◆Delphi多线程编程之五不同类线程读写全局变量-阻塞和锁定◆（乌龙哈里2008-10-13）
（调试环境：Delphi2007+WinXPsp3 例程：Tst_Thread5.dpr）
前面的例子都是同类线程的不同实例来读写全局变量，用临界区、互斥等来锁住同段代码。现在碰到的问题是，A,B两个不同类型的线程，如何安全地来读写全局变量。
unit Tst_Thread5U;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls,SyncObjs;
type
TForm1 = class(TForm)
Memo1: TMemo;
Button1: TButton;
Button2: TButton;
Button3: TButton;
procedure Button1Click(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure Button2Click(Sender: TObject);
procedure Button3Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
TFirstThread=Class(TThread)
protected
procedure Execute;override;
procedure ShowNum;
end;
TSecondThread=Class(TThread)
protected
procedure Execute;override;
procedure ShowNum;
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
const
MaxSize=12;
var
WhichRun:integer;
GlobalArry:array[1..MaxSize] of Integer;
LockCS:TCriticalSection; //uses SyncObjs
LockMREWS:TMultiReadExclusiveWriteSynchronizer; //uses SysUtils
//===============
procedure AddNum;
var
i,j:integer;
begin
if WhichRun=2 then LockCS.Acquire;
if WhichRun=3 then LockMREWS.BeginRead;
j:=GlobalArry[MaxSize];
if WhichRun=3 then LockMREWS.EndRead;
if WhichRun=3 then LockMREWS.BeginWrite;
for i := 1 to MaxSize do
begin
GlobalArry[i]:=j;
inc(j);
Sleep(5);
end;
if WhichRun=2 then LockCS.Release;
if WhichRun=3 then LockMREWS.EndWrite;
end;
procedure TFirstThread.Execute;
begin
FreeOnTerminate:=True;
AddNum;
Synchronize(ShowNum);
end;
procedure TFirstThread.ShowNum;
var
i:integer;
begin
for i := 1 to MaxSize do
Form1.Memo1.Lines.Add(inttostr(GlobalArry[i]));
end;
procedure TSecondThread.Execute;
begin
FreeOnTerminate:=True;
AddNum;
Synchronize(ShowNum);
end;
procedure TSecondThread.ShowNum;
var
i:integer;
begin
for i := 1 to MaxSize do
Form1.Memo1.Lines.Add(inttostr(GlobalArry[i]));
end;
//未阻塞
procedure TForm1.Button1Click(Sender: TObject);
begin
WhichRun:=1;
TFirstThread.Create(False);
TSecondThread.Create(False);
end;
//阻塞 TCriticalSection
procedure TForm1.Button2Click(Sender: TObject);
begin
WhichRun:=2;
TFirstThread.Create(False);
TSecondThread.Create(False);
end;
//TMREWS
procedure TForm1.Button3Click(Sender: TObject);
begin
WhichRun:=3;
TFirstThread.Create(False);
TSecondThread.Create(False);
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
LockCS:=TCriticalSection.Create;
LockMREWS:=TMultiReadExclusiveWriteSynchronizer.Create;
end;
procedure TForm1.FormDestroy(Sender: TObject);
begin
LockCS.Free;
LockMREWS.Free;
end;
end.
一、用TCriticalSection类来阻塞。（ps：这里的咚咚都涉及Win32编程，我对这些不了解，先掌握如何运用吧。）这个有点类似临界区。要Uses SyncObjs单元。
1、声明一个全局的TCriticalSection类的实例。
2、建立TCriticalSection.Create，最好是在程序onCreate里面，这样才能保证对所有线程进行控制。
3、在全局变量访问前用TCriticalSection.Acquire或TCriticalSection.Enter来阻塞。
4、访问完毕，用TCriticalSection.Release或TCriticalSection.Leave来解除阻塞。
5、在恰当的位置进行TCriticalSection.Free，一般在程序的onDestroy里。
二、用TMultiReadExclusiveWriteSynchronizer类来阻塞。可以写成TMREWSync。这个类在SysUtils单元里。(ps:天哪，那么长的单词我第一次见)。TMultiReadExclusiveWriteSynchronizer和TCriticalSection不同的是，它允许多个线程同时读一个变量，只是在写一个变量时才需要事先阻塞，因为只有多个线程同时写一个变量才有可能造成冲突。
使用TMREWS的优势是它允许多线程的并发读取，同时又与CriticalSection一样允许读的时候只有一个线程访问。劣势是TMREWS用起来要费更高的代价。
1、声明一个全局的TMultiReadExclusiveWriteSynchronizer类的实例。
2、建立TMultiReadExclusiveWriteSynchronizer.Create。
3、每个线程在读一个全局变量前要先调用该类的BeginRead()来检查是否有其他线程在写这个变量。如果有，就