While finding the bug #12897 I found that a memory stream position could be set to before the start of the file without giving an error, and subsequent reads would appear to work (unless the position was so far off an invalid region of memory was read). I found the precise problem by patching TCustomMemoryStream to raise an exception if the position is set before the start of the file. Should an exception be raised in this circumstance, which would make finding bugs easier? Or should the position just be set to 0, which would be more consistent with the behaviour when going beyond the end of the stream?

See attached patch for the sort of thing I am thinking of.

Colin

I think that an exception is better. If you set the position to a non-existing position, there is an error in your logic anyway.

Michael.

Just tested with delphi

procedure TForm1.Button1Click(Sender: TObject);
var
  S: TMemoryStream;
begin
  S := TMemoryStream.Create;
  S.Position := 10;
  ShowMessage(IntToStr(S.Position));
  S.Position := -1;
  ShowMessage(IntToStr(S.Position));
  S.Free;
end;

It shows 10 and -1.

If you do an exception or silent change of position it can cause incompatibilities. Maybe read operation must be fixed to ignore positions out of the range 0..Size -1?

Best regards,
Paul Ishenin.

What does Delphi do in such a case ?

Michael.

Delphi reads and writes silently do nothing for position < 0, though I would prefer an exception.

Colin

Tested so:

procedure TForm1.Button1Click(Sender: TObject);
var
  S: TMemoryStream;
  B: Byte;
  Count: Integer;
begin
  S := TMemoryStream.Create;
  B := 1;
  S.Write(B, SizeOf(B));
  S.Position := -1;
  Count := S.Read(B, SizeOf(B));
  ShowMessage(IntToStr(Count));
  S.Free;
end;

Count = 0. IOW, delphi does not read if position < 0

Best regards,
Paul Ishenin.

And writing ?

Michael.

Michael Van Canneyt wrote

And writing ?

Silently do nothing:

procedure TForm4.Button1Click(Sender: TObject);
var
  S: TMemoryStream;
  B: Byte;
  Count: Integer;
begin
  S := TMemoryStream.Create;
  S.Position := -1;
  B := 1;
  S.Write(B, SizeOf(B));
  ShowMessage(IntToStr(S.Size));
  S.Free;
end;

it shows 0

Best regards,
Paul Ishenin.

I have submitted a patch:

<http://bugs.freepascal.org/view.php?id=13318>

Colin