (p) as an argument. p must take a Boolean argument. p
       will immediately be called with False given as its argument. When
       a runtime error would otherwise be caused while p is active, p
       will instead be called again with True as its argument. After p
       returns, runtime error trapping ends.
       When the program terminates (e.g. by reaching its end or by a Halt
       statement) and a runtime error was trapped during the run, Trap
       will set the ExitCode and ErrorAddr variables to indicate the
       trapped error.
       - After trapping a runtime error, your program might not be in a
         stable state. If the runtime error was a "minor" one (such as a
         range checking or arithmetic error), it should not be a problem.
         But if you, e.g., write a larger application and use Trap to
         prevent a sudden abort caused by an unexpected runtime error,
         you should make the program terminate regularly as soon as
         possible after a trapped error (perhaps by telling the user to
         save the data, then terminate the program and report the bug to
       - Since the trapping mechanism *jumps* back, it has all the
         negative effects that a (non-local!) goto can have! You should
         be aware of the consequences of all active procedures being
         terminated at an arbitrary point!
       - Nested traps are supported, i.e. you can call TrapExec again
         within a routine called by another TrapExec instance. Runtime
         errors trapped within the inner TrapExec invocation will be
         trapped by the inner TrapExec, while runtime errors trapped
         after its termination will be trapped by the outer TrapExec
       Copyright (C) 1996-2005 Free Software Foundation, Inc.
       Author: Frank Heckenbach <>
       This file is part of GNU Pascal.
       GNU Pascal is free software; you can redistribute it and/or modify
       it under the terms of the GNU General Public License as published
       by the Free Software Foundation; either version 2, or (at your
       option) any later version.
       GNU Pascal is distributed in the hope that it will be useful, but
       WITHOUT ANY WARRANTY; without even the implied warranty of
       General Public License for more details.
       You should have received a copy of the GNU General Public License
       along with GNU Pascal; see the file COPYING. If not, write to the
       Free Software Foundation, 59 Temple Place - Suite 330, Boston, MA
       02111-1307, USA.
       As a special exception, if you link this file with files compiled
       with a GNU compiler to produce an executable, this does not cause
       the resulting executable to be covered by the GNU General Public
       License. This exception does not however invalidate any other
       reasons why the executable file might be covered by the GNU
       General Public License. }
     {$if __GPC_RELEASE__ < 20030303}
     {$error This unit requires GPC release 20030303 or newer.}
     unit Trap;
     uses GPC;
       TrappedExitCode: Integer = 0;
       TrappedErrorAddr: Pointer = nil;
       TrappedErrorMessageString: TString = '';
     { Trap runtime errors. See the comment at the top. }
     procedure TrapExec (procedure p (Trapped: Boolean));
     { Forget about saved errors from the innermost TrapExec instance. }
     procedure TrapReset;