Path: utzoo!attcan!uunet!lll-winken!ncis.llnl.gov!ncis!helios.ee.lbl.gov!pasteur!ucbvax!hplabs!sm.unisys.com!randvax!florman
From: florman@randvax.UUCP (Bruce Florman)
Newsgroups: comp.sys.mac.programmer
Subject: Re: serious code generation bug in Lightspeed C
Message-ID: <1858@randvax.UUCP>
Date: 17 Jan 89 18:54:36 GMT
References: <6275@hoptoad.uucp> <1676@helios.ee.lbl.gov> <6315@hoptoad.uucp> <24012@apple.Apple.COM>
Reply-To: florman@rand-unix.UUCP (Bruce Florman)
Organization: Rand Corp., Santa Monica, Ca.
Lines: 48

In article <24012@apple.Apple.COM> dowdy@Apple.COM (Tom Dowdy) writes:

>A subtle bug that is hinted at here effects users of Object Pascal
>more often, but can bite others as well:
>  
>If you call a function with a field from a handle based record, or
>Object Pascal object, as one of the parameters and the function
>or procedure you are calling is in another, unloaded, segment
>you will be in for a crash.  The MPW 3.0 compilers check for
>this condition and warn you about it.  

    I'm not sure that I understand this statement.  Are you saying that 
the following code will crash if Foo and Bar are in different segments, 
and the segment containing Foo is unloaded when Bar is executed?

------------------------------------------------------------------------
TYPE
    SomeRecord = RECORD
	field1, field2 : INTEGER;
      END;

    SomeRecPtr = ^SomeRecord;
    SomeRecHdl = ^SomeRecPtr;

FUNCTION Foo(a : INTEGER) : BOOLEAN;
BEGIN
  Foo := a > 37;
END;

PROCEDURE Bar(srH : SomeRecHdl);
BEGIN
  IF Foo(srH^^.field2) THEN
    SysBeep(10);
END;
------------------------------------------------------------------------

    I can see where there would be a problem if Foo's parameter were 
being passed by reference, or if Foo's result were being assigned to 
a field of the record, but I don't understand how the code above can 
cause a problem.  Am I just being dense, or what?

>  Tom Dowdy                 Internet: dowdy@apple.COM
>  Apple Computer MS:27Y     AppleLink:DOWDY1
>  20525 Mariani Ave         UUCP:     {sun,voder,amdahl,decwrl}!apple!dowdy
>  Cupertino, CA 95014    
>  "The 'Oo-Ah' Bird is so called because it lays square eggs."   

Bruce Florman			florman@rand.org