多线程编程(10) - 多线程同步之 Mutex (互斥对象)

原理分析:
互斥对象是系统内核对象, 各线程都可以拥有它, 谁拥有谁就能执行;
执行完毕, 用 ReleaseMutex 函数释放拥有权, 以让其他等待的线程使用.
其他线程可用 WaitForSingleObject 函数排队等候(等候也可以理解为排队申请).

使用过程:

var hMutex: THandle; {应该先声明一个全局的互斥句柄}

CreateMutex          {建立一个互斥对象}

WaitForSingleObject  {用等待函数排队等候}

ReleaseMutex         {释放拥有权}

CloseHandle          {最后释放互斥对象}


 
   

ReleaseMutex、CloseHandle 的参数都是 CreateMutex 返回的句柄, 关键是 CreateMutex 函数:

function CreateMutex(

  lpMutexAttributes: PSecurityAttributes;

  bInitialOwner: BOOL; {是否让创建者(此例中是主线程)拥有该互斥对象}

  lpName: PWideChar    {可以给此互斥对象取个名字, 如果不要名字可赋值为 nil}

): THandle;

{

1、第一个参数前面说过.

2、第二个参数在这里一定要是 False, 如果让主线程拥有互斥, 从理论上讲, 得等程序退出后其他线程才有机会;

   取值 False 时, 第一个执行的线程将会最先拥有互斥对象, 一旦拥有其他线程就得先等等.

3、第三个参数, 如果给个名字, 函数将从系统中寻找是否有重名的互斥对象, 如果有则返回同名对象的存在的句柄;

   如果赋值为 nil 将直接创建一个新的互斥对象; 下个例子将会有名字. }


 
   

本例效果图:

---

代码文件:

---

unit Unit1;



interface



uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls;



type

  TForm1 = class(TForm)

    Button1: TButton;

    procedure Button1Click(Sender: TObject);

    procedure FormCreate(Sender: TObject);

    procedure FormDestroy(Sender: TObject);

  end;



var

  Form1: TForm1;



implementation



{$R *.dfm}



var

  f: Integer;      {用这个变量协调一下各线程输出的位置}

  hMutex: THandle; {互斥对象的句柄}



function MyThreadFun(p: Pointer): DWORD; stdcall;

var

  i,y: Integer;

begin

  Inc(f);

  y := 20 * f;

  for i := 0 to 50000 do

  begin

    if WaitForSingleObject(hMutex, INFINITE) = WAIT_OBJECT_0 then

    begin

      Form1.Canvas.Lock;

      Form1.Canvas.TextOut(20, y, IntToStr(i));

      Form1.Canvas.Unlock;

      Sleep(0); {稍稍耽搁一点, 不然有时 Canvas 会协调不过来}

      ReleaseMutex(hMutex);

    end;

  end;

  Result := 0;

end;



procedure TForm1.Button1Click(Sender: TObject);

var

  ThreadID: DWORD;

begin

  Repaint;

  f := 0;

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

end;



procedure TForm1.FormCreate(Sender: TObject);

begin

  hMutex := CreateMutex(nil, False, nil);

end;



procedure TForm1.FormDestroy(Sender: TObject);

begin

  CloseHandle(hMutex);

end;



end.


 
   

窗体文件:

---

object Form1: TForm1

  Left = 0

  Top = 0

  Caption = 'Form1'

  ClientHeight = 140

  ClientWidth = 192

  Color = clBtnFace

  Font.Charset = DEFAULT_CHARSET

  Font.Color = clWindowText

  Font.Height = -11

  Font.Name = 'Tahoma'

  Font.Style = []

  OldCreateOrder = False

  OnCreate = FormCreate

  PixelsPerInch = 96

  TextHeight = 13

  object Button1: TButton

    Left = 109

    Top = 107

    Width = 75

    Height = 25

    Caption = 'Button1'

    TabOrder = 0

    OnClick = Button1Click

  end

end


 
   

SyncObjs 单元下有封装好的 TMutex 类, 好像不如 Api 快, 内部机制也稍有区别, 但使用方法差不多:

---

unit Unit1;



interface



uses

  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,

  Dialogs, StdCtrls;



type

  TForm1 = class(TForm)

    Button1: TButton;

    procedure Button1Click(Sender: TObject);

    procedure FormCreate(Sender: TObject);

    procedure FormDestroy(Sender: TObject);

  end;



var

  Form1: TForm1;



implementation



{$R *.dfm}



uses SyncObjs;

var

  f: Integer;

  MyMutex: TMutex;



function MyThreadFun(p: Pointer): DWORD; stdcall;

var

  i,y: Integer;

begin

  Inc(f);

  y := 20 * f;

  for i := 0 to 50000 do

  begin

    if MyMutex.WaitFor(INFINITE) = wrSignaled then

    begin

      Form1.Canvas.Lock;

      Form1.Canvas.TextOut(20, y, IntToStr(i));

      Form1.Canvas.Unlock;

      MyMutex.Release;

    end;

  end;

  Result := 0;

end;



procedure TForm1.Button1Click(Sender: TObject);

var

  ThreadID: DWORD;

begin

  Repaint;

  f := 0;

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

  CreateThread(nil, 0, @MyThreadFun, nil, 0, ThreadID);

end;



procedure TForm1.FormCreate(Sender: TObject);

begin

  MyMutex := TMutex.Create(False);

end;



procedure TForm1.FormDestroy(Sender: TObject);

begin

  MyMutex.Free;

end;



end.