How to Load a DLL in multiple threads in Delphi?

Question

Maybe there is something that I missed, I can't figure what is happening here.

I'm trying to load the same DLL in multiple instances of a TThread Object.

Here is my DLL code:

library MyCalcFor32;

uses
  SysUtils,
  Classes,
  uRunner in 'uRunner.pas';

Exports EVal;

{$R *.res}

begin
end.

This is the uRunner.pas:

unit uRunner;

interface

uses SysUtils,
     Classes;

function EVal(Valor: WideString): WideString; stdcall; export;

implementation

function EVal(Value: WideString): WideString; stdcall; export;
begin
  Result := Value+' xxx';
end;

initialization

finalization

end.

This is the program to Load the DLL:

procedure TfrmMain.FormCreate(Sender: TObject);
var I: Integer;
begin
  SetLength(Threads, 10);
  for I:= 0 to 9 do
  begin
    Threads[I] := TWorker.Create(Self.Handle, I+1, Memo1.Text, ExtractFilePath(ParamStr(0)));
  end;
end;


procedure TfrmMain.btnExecuteThreadsClick(Sender: TObject);
var I: Integer;
begin
  ClearMemos([MT1, MT2, MT3, MT4, MT5, MT6, MT7, MT8, MT9, MT10]);

  for I:= 0 to 0 do //to 9, for multiple
  begin
    if Threads[I].Suspended then
      Threads[I].Resume
    else
      ShowMessage('Thread already in execution');
  end;
end;

procedure TWorker.Execute;
var I: Integer;
    J: Cardinal;

    Ret: WideString;

    A,B,C: Extended;       
begin
  CoInitialize(nil);
  try
    LoadDll;

    while not Terminated do
    begin
      if not (Suspended or Terminated) then
      begin
        A := 310132041025;
        B := 17592186044416;
        C := 0;

        for I:= 0 to 10 do
        begin

          if (Terminated) then begin
            Break;
          end;

          for J:= 0 to 9999999 do
          begin

            if (Terminated) then begin
              Break;
            end;

            A:= Sqrt(A);

            if A <= 0 then begin
              A:= 310132041025;
            end
            else begin
              A:= Math.Power(A, 2);
            end;

            C:= C + (B-34 / 4);

            B:= B / 2;

            if B <= 0 then begin
              B:= 17592186044416;
            end;
          end;

          Ret := FEvalProcAddress(FEValValue);

          NotifyMainForm(Format('Evaluate %s, resulted in %s', [IntToStr(I), Ret]));
        end;
        Suspend;
      end;

      Sleep(5000);
    end;
  finally
    CoUninitialize;
  end;
end;  

procedure TWorker.LoadDll;
begin
  //GlobalLock.Enter;
  //try
    FDLLHandle := LoadLibraryA(PChar(FPathApp + 'MyCalcFor32.dll'));
  //finally
  //  GlobalLock.Leave;
  //end;

  if GetLastError <> 0 then
  begin
    NotifyTerminateThread;
  end
  else
  begin
    FEvalProcAddress := GetProcAddress(FDLLHandle, PChar('EVal'));

    if GetLastError <> 0 then
    begin
      NotifyTerminateThread;
    end;
  end;
end;

When I have only 1 thread, it works just fine, but when I use multiple threads It raises the following exception:

System Error.  Code: 87.
Incorrect Parameter

Note: The above code is just for reproduction;
I am aware of WideString + AnsiString problem.

Answer 1

You are performing the error checking incorrectly. You are only meant to call GetLastError if the function fails. I expect that you are calling GetLastError after an API call that succeeded and not all API calls do SetLastError(0) when they return success. So you are picking up a stale error code that does not apply to the function call that you made.

To check for failure, for these functions, you need to examine the return value.

• LoadLibrary reports failure by returning 0.
• GetProcAddress reports failure by returning nil.

You have to read the documentation of the functions carefully, but this is a very common theme. Each Win32 API function may potentially handle errors differently. Read the docs for each function individually.