Delphi Assembly Function Returning a Long String

问题:

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.).

回答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.

回答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; 

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