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)
