How to Load a DLL in multiple threads in Delphi? - windows

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.