动态初始化并仅按需要从TThread调用LoadLibrary一次

Question

我有一个Delphi DLL,需要从我的主UI应用程序或工作线程调用.

每次调用DLL时我都不想调用LoadLibrary/FreeLibrary.但是,我也不想在我的应用程序初始化部分加载它.因为在应用程序的生命周期中我可能根本不使用DLL.

所以我需要的是初始化和加载DLL的第一个调用者(线程或主UI).DLL将在完成部分中卸载.我意识到我需要一些同步.所以我使用了一个关键部分,但我似乎无法使它工作.

只有一个线程应该尝试并加载DLL.如果失败,其他线程不应该再次尝试加载DLL.

同步未按预期工作!
有人可以建议为什么？

MCVE:

program Project1;
{$APPTYPE CONSOLE}
uses
  Windows,
  SysUtils,
  Classes;

const
  MyDLL = 'MyDLL.dll';

type
  TDLLProcessProc = function(A: Integer): Integer; stdcall;

var
  DLLProc: TDLLProcessProc = nil;
  DLLModule: HMODULE = 0;
  DLLInitialized: Boolean = False;
  DLLInitialized_OK: Boolean = False;
  CS: TRTLCriticalSection;

procedure InitDLLByFirstCall;
begin
  if DLLModule = 0 then
  begin
    if DLLInitialized then Exit;
    EnterCriticalSection(CS);
    try
      if DLLInitialized then Exit;
      DLLInitialized := True;
      DLLModule := LoadLibrary(MyDLL);
      if DLLModule = 0 then RaiseLastWin32Error;
      DLLProc := GetProcAddress(DLLModule, 'Process');
      if @DLLProc = nil then RaiseLastWin32Error;
      DLLInitialized_OK := True;
    finally
      LeaveCriticalSection(CS);
    end;
  end;
end;

function DLLProcess(A: Integer): Integer;
begin
  InitDLLByFirstCall;
  if not DLLInitialized_OK then
    raise Exception.Create('DLL was not initialized OK');
  Result := DLLProc(A);
end;

type
  TDLLThread = class(TThread)
  private
    FNum: Integer;
  public
    constructor Create(CreateSuspended: Boolean; ANum: Integer);
    procedure Execute; override;
  end;

constructor TDLLThread.Create(CreateSuspended: Boolean; ANum: Integer);
begin
  FreeOnTerminate := True;
  FNum := ANum;
  inherited Create(CreateSuspended);
end;

procedure TDLLThread.Execute;
var
  RetValue: Integer;
begin
  try
    RetValue := DLLProcess(FNum);
    Sleep(0);
    Writeln('TDLLThread Result=> ' + IntToStr(RetValue));
  except
    on E: Exception do
    begin
      Writeln('TDLLThread Error: ' + E.Message);
    end;
  end;
end;

var
  I: Integer;

begin
  InitializeCriticalSection(CS);
  try
    // First 10 thread always fail!  
    for I := 1 to 10 do
      TDLLThread.Create(False, I);
    Readln;

    for I := 1 to 10 do
      TDLLThread.Create(False, I);
    Readln;
  finally
    DeleteCriticalSection(CS);
  end;
end. 

DLL:

library MyDLL;

uses
  Windows;

{$R *.res}        

function Process(A: Integer): Integer; stdcall;
begin
  Result := A;
end;

exports
  Process;

begin
  IsMultiThread := True;
end.

Answer 1

您需要修改代码,使得InitDLLByFirstCall只有在完成所有初始化之后才设置在开始时检查的条件变量.因此DLL句柄是一个糟糕的选择.

其次,您需要使用相同的条件变量外部和关键部分的内-如果你使用DLLInitialized了这一点,那么确实没有为任何一个使用DLLInitialized_OK也不DLLModule.

为了让事情更容易推理,你应该尽量避免使用最少数量的变量.像下面这样的东西应该工作:

var
  DLLProc: TDLLProcessProc = nil;
  DLLInitialized: Boolean = False;
  CS: TRTLCriticalSection;

procedure InitDLLByFirstCall;
var
  DLLModule: HMODULE;
begin
  if DLLInitialized then
    Exit;

  EnterCriticalSection(CS);
  try
    if not DLLInitialized then
    try
      DLLModule := LoadLibrary(MyDLL);
      Win32Check(DLLModule <> 0);

      DLLProc := GetProcAddress(DLLModule, 'Process');
      Win32Check(Assigned(DLLProc));
    finally
      DLLInitialized := True;
    end;
  finally
    LeaveCriticalSection(CS);
  end;
end;

function DLLProcess(A: Integer): Integer;
begin
  InitDLLByFirstCall;
  if @DLLProc = nil then
    raise Exception.Create('DLL was not initialized OK');
  Result := DLLProc(A);
end;

如果您不想检查其中的函数地址,DLLProcess那么您还可以对DLLInitialized变量使用整数或枚举,具有未初始化,失败和成功的不同值.