How to - string in DLL

[WojTec]

This is the bigest problem I currently have: how to use strings in DLL.

I know have to use PChar in exports. How about internal code that operates on Delphi's strings? I want to share this DLL with any language, so I can't use external memory manager (as I did with DLLs that can be used by my software only). For example function returns string or parameter is string or variable or constant is string - what to do whit this?

With numbers there is no problem. Please help me with strings :cry:

[Luckie]

Here you go: http://www.michael-puff.de/Programmi...tringDLL.shtml

[geskill]

i always use the WideString type, it is fully COM compatible and i don't have to cast. for me the most easiest way!

[jaenicke]

You can use PWideChar or PAnsiChar just as the Windows API internally uses too.

If you want to return such a value:
Just give a PWideChar and its size as variable parameter. If the size is not sufficient, you return the size you need. If the size is sufficient you copy the data into the buffer.

This is the way the API manages it. And it works well.

[himitsu]

WideString = SysAllocStringLen, SysReAllocStringLen, SysFreeString and SysStringLen

- no delphi type > OLE-String
- no delphi memory manager
- no RTTI required
- no problems

- without reference counting

[WojTec]

Thanks @Luckie!

Can I ask for example how to use functions @himitsu listed? Or just use WideString as Delphi's string?

[WojTec]

I tested @Luckies code and it working in example, but don't work in this case:

Delphi-Quellcode:

function func1(s: PChar; Buffer: PChar; lenBuffer: Integer): Integer; external 'StringDLL.dll';

var
  Buffer: array [0..MAX_PATH] of Char;
  BufferSize: DWORD;
begin
  BufferSize := High(Buffer);
  func1('bar', Buffer, BufferSize);
  ShowMessage(Buffer);
end;

Raises AV. Windows functions working, this one not. Why?

[jaenicke]

One inportant issue first:
Always use PWideChar or PAnsiChar in DLL interfaces. Otherwise it depends on the Delphi version and what PChar means in it whether your program works or not...

Regarding your problem:
I don't see an error handling (you ignore the return value). And where does the exception occur? Inside the DLL function?

And could you please post the source of the function inside the DLL too?

[WojTec]

Code (function from @Luckie article).

Delphi-Quellcode:

library StringDLL;

uses
  SysUtils;

function func1(s: PWideChar; Buffer: PWideChar; lenBuffer: Integer): Integer; stdcall;
var
  foo: String;
begin
  foo := 'foo'+ s;
  if Assigned(Buffer) then
    StrLCopy(Buffer, PWideChar(foo), lenBuffer);
  Result := Length(foo);
end;

exports
  func1;

begin
end.

function func1(s: PWideChar; Buffer: PWideChar; lenBuffer: Integer): Integer; external 'StringDLL.dll';

procedure TForm1.Button1Click(Sender: TObject);
var
  Buffer: array [0..MAX_PATH] of Char;
  BufferSize: DWORD;
begin
  BufferSize := High(Buffer);
  func1('bar', Buffer, BufferSize);
  ShowMessage(Buffer);
end;

Windows functions I'm using in the sam way, exception in DLL.

Original example is working:

Delphi-Quellcode:

procedure TForm1.FormCreate(Sender: TObject);
var
  hLib: THandle;
  s: String;
  foo1: function(s: PChar; Buffer: PChar; lenBuffer: Integer): Integer; stdcall;
  len: Integer;
  Buffer: PChar;
begin
  Buffer := nil;
  hLib := LoadLibrary('StringDLL.dll');
  if hLib = 0 then
  begin
    Str(GetLastError, s);
    ListBox1.Items.Add('LE: ' + s);
    exit;
  end;
  Str(hLib, s);
  ListBox1.Items.Add('hlib: ' + s);
  @foo1 := GetProcAddress(hLib, 'func1');
  if (not Assigned(foo1)) then
  begin
    Str(GetLastError, s);
    ListBox1.Items.Add('AE: ' + s);
    exit;
  end;
  Str(Integer(@foo1), s);
  ListBox1.Items.Add('@func1: ' + s);
  len := foo1('', nil, 0);
  Str(len, s);
  ListBox1.Items.Add('len: ' + s);
  try
    GetMem(Buffer, len + 1);
    len := foo1('', Buffer, len + 1);
    Str(len, s);
    ListBox1.Items.Add(String(Buffer)+ ' [' + s + ']');
  finally
    FreeMem(Buffer);
  end;
end;

I prefer 1st method (as in WinAPI). Why it don't working? :(

[jaenicke]

Zitat:

Zitat von WojTec (Beitrag 1107352)

Delphi-Quellcode:

  foo := 'foo'+ s;
  if Assigned(Buffer) then
    StrLCopy(Buffer, PWideChar(foo), lenBuffer);

You try to copy lenBuffer Bytes from foo, but it does not have so many Bytes. ;-)

By the way: I prefer the way most of the API functions work:
First I ask for the buffer size needed, then I reserve enough memory and ask for the data itself.