Delphi Assembly Function Returning a Long String

Question

I am trying to learn inline assembly programming in Delphi, and to this end I have found this article highly helpful.

Now I wish to write an assembly function returning a long string, specifically an AnsiString (for simplicity). I have written

function myfunc: AnsiString;
asm
  // eax = @result
  mov edx, 3
  mov ecx, 1252
  call System.@LStrSetLength
  mov [eax + 0], ord('A')
  mov [eax + 1], ord('B')
  mov [eax + 2], ord('C')
end;

Explanation:

A function returning a string has an invisible var result: AnsiString (in this case) parameter, so, at the beginning of the function, eax should hold the address of the resulting string. I then set edx and ecx to 3 and 1252, respectively, and then call System._LStrSetLength. In effect, I do

  _LStrSetLength(@result, 3, 1252)

where 3 is the new length of the string (in characters = bytes) and 1252 is the standard windows-1252 codepage.

Then, knowing that eax is the address of the first character of the string, I simply set the string to "ABC". But it does not work - it gives me nonsense data or EAccessViolation. What is the problem?

Update

Now we have two seemingly working implementations of myfunc, one employing NewAnsiString and one employing LStrSetLength. I cannot help but wonder if both of them are correct, in the sense that they do not mess upp Delphi's internal handling of strings (reference counting, automatic freeing, etc.).

Answer 1

You have to use some kind of:

function myfunc: AnsiString;
asm
  push eax // save @result
  call system.@LStrClr
  mov    eax,3                 {Length}
{$ifdef UNICODE}
  mov    edx,1252 // code page for Delphi 2009/2010
{$endif}
  call   system.@NewAnsiString
  pop edx
  mov [edx],eax
  mov [eax],$303132
end;

It will return a '210' string...

And it's always a good idea of putting a {$ifdef UNICODE} block to have your code compatible with version of Delphi prior to 2009.

Answer 2

With the excellent aid of A.Bouchez, I managed to correct my own code, employing LStrSetLength:

function myfunc: AnsiString;
asm

  push eax

  // eax = @result
  mov edx, 3
  mov ecx, 1252
  call System.@LStrSetLength

  pop eax

  mov ecx, [eax]

  mov [ecx], 'A'
  mov [ecx] + 1, 'B'
  mov [ecx] + 2, 'C'

end;