多线程同步之 CriticalSection(临界区)

先看一段程序, 代码文件:unit Unit1;
interface
uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs, StdCtrls;
type
 TForm1 = class(TForm)
  ListBox1: TListBox;
  Button1: TButton;
  procedure FormCreate(Sender: TObject);
  procedure Button1Click(Sender: TObject);
 end;
var
 Form1: TForm1;
implementation
{$R *.dfm}
function MyThreadFun(p: Pointer): DWORD; stdcall;
var
 i: Integer;
begin
 for i := 0 to 99 do Form1.ListBox1.Items.Add(IntToStr(i));
 Result := 0;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
 ID: DWORD;
begin
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
 ListBox1.Align := alLeft;
end;
end.

在这段程序中, 有三个线程几乎是同时建立, 向窗体中的 ListBox1 中写数据, 最后写出的结果是这样的:

 多线程编程(8) - 多线程同步之 CriticalSection(临界区)

  能不能让它们别打架, 一个完了另一个再来? 这就要用到多线程的同步技术.

  前面说过, 最简单的同步手段就是 "临界区".

  先说这个 "同步"(Synchronize), 首先这个名字起的不好, 我们好像需要的是 "异步"; 其实异步也不准确...

  管它叫什么名字呢, 它的目的就是保证不冲突、有次序、都发生.

  "临界区"(CriticalSection): 当把一段代码放入一个临界区, 线程执行到临界区时就独占了, 让其他也要执行此代码的线程先等等; 这和前面用的 Lock 和 UnLock 差不多; 使用格式如下:

var CS: TRTLCriticalSection;  {声明一个 TRTLCriticalSection 结构类型变量; 它应该是全局的}
InitializeCriticalSection(CS); {初始化}
EnterCriticalSection(CS);   {开始: 轮到我了其他线程走开}
LeaveCriticalSection(CS);   {结束: 其他线程可以来了}
DeleteCriticalSection(CS);   {删除: 注意不能过早删除}
//也可用 TryEnterCriticalSection 替代 EnterCriticalSection.

  用上临界区, 重写上面的代码, 运行效果图:

  代码文件:unit Unit1;
interface
uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs, StdCtrls;
type
 TForm1 = class(TForm)
  ListBox1: TListBox;
  Button1: TButton;
  procedure FormCreate(Sender: TObject);
  procedure FormDestroy(Sender: TObject);
  procedure Button1Click(Sender: TObject);
 end;
var
 Form1: TForm1;
implementation
{$R *.dfm}
var
 CS: TRTLCriticalSection;
function MyThreadFun(p: Pointer): DWORD; stdcall;
var
 i: Integer;
begin
 EnterCriticalSection(CS);
 for i := 0 to 99 do Form1.ListBox1.Items.Add(IntToStr(i));
 LeaveCriticalSection(CS);
 Result := 0;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
 ID: DWORD;
begin
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
 ListBox1.Align := alLeft;
 InitializeCriticalSection(CS);
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
 DeleteCriticalSection(CS);
end;
end.

Delphi 在 SyncObjs 单元给封装了一个 TCriticalSection 类, 用法差不多, 代码如下:unit Unit1;
interface
uses
 Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
 Dialogs, StdCtrls;
type
 TForm1 = class(TForm)
  ListBox1: TListBox;
  Button1: TButton;
  procedure FormCreate(Sender: TObject);
  procedure FormDestroy(Sender: TObject);
  procedure Button1Click(Sender: TObject);
 end;
var
 Form1: TForm1;
implementation
{$R *.dfm}
uses SyncObjs;
var
 CS: TCriticalSection;
function MyThreadFun(p: Pointer): DWORD; stdcall;
var
 i: Integer;
begin
 CS.Enter;
 for i := 0 to 99 do Form1.ListBox1.Items.Add(IntToStr(i));
 CS.Leave;
 Result := 0;
end;
procedure TForm1.Button1Click(Sender: TObject);
var
 ID: DWORD;
begin
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
 CreateThread(nil, 0, @MyThreadFun, nil, 0, ID);
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
 ListBox1.Align := alLeft;
 CS := TCriticalSection.Create;
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
 CS.Free;
end;
end.

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