Path: utzoo!attcan!uunet!snorkelwacker!bloom-beacon!eru!luth!sunic!draken!d88-eli
From: d88-eli@nada.kth.se (Erik Liljencrantz)
Newsgroups: comp.lang.pascal
Subject: Re: exiting some calling ancestor in Turbo Pascal 5
Message-ID: <2551@draken.nada.kth.se>
Date: 17 Dec 89 14:12:12 GMT
References: <8912141752.aa23459@ICS.UCI.EDU> <122@crucible.UUCP>
Reply-To: d88-eli@nada.kth.se (Erik Liljencrantz)
Organization: Royal Institute of Technology, Stockholm, Sweden
Lines: 76

In article <122@crucible.UUCP> al@crucible.UUCP (Al Evans) writes:
>In article <8912141752.aa23459@ICS.UCI.EDU> milne@ICS.UCI.EDU (Alastair Milne) writes:
>[description of need for multi-level "EXIT" deleted....]
>>   Turbo Pascal gives only, as far as I know, EXIT (which exits the immediate
>>   block, but nothing further) and HALT (which kills the whole program).
>>   Does anybody know of a way in Turbo Pascal to obtain the effect of UCSD
>>   Pascal's EXIT?
>I wish there were. Actually, in UCSD, I think you can even exit procedures
>that HAVEN'T been called -- this gives interesting results. I do things
>as shown below, in about 60K lines of Pascal that exists under both Turbo and
>UCSD (don't ask:-).
>[code with boolean exitflag deleted]
>It ain't pretty, but it's the only way I've found to accomplish a multilevel
>exit under Turbo Pascal.
>...
>Al Evans					"You'd grep to know what
>...uunet!execu!sequoia!crucible!al			 you really sed."
>							--Referent Blob

  In C there are a standard method of exiting from several levels of calls.
setjmp and longjmp works together to accomplish this. A call to setjmp saves
the state of the stack (i.e. SP) and some registers (only BP needed for Turbo)
and a returnaddress just after setjmp. A call to longjmp from a deeper level
restores the stack and BP and jumps to the setjmp location. This would be
possible to accomplish in Turbo Pascal too (just a little assembler). A small
example of how it could work:

Declarations:
  FUNCTION SetJmp  (Buffer:Save_SP_BP_CS_IP_Record):INTEGER;
  { Return 0 when explicitly called, or ReturnValue from LongJmp if
    LongJmp is used. }
  PROCEDURE LongJmp(Buffer:Save_SP_BP_CS_IP_Record; ReturnValue:INTEGER);
  { Buffer must have been initialized by a previous call to SetJmp. ReturnValue
    is the result for SetJmp. It shouldn't be 0 (reserved for explicit call to
    SetJmp). }

Usage:
  PROGRAM JustATest;
  USES
    The_Imaginary_SetJmp_And_LongJmp_Unit;
  VAR
    ReturnToMainLoop:Save_SP_BP_CS_IP_Record;

    PROCEDURE NoReturn;
    BEGIN
      LongJmp(ReturnToMainLoop,1);
      { Transfer control to the mainloop! (Two level exit) }
    END;

    PROCEDURE DoSomething;
    BEGIN
      NoReturn;
      WriteLn('It never happens');
    END;

  BEGIN
    IF SetJmp(ReturnToMainLoop)<>0 THEN BEGIN
      { SetJmp returns 0 on initialization or something else if LongJmp
        was called. }
      WriteLn('A call to LongJmp returned control here!');
    END;
    REPEAT                         { Never ending main loop }
      CASE UpCase(ReadKey) OF
        'D':DoSomething;
        'Q':Halt;
      END;
    UNTIL FALSE;
  END.

Well, I hope you get the idea... This isn't a multilevel exit, but it's a
method to jump back in the call-chain at least. In C it can be used to exit
callingchains when an error is encountered. I hope I will be able to find
some time to write the procedures. If you are interested, just email!
Comments? Email them too!
-- 
Erik Liljencrantz     | "No silly quotes!!"
d88-eli@nada.kth.se   |  Embraquel D. Tuta