[fpc-devel] Overflow in TMemoryStream?

[Martok]

Hi,

yes, I can confirm this as an overflow, but on its own, it should be safe. Above
430MB, the stream doesn't grow by a quarter but just by however much was
requested, luckily the branch fails before the wrong capacity could be set.

Test:
type
  TMS2 = class(TMemoryStream) end;
var
  ms: TMS2;
  ds: Int64;
begin
  ds:= 100*1000*1000;
  ms:= TMS2.Create;
  ms.SetSize(ds);
  WriteLn(ds:15,' ', ms.Size:15, ' ', ms.Capacity:15);
  inc(ds, ds div 10); // grow by less than 25%
  ms.SetSize(ds);
  WriteLn(ds:15,' ', ms.Size:15, ' ', ms.Capacity:15);
end.

with ds=100M, prints:
      100000000       100000000       100003840
      110000000       110000000       125005824    << grew by 1/4*100M

with ds=500M, prints:
      500000000       500000000       500002816
      550000000       550000000       550002688    << bug, grew by 1/10*500M

However, with ds=869M, prints:
      869000000       869000000       869003264
      955900000 18666185569013440       955904000
and mostly crashes with Runtime Error 203 except when I'm step-by-step-debugging
it...
That looks like a *separate* overflow to me, probably caused by the wild mix of
Int64 and Longint that our Streams inherited from Delphi...

I don't have RTL built with full symbols right now, maybe someone else can
investigate?


Martok