哲学家就餐问题

/*inux进程的实现:哲学家就餐问题在 linux 上的程序实现

设有5个哲学家,共享一张放油把椅子的桌子,每人分得一吧椅子.但是桌子上总共只有5支筷子,在每个人两边分开各放一支.哲学家只有在肚子饥饿时才试图分两次从两边拾起筷子就餐.

就餐条件是:

1)哲学家想吃饭时,先提出吃饭的要求;

2)提出吃饭要求,并拿到2支筷子后,方可吃饭;

3)如果筷子已被他人获得,则必须等待该人吃完饭之后才能获取该筷子;

4)任一哲学家在自己未拿到2支筷子吃饭之前,决不放下手中的筷子;

5)刚开始就餐时,只允许2个哲学家请求吃饭.

试问:

1)描述一个保证不会出现两个邻座同时要求吃饭的算法;

2)描述一个既没有两邻座同时吃饭,又没有人饿死的算法;

3)在什么情况下,5个哲学家全都吃不上饭?

哲学家进餐问题是典型的同步问题.它是由Dijkstra提出并解决的.该问题是描述有五个哲学家,他们的生活方式是交替地进行思考和进餐.哲学家们共用一张圆桌,分别坐在周围的五张椅子上.在圆桌上有五个碗和五支筷子,平时一个哲学家进行思考,饥饿时便试图取用其左右岁靠近他的筷子,只有在他拿到两支筷子时才能进餐.进餐完毕,放下筷子继续思考.

*/

#include

#include

#include

#include

#include

#include

//#include "RasUtil.h"

using namespace std;

const unsigned int PHILOSOPHER_NUM=5;

const char THINKING=1;

const char HUNGRY=2;

const char DINING=3;

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

pthread_mutex_t mutex; // Mutex for printing

void* philosopherProc(void* param);

int main(int argc, char* argv[])

int i;

srand(getpid());

pthread_t philosopherThread[PHILOSOPHER_NUM]; //定义线程数组

int phId[PHILOSOPHER_NUM];

pthread_mutex_init(&mutex, NULL);

for (i=0; i

{

phId[i] = i;

sem_init(&semph[i], 0, 1); //对每只筷子信号量进行初始化为当前进程的局部信号量

pthread_create(&philosopherThread[i], NULL, philosopherProc, (void*)(&phId[i])); //以philosopherProc为原型创建PHILOSOPHER_NUM个线程

usleep(5000);

}

sleep(30);

return 0;

}

void* philosopherProc(void* param)

{

int myid;

char stateStr[128];

char mystate;

int ret;

unsigned int leftFork;

unsigned int rightFork;

myid = *((int*)(param));

cout << "philosopher " << myid << " begin......" << endl;

usleep(1000000); //把进程挂起一段时间，单位是微秒（百万分之一秒）；

// 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 = sem_trywait(&semph[leftFork]);

if (ret == 0)

{

// left fork is ok, take it up ,then test the right fork ...

ret = sem_trywait(&semph[rightFork]);

if (ret == 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. sem_post(&semph[leftFork]);

}

}

break;

case DINING:

// put down both the left and right fork

sem_post(&semph[leftFork]);

sem_post(&semph[rightFork]);

// changing my state

mystate = THINKING;

strcpy(stateStr, "THINKING");

break;

}

// print my state

pthread_mutex_lock(&mutex);

cout << "philosopher " << myid << " is : " << stateStr << endl;

pthread_mutex_unlock(&mutex);

// sleep a random time : between 1 - 5 s

int sleepTime;

sleepTime = 1 + (int)(5.0*rand()/(RAND_MAX+1.0));

usleep(sleepTime*100000);

}

}