多线程同步中的哲学家就餐问题 － windows 平台实现

哲学家就餐问题是多线程同步中的经典问题，本文在 windows 上用 C/C++  实现该问题。

之前在网上看到一个该问题的实现：http://dev.csdn.net/article/77/77383.shtm，是用临界区作的，但是感觉不是太好，因为当一个哲学家在试图取叉子用餐的时候，其他哲学家必须等待，而实际上，当 1 号哲学家

取叉子时，3号哲学家也可以取叉子，他们是不冲突的。

假设有 5 个哲学家，有 5 把叉子，每个哲学家有思考，饥饿，和吃饭三个状态。我们用 5 个信号量分别表示 5 把叉子。在打印哲学家状态时，使用一个互斥体 mutex ，否则打印出来是乱的。

源代码如下：

// PhilosopherDining.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include "windows.h"

#include <process.h>
#include <time.h>
#include <stdlib.h>
#include <stdio.h>

#include <iostream>

using namespace std;

const unsigned int PHILOSOPHER_NUM=5;
const char THINKING=1;
const char HUNGRY=2;
const char DINING=3;

// each fork has a semaphore
HANDLE semph[PHILOSOPHER_NUM];

// Mutex for printing
HANDLE mutex;

void philosopherProc(void* param);

int main(int argc, char* argv[])
{
       int i;
       srand(time(0));

       mutex = CreateMutex(NULL, false, NULL);
       for (i=0; i<PHILOSOPHER_NUM; i++)
       {
              semph[i] = CreateSemaphore(NULL, 1, 1, NULL);
              _beginthread(philosopherProc, 0, (void*)&i);
              Sleep(10);
       }

       Sleep(2000);
       return 0;
}

void philosopherProc(void* param)
{
       int myid;
       char idStr[128];
       char stateStr[128];
       char mystate;
       int ret;
       unsigned int leftFork;
       unsigned int rightFork;

       myid = *((int*)(param));
       itoa(myid, idStr, 10);

       //cout << "philosopher " << myid << " begin......" << endl;
       Sleep(10);

       // initial state is THINKING
       mystate = THINKING;
       leftFork = (myid) % PHILOSOPHER_NUM;
       rightFork = (myid + 1) % PHILOSOPHER_NUM;

       while (true)
       {
              switch(mystate)
              {
              case THINKING:
                     // changing my state
                     mystate = HUNGRY;
                     strcpy(stateStr, "HUNGRY");
                     break;

              case HUNGRY:
                     strcpy(stateStr, "HUNGRY");
                     // first test the left fork ...
                     ret = WaitForSingleObject(semph[leftFork], 0);
                     if (ret == WAIT_OBJECT_0)
                     {
                            // left fork is ok, take it up !
                            // then test the right fork ...
                            ret = WaitForSingleObject(semph[rightFork], 0);
                            if (ret == WAIT_OBJECT_0)
                            {
                                   // right fork is also ok !
                                   // changing my state
                                   mystate = DINING;
                                   strcpy(stateStr, "DINING");
                            }
                            else
                            {
                                   // right fork is being used by others, so I must put down
                                   // the left fork.
                                   ReleaseSemaphore(semph[leftFork], 1, NULL);
                            }
                     }
                     break;

              case DINING:
                     // put down both the left and right fork
                     ReleaseSemaphore(semph[leftFork], 1, NULL);
                     ReleaseSemaphore(semph[rightFork], 1, NULL);

                     // changing my state
                     mystate = THINKING;
                     strcpy(stateStr, "THINKING");
                     break;
              }

              // print my state
              WaitForSingleObject(mutex, INFINITE);
              cout << "philosopher " << myid << " is : " << stateStr << endl;  
              ReleaseMutex(mutex);
  
              // sleep a random time : between 1 - 5 s
              int sleepTime; 
              sleepTime = 1 + (int)(5.0*rand()/(RAND_MAX+1.0)); 
              Sleep(sleepTime*10);
       }
}