The results from DLL in Delphi where the parameters are PAnsiChar

Question

i've got a small problem. In my DLL i've got:

uses
  ShareMem,
  SysUtils,
  Classes,
  Dialogs;

function My_func (Param1, Param2, Param3: PAnsiChar) : Integer;

var
  s1,s2,s3 : string;

begin

     s1 := string(Param1);
     s2 := string(Param2);
     s3 := string(Param3);
     ShowMessage(s1);
     ShowMessage(s2);
     ShowMessage(s3);

     My_func := 0;
end;

And the call from my TestUnit.pas

unit Utestunit;

interface

uses
  ShareMem, Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,  Dialogs;

procedure Button1Click(Sender: TObject);
type
  TMy_func = function (Param1, Param2, Param3: PAnsiChar) : Integer; StdCall;
var
  my_func : TMyfunc;

  error_code:integer;
  My_library : THandle;
  the_end : Boolean;
  path_library,
  path_library_full : string;

  test1,
  test2,
  test3 : array [0..255] of AnsiChar;
begin
  // open the DLL
  path_library := ExtractFilePath(Application.ExeName)+'DLLRS_DLL.dll';
  path_library_full := ExtractFilePath(Application.ExeName)+'DLL';
  SetCurrentDir(path_library_full);
  try
    the_end := False;
    My_library := SafeLoadLibrary(PChar(path_library));
    if My_library > 32 then
    begin
        @My_func := GetProcAddress(My_library, PChar('My_func'));
        if @My_func = nil then
        begin
          ShowMessage('There is no library in '+path_library);
          the_end := True;
        end;
    end
    else
    begin
      error_code := GetLastError;
      ShowMessage('B??d biblioteki '+ sciezka_biblioteki+' nr:'+IntToStr(error_code));
      the_end := True;
    end;

    if not the_end then
    begin
      // the calling

      test1 := 'My string nr 1';
      test2 := 'My string nr 2';
      test3 := 'My string nr 3';

      kod_bledu := My_func(
          test1,
          test2,
          test3
      );      
    end;

  finally
    FreeLibrary(My_library);
    SetCurrentDir(ExtractFilePath(Application.ExeName))
  end;
end;

The results are:

My string nr 1

trash

trash

as the messages from my DLL.

Why only the first result is good while the rest trash is? It's only the test in my app.

question from:https://stackoverflow.com/questions/65643673/the-results-from-dll-in-delphi-where-the-parameters-are-pansichar

Answer 1

There are a lot of very bad habits in this code, that I don't want to get into because it will take so much effort.

However, the reason for the behaviour is that you did not declare the function in the DLL as stdcall. Fix that and the text will be passed correctly.