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 strin开发者_如何转开发g 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.).

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.

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;