Delphi XE5 string size exchange limitation between DLL and EXE - delphi

Question

I have a Delphi 7 DLL function that returns large string and it works fine but in Delphi XE5 I get an access violation after a specific size.
I have written a sample demo, that reflects my actual code, that generates also a AV in Delphi XE5 that returns also a large string but again after a specific size, I get an Access Violation ?
13000 lines of 20 chars, it works fine but with 14000 lines it crashes.
I did some tests with Delphi 7 and it works fine also.
What am I doing wrong ? Can anyone help me out ?
Thanks.
Here is the code of my DLL :
function RetLargeStr(Buffer : pAnsiChar; var BufferSize: Integer) : boolean ; stdcall;
var l_ansiStr : string;
loop : integer;
begin
Result := False;
//13000 ok 14000+ fail ???
for loop := 1 to 15000 do
begin
l_AnsiStr := l_AnsiStr + 'String of 20 chars' + Char($0D) + Char($0A) ;
end;
if Assigned(Buffer) and (BufferSize >= Length(l_ansiStr) + 1) then
begin
//Buffer := pAnsiChar(AnsiString(l_AnsiStr));
move(l_AnsiStr, Buffer^, length(l_AnsiStr) + 1);
Result := True;
end;
//Return actual size of output string.
BufferSize := Length(l_AnsiStr) + 1;
end ;
Here’s the call from my EXE :
procedure TForm1.Button7Click(Sender: TObject);
var l_StrOut : pAnsiChar;
l_Str : ansistring;
p_Size : integer;
begin
p_Size := 600000;
SetLength(l_Str, p_Size);
l_strout := pAnsiChar(l_str);
Memo2.Lines.Clear;
if RetLargeStr(l_StrOut, p_Size)
then Memo2.Lines.Add( l_StrOut );
end;

Answer 1

The way you have it here it's probably just luck that it works at all.
In the DLL, when you do this:
Buffer := pAnsiChar(AnsiString(l_AnsiStr));
you are actually returning the string buffer allocated in the DLL to the calling EXE, even though you've explicitly allocated a receive buffer before the call. That receive buffer pointer gets overwritten.
The crash most likely occurs because the heap manager in the EXE is unprepared for freeing a memory block, which was allocated somewhere else (in the DLL).
Instead of assigning to buffer, you might try copying the content of the string to it, like this:
if Assigned(Buffer) and (BufferSize >= Length(l_ansiStr) + 1) then
begin
move(AnsiStr[1], Buffer^, length(AnsiStr) + 1));
Result := True;
end;
Test code (DLL):
library Project2;
uses
SysUtils,
Classes;
function RetLargeStr(Buffer : pAnsiChar; var BufferSize: Integer) : boolean ; stdcall;
var l_ansiStr : string;
loop : integer;
begin
Result := False;
//13000 ok 14000+ fail ???
for loop := 1 to 15000 do
begin
l_AnsiStr := l_AnsiStr + 'String of 20 chars' + Char($0D) + Char($0A) ;
end;
if Assigned(Buffer) and (BufferSize >= Length(l_ansiStr) + 1) then
begin
//Buffer := pAnsiChar(AnsiString(l_AnsiStr));
move(l_AnsiStr[1], Buffer^, length(l_AnsiStr) + 1);
Result := True;
end;
//Return actual size of output string.
BufferSize := Length(l_AnsiStr) + 1;
end ;
exports
RetLargeStr;
begin
end.
Test code (EXE):
unit Unit3;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;
type
TForm3 = class(TForm)
Button1: TButton;
Memo1: TMemo;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form3: TForm3;
implementation
{$R *.dfm}
function RetLargeStr(Buffer : pAnsiChar; var BufferSize: Integer) : boolean ; stdcall; external 'project2.dll';
procedure TForm3.Button1Click(Sender: TObject);
var l_StrOut : pAnsiChar;
l_Str : ansistring;
p_Size : integer;
begin
p_Size := 600000;
SetLength(l_Str, p_Size);
l_strout := pAnsiChar(l_str);
Memo1.Lines.Clear;
if RetLargeStr(l_StrOut, p_Size)
then Memo1.Lines.Add( l_StrOut );
end;
end.