Error calling DLL with Unicode Delphi

Question

I have a DLL with this function signature:

function KP_RecuperarDadosFabricante(EnderecoIp, ChaveAcesso, 
  Porta: string; Return: PAnsiChar): Integer; stdcall; external 'Key.dll';

This is the sample VB.NET code from the manual of the dll:

Dim ret As Integer 
Dim ptrStr As IntPtr 
Dim DadosFab As String 

ptrStr = System.Runtime.InteropServices.Marshal.AllocHGlobal(256) 
ret = KP_RecuperarDadosFabricante("192.168.1.201", "0", "4370", ptrStr) 
DadosFab = System.Runtime.InteropServices.Marshal.PtrToStringAnsi(ptrStr)
System.Runtime.InteropServices.Marshal.FreeHGlobal(ptrStr)

This is my Delphi code:

procedure TForm1.BitBtn1Click(Sender: TObject);
var
  Return: string;
  pReturn: PAnsiChar;
begin
  GetMem(pReturn, 512);
  FillChar(pReturn^, 512, 0);
  Memo1.Text:='';
  if KP_RecuperarDadosFabricante("192.168.1.201", "0", "4370", pReturn) = 1 then
  begin
    Return := StrPas(pReturn);
    ShowMessage(Return);
  end
  else
  begin
    ShowMessage('Error');
  end;
end;

This code works well in Delphi 7, but not works in Delphi XE. It always returns "Error". How to make this code works in Delphi XE?

the **far** directive is deprecated since... mmmm, maybe since Delphi 3 if not Delphi 1, why are you using it? what about freeing the memory you get? The vb example frees it, but your code leaks 512 bytes per call! — jachguate, Mar 15 '13 at 23:27
@jachguate I removed "far" code, but not solved the problem. The memory is not problem yet. thanks — dataol, Mar 15 '13 at 23:54
I didn't say it solves your problem, but they are other _problems_ you have. — jachguate, Mar 16 '13 at 04:34
@DavidHeffernan In Delphi 7 it works with this code, I don't have more code. But I don't know how to work with P/Invoke. Can you help me? Thanks! — dataol, Mar 16 '13 at 14:08
We need to see the VB code which defines `KP_RecuperarDadosFabricante` — David Heffernan, Mar 16 '13 at 14:10
@DavidHeffernan OK. I will request this code to the device manufacturer. — dataol, Mar 16 '13 at 14:51

Rob Kennedy · Accepted Answer · 2013-03-15T22:43:04.000

7

If the DLL was written in Delphi 7, then you cannot use that DLL from Delphi XE. Even if you change the string parameters to AnsiString to make the character sizes the same, you're still stuck with the fact that the internal structure of strings changed somewhere between those versions, so the AnsiString you send from the Delphi XE code will not look the way the Delphi 7 code expects an AnsiString to look.

If you have the power to change and recompile the DLL, you should make the parameters be PAnsiChar instead, just like you'd see from nearly any other DLL on Windows. Also remember to change the final PChar parameter to PAnsiChar.

If the DLL was written in some non-Delphi language, then you might be able to fix it simply by changing the function signature. Non-Delphi languages don't have Delphi's string type, so the declaration was wrong all the time; you were just lucky when it worked from Delphi 7. Change all the parameters to have type PAnsiChar (including the final parameter); then they'll match the char* type they probably have from the original C code.

edited Mar 15 '13 at 22:43

answered Mar 15 '13 at 21:07

Rob Kennedy

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+1. Never expose anything from a DLL other than very basic non-Delphi specific types like 8, 16, 32 and 64 bit integers, 32 bit and 64 bit floating point types, records containing such types and pointers. – Rudy Velthuis Mar 15 '13 at 21:13
Thanks Rob. I changed the signature from Pchar to PAnsiChar, but I don't solved the problem. This is the sample code from the manual of the dll, in VB.NET: >>> Dim ret As Integer Dim ptrStr As IntPtr Dim DadosFab As String ptrStr = System.Runtime.InteropServices.Marshal.AllocHGlobal(256) ret = KP1510_RecuperarDadosFabricante("192.168.1.201", "0", "4370", ptrStr) DadosFab = System.Runtime.InteropServices.Marshal.PtrToStringAnsi(ptrStr) System.Runtime.InteropServices.Marshal.FreeHGlobal(ptrStr) – dataol Mar 15 '13 at 22:13
I do not know if the DLL was written in Delphi 7 – dataol Mar 15 '13 at 22:20
2

It's not enough just to change the PChar parameter to PAnsiChar. I told you to change the *string* parameters to that type, too. – Rob Kennedy Mar 15 '13 at 22:44
Thanks Rob. I did this, and already updated my code in question. Could you suggest some other solution? – dataol Mar 15 '13 at 22:56
1

@dataol: The sample call should have been in your original question. I've added it, but next time this type of information is important to include when you ask a question (in the question itself, not in comments where people might or might not see it). – Ken White Mar 15 '13 at 23:00

Error calling DLL with Unicode Delphi

1 Answers1