[fpc-pascal] Stack corruption setting UnicodeString Char

[Mattias Gaertner]

Felipe Monteiro de Carvalho <felipemonteiro.carvalho at gmail.com> hat am 19.
Oktober 2011 um 09:13 geschrieben:

> Hello,
>
> Usually when we work with PChars we do things like this to set the
> final terminator:
>
> var
>   ansistr: ansistring;
> begin
>   // Copy the originating string taking into account the specified length
>   SetLength(ansistr, len+1);
>   System.Move(source^, ansistr[1], len);
>   ansistr[len+1] := #0;
>   SetLength(ansistr, len);
>
> And it works fine. You get a string with correct length and a final #0
> terminator. 
I never saw such code for ansistring. Only for shortstring. 
SetLength(ansistring) already allocates memory for the header+len+1, creates the
header and sets the len+1 character to #0. 
Simply do:
  SetLength(ansistr, len);
  if len>1 then
    System.Move(source^, ansistr[1], len);
 

>
> Now, I simply tryed the same code for a unicodestring, and it corrupts my
> stack:
>
> procedure Unicode2AnsiMove(source:pwidechar;var dest:ansistring;len:SizeInt);
> var
>   widestr: unicodestring;
> begin
>   {$ifdef PASWSTRING_VERBOSE}WriteLn('Unicode2AnsiMove START');{$endif}
>   // Copy the originating string taking into account the specified length
>   SetLength(widestr, len+1);
>   System.Move(source^, widestr[1], len*2);
>   PWideChar(@widestr)[len] := #0; // <--- This corrupts the stack 
This would corrupt the stack for ansistring too. This would only work for
shortstring.
widestr is a unicodestring is a pointer to widechar.  
 
  PWideChar(widestr)[len] := #0; 
 
 

>   SetLength(widestr, len);>
> I already fixed the problem by simply not setting this last widechar
> to #0, but now I got puzzled: Any ideas why this doesn't work?
>
> It crashes only a bit after this procedure, but I found that this was
> the statement which caused the problem.

Mattias
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