Delphi stack misalignment + com marshalling = wrong marshalling

Question

This is not exactly a straight-out question because I have just solved it, but more like "am I getting it right" type of question and a reminder for those who might get stuck into that.

Turns out, Delphi does not align variables on stack and there are no directives/options to control this behavior. Default COM marshaller on my XP SP3 seem to require 4-byte alignment when marshaling records though. Worse, when it encounters unaligned pointer, it does not return an error, oh no: it rounds the pointer down to the nearest 4-byte boundary and continues like that.

Therefore, if you pass a record you have allocated on stack into COM-marshaled function by reference, you're screwed and you won't even know.

The problem can be solved by using New/Dispose to allocate records, as memory managers tend to align everything at 8 bytes or better, but god, this is annoying, both the misalignment part and the "trim-down-pointers" part.

Is this really the reason, or am I wrong somewhere?

Update: How to reproduce (Delphi 2007 for Win32).

uses SysUtils;

type
  TRec = packed record
    a, b, c, d, e: int64;
  end;

  TDummy = class
  protected
    procedure Proc(param1: integer);
  end;

procedure TDummy.Proc(param1: integer);
var a, b, c: byte;
  rec: TRec;
begin
  a := 5;
  b := 9;
  c := 100;

  rec.a := param1;
  rec.b := a;
  rec.c := b;
  rec.d := c;
  writeln(IntToHex(integer(@rec), 8));
  readln;
end;

var Obj: TDummy;
begin
  obj := TDummy.Create;
  try
    obj.Proc(0);
  finally
    FreeAndNil(obj);
  end;
end.

This gives odd result address, clearly not aligned on anything. If it doesn't, try adding more byte variables to "a, b, c: byte" (and don't forget to simulate some work with them at the end of the function).

The part with COM is easier to reproduce but longer to explain. Create a new VCL app called Sample Server, add a COM object SampleObject implementing ISampleObject, with a type library, free-threaded, single instance (make sure to check ISampleObject is marked as Ole Automation in type library). Open the type library, declare a new SampleRecord with five __int64 fields. Add a SampleFunction with a single SampleRecord* out parameter to ISampleObject. Implement SampleFunction in TSampleObject by returning fixed values:

function TSampleObject.SampleFunction(out rec: SampleRecord): HResult;
begin
  rec.a := 1291;
  rec.b := 742310;
  //...
  Result := S_OK;
end;

Note how Delphi declares SampleRecord as "packed record" in automatically generated type library header code:

SampleRecord = packed record
  a: Int64;
  b: Int64;
  //...
end;

I have checked, and this, at least, was fixed in Delphi 2010. Automatically generated records are not packed there.

Register the COM server. Run it.

Now modify the source above (sample 1) to call this server instead of just doing writeln:

uses SysUtils, Windows, ActiveX, SampleServer_TLB;

procedure TDummy.Proc(param1: integer);
var a, b, c: byte;
  rec: SampleRecord;
  Server: ISampleObject;
begin
  a := 5;
  b := 9;
  c := 100;

  rec.a := param1;
  rec.b := a;
  rec.c := b;
  rec.d := c;
  Server := CoSampleObject.Create;
  hr := Server.SampleFunction(rec);
  writeln('@: 'IntToHex(integer(@rec), 8)+', rec.a='+IntToStr(rec.a));
  readln;
end;

var Obj: TDummy;
begin
  CoInitializeEx(nil, COINIT_MULTITHREADED);
  obj := TDummy.Create;
  try
    obj.Proc(0);
  finally
    FreeAndNil(obj);
    CoUninitialize();
  end;
end.

Observe that when the address of rec is not aligned, values of rec fields are wrong (specifically, bitshifted to 8, 16 or 24 bits, sometimes wrapped over to the next value).

What version of Delphi, what Memory manager? Can you provide an example we can try? — Francesca
Please enter a report into Quality Central with this test case; qc.embarcadero.com — Allen Bauer

Allen Bauer Allen Bauer · Accepted Answer · 2010-06-24T17:08:43

Do you have an example of the stack being misaligned? Delphi should be aligning everything to 4 byte boundaries. The only case I can think of that would cause misalignment is if someplace along the call-chain is some assembler code that has explicitly done something to the stack to mis-align it.

Delphi stack misalignment + com marshalling = wrong marshalling

1 Answers