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.