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From: dgil@pa.reuter.COM (Dave Gillett)
Newsgroups: comp.lang.pascal
Subject: Turbo bug with VAR parameters
Keywords: bug,var,borland
Message-ID: <683@saxony.pa.reuter.COM>
Date: 29 Jan 91 01:22:08 GMT
Distribution: na
Organization: Reuter:file Inc  (A Reuter Company) Palo Alto, CA
Lines: 44


     Consider the following trivial example:

var
  Global : String [255];

  procedure Foo (VAR FooStr : String [255]);

  begin
    FooStr := 'abcdefghijklmnopqrstuvwxyz';
  end { Foo };

  procedure Bar (VAR BarStr : String [255]);

  begin
    Foo (BarStr);
  end { Bar };

begin
  Bar (Global);
end.

     When I compile this using version 5.5 of Turbo Pascal (okay, this might
be fixed in 6.0 for all I know), Foo gets compiled to access FooStr by
reference rather than by value; apparently, as a pointer back into the
previous stack frame.  Similarly, Bar gets compiled to generate such access
for BarStr.
     The problem is that, at run-time, Foo trashes Bar's stack frame, instead
of reaching all the way back to the global context to resolve FooStr -> Global.

     I can understand that at the point that Foo is being compiled, it cannot
tell what the calling stack will look like.  And presumably the code to look
back a single level can easily be generated inline and is pretty efficient.
So generating "correct" code for this case may be impractical.
     However:  When Bar is being compiled, the compiler *must* already know
that Foo expects a VAR argument (it does other kinds of type-checking on the
call!).  And so if the compiler cannot generate correct code for this case,
it's not unreasonable to expect *at least* a diagnostic warning that the
passing of VAR parameters as VAR parameters is not supported.

     It *is* unreasonable to expect the programmer not only to manually
program around this (via trivial use of temporaries), but also manually
diagnose and locate instances of this.
                                              Dave