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撰文：周翔
这是我在上操作系统课的那个学期写的一段程序，并组织成了一篇文章。当初被我的挚友曾毅发表在cstc的论坛上：http://cstc.net.cn/bbs/viewtopic.php?t=457，在此，我把它贴在这儿，希望对大家有所裨益。
学操作系统的进程同步都要涉及到三个经典问题：生产者-消费者问题、读者-写者问题和哲学家就餐问题。下面来介绍一下哲学家就餐问题：
哲学家就餐问题中，一组哲学家围坐在一个圆桌旁，每个哲学家的左边都只有一只筷子（当然他的右边也有一只筷子，但是这是他右边哲学家的左边的筷子），他们吃完了就思考，思考了一会就会饿，饿了就想吃，然而，为了吃饭，他们必须获得左边和右边的筷子。当每个哲学家只拿有一只筷子的时候，会坐者等另一只筷子，在每个哲学家都只拿一个筷子的时候，就会发生死锁。传统的解决死锁问题的方法是引用管程的概念，但是在c#中来实现的话可以使system.threading中的mutex为每个哲学家来声名两个信号量rightchopstick和leftchopstick，在主程序中用5个mutex赋值给它，用waithandle来实现对筷子的独占访问。这个例子是用windows图形界面实现，用事件来通知界面哲学家的状态。
以下是代码（在vs.net 下运行通过）：
//diningphilosophers.cs———-code:seafrog—————————————————–
using system;
using system.threading;
using system.windows.forms;
using seafrog.threading;
using seafrog.philosopher;
namespace diningphilosophers
{
public class form1 : system.windows.forms.form
{
private system.windows.forms.button button1;
private system.componentmodel.container components = null;
private system.windows.forms.listbox listbox1;
private philosopher[] p=new philosopher[5];
public form1()
{
initializecomponent();
mutex[] chopsticks=new mutex[5];
for(int i=0;i<5;i++)
{
chopsticks[i]=new mutex(false);
}
for(int i=0;i<5;i++)
{
philosopherdata pd;
pd.philosopherid=i;
pd.rightchopstick=chopsticks[(i+1)%5];
pd.leftchopstick=chopsticks[(i+4)%5];
pd.amounttoeat=5;
pd.totalfood=35;
p[i]=new philosopher(pd);
p[i].messagearrival+=new philosopher.messagearrivedhandler(showmessage);
}
}
protected override void dispose( bool disposing )
{
if( disposing )
{
if (components != null)
{
components.dispose();
}
}
base.dispose( disposing );
}
#region windows form designer generated code
private void initializecomponent()
{
this.button1 = new system.windows.forms.button();
this.listbox1 = new system.windows.forms.listbox();
this.suspendlayout();
//
// button1
//
this.button1.location = new system.drawing.point(8, 224);
this.button1.name = "button1";
this.button1.size = new system.drawing.size(272, 40);
this.button1.tabindex = 1;
this.button1.text = "go to restaurant";
this.button1.click += new system.eventhandler(this.button1_click);
//
// listbox1
//
this.listbox1.itemheight = 12;
this.listbox1.name = "listbox1";
this.listbox1.size = new system.drawing.size(296, 220);
this.listbox1.tabindex = 2;
//
// form1
//
this.autoscalebasesize = new system.drawing.size(6, 14);
this.clientsize = new system.drawing.size(292, 273);
this.controls.addrange(new system.windows.forms.control[] {
this.listbox1,
this.button1});
this.name = "form1";
this.text = "form1";
this.resumelayout(false);
}
#endregion
[stathread]
static void main()
{
application.run(new form1());
}
private void button1_click(object sender, system.eventargs e)
{
for(int i=0;i<5;i++)
p[i].start();
}
public void showmessage(object sender,messagearrivedeventargs e)
{
switch(e.type)
{
case philosopher.ready:
listbox1.items.add("philosopher("+e.philosopherdata.philosopherid+") ready.");
break;
case philosopher.eating:
listbox1.items.add("philosopher("+
e.philosopherdata.philosopherid+") eating "+
e.philosopherdata.amounttoeat+" of "+
e.philosopherdata.totalfood+" food.");
break;
case philosopher.thinking:
listbox1.items.add("philosopher("+e.philosopherdata.philosopherid+") thinking.");
break;
case philosopher.finished:
listbox1.items.add("philosopher("+e.philosopherdata.philosopherid+") finished.");
break;
}
}
}
}
//basethread.cs———-code:seafrog——————————————————–
using system;
using system.threading;
namespace seafrog.threading
{
//工作线程抽象类，作为对线程操作的封装。
public abstract class workerthread
{
private object threaddata;
private thread thisthread;
public object data
{
get{return threaddata;}
set{threaddata=value;}
}
public object isalive
{
get{return thisthread==null?false:thisthread.isalive;}
}
public workerthread(object data)
{
this.threaddata=data;
}
public workerthread()
{
threaddata=null;
}
public void start()
{
thisthread=new thread(new threadstart(this.run));
thisthread.start();
}
public void stop()
{
thisthread.abort();
while(thisthread.isalive);
thisthread=null;
}
protected abstract void run();
}
}
//philosophers.cs———-code:seafrog——————————————————–
using system;
using system.threading;
using seafrog.threading;
namespace seafrog.philosopher
{
//封装哲学家数据的结构
public struct philosopherdata
{
public int philosopherid;
public mutex rightchopstick;
public mutex leftchopstick;
public int amounttoeat;
public int totalfood;
}
public class philosopher : seafrog.threading.workerthread
{
public const int ready=0;
public const int eating=1;
public const int thinking=2;
public const int finished=3;
public philosopher(object data):base(data){}
public delegate void messagearrivedhandler(object sender,messagearrivedeventargs args);
public event messagearrivedhandler messagearrival;
public static int finished=0;
protected override void run()
{
philosopherdata pd=(philosopherdata)data;
random r=new random(pd.philosopherid);
messagearrival(this,new messagearrivedeventargs(ready,pd));
waithandle[] chopsticks=new waithandle[]{pd.leftchopstick,pd.rightchopstick};
while(pd.totalfood>0)
{
//如果两边的哲学家拿着筷子，则等待。
waithandle.waitall(chopsticks);
//否则，吃饭。
messagearrival(this,new messagearrivedeventargs(eating,pd));
//把饭吃掉一部分。
pd.totalfood-=pd.amounttoeat;
thread.sleep(r.next(1000,5000));
messagearrival(this,new messagearrivedeventargs(thinking,pd));
//放下左边和右边的筷子。
pd.rightchopstick.releasemutex();
pd.leftchopstick.releasemutex();
thread.sleep(r.next(1000,5000));
}
//饭都吃完了。
messagearrival(this,new messagearrivedeventargs(finished,pd));
if(++finished==4)
system.windows.forms.messagebox.show("all finished!");
}
}
//事件：用来通知主窗体现在哲学家的状态。
public class messagearrivedeventargs : eventargs
{
public int type;
public philosopherdata philosopherdata;
public messagearrivedeventargs(int t,philosopherdata pd)
{
type=t;
philosopherdata=pd;
}
}
}
( 完)
