哲学家就餐问题——and型信号量机制

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <stdint.h>

#include <stdbool.h>

#include <errno.h>

#include <unistd.h>

#include <sys/types.h>

#include <sys/stat.h>

#include <sys/ipc.h>

#include <sys/sem.h>

#include <sys/wait.h>

#ifdef _SEM_SEMUN_UNDEFINED

union semun

{
int val;
struct semid_ds *buf;
unsigned short *array;
struct seminfo *__buf;

};

#endif

#define ERR_EXIT(m) \
do { \
perror(m); \
exit(EXIT_FAILURE); \
} while(0)

//相当于P操作

int wait_1chopstick(int no,int semid)

{
struct sembuf sb = {no,-1,0};
int ret;
ret = semop(semid,&sb,1);//semop()系统调用在semid标识的信号量集中的信号量上执行一个或多个up或down操作，可用于进程间的同步和互斥。
if(ret < 0) {
ERR_EXIT("semop");
}
return ret;

}

//相当于V操作

int free_1chopstick(int no,int semid)

{
struct sembuf sb = {no,1,0};
int ret;
ret = semop(semid,&sb,1);
if(ret < 0) {
ERR_EXIT("semop");
}
return ret;

}

#define DELAY (rand() % 5 + 1)

//相当于P操作

void wait_for_2chopstick(int no,int semid)

{
//哲学家左边的筷子号数
int left = no;
//右边的筷子
int right = (no + 1) % 5;

//筷子值是两个
//注意第一个参数是编号
struct sembuf buf[2] = {
{left,-1,0},   //左右两只筷子都能用时才进餐
{right,-1,0}
};
//信号集中有5个信号量，只是对其中的
//资源sembuf进行操作
semop(semid,buf,2);

}

//相当于V操作

void free_2chopstick(int no,int semid)

{
int left = no;
int right = (no + 1) % 5;
struct sembuf buf[2] = {
{left,1,0},
{right,1,0}
};
semop(semid,buf,2);

}

void philosophere(int no,int semid)

{
srand(getpid());
for(;;) {

       #if 1
//这里采取的措施是当两筷子都可用的时候
//哲学家才能吃饭，这样不相邻的哲学家就可
//吃上饭
printf("%d is thinking\n",no);
sleep(DELAY);
printf("%d is hungry\n",no);
wait_for_2chopstick(no,semid);//拿到筷子才能吃饭
printf("%d is eating\n",no);
sleep(DELAY);
free_2chopstick(no,semid);//释放筷子
#else
//这段代码可能会造成死锁
int left = no;
int right = (no + 1) % 5;
printf("%d is thinking\n",no);
sleep(DELAY);
printf("%d is hungry\n",no);
wait_1chopstick(left,semid);
sleep(DELAY);
wait_1chopstick(right,semid);
printf("%d is eating\n",no);
sleep(DELAY);
free_1chopstick(left,semid);

                free_1chopstick(right,semid);
#endif
}

}

int

main(int argc,char *argv[])

{
int semid;
//创建信号量
semid = semget(IPC_PRIVATE,5,IPC_CREAT | 0666);
if(semid < 0) {
ERR_EXIT("semi
d");
}
union semun su;
su.val = 1;
int i;
for(i = 0;i < 5;++i) {
//注意第二个参数也是索引
semctl(semid,i,SETVAL,su);//semctl()系统调用在一个信号量集或集合中的单个信号量上执行各种控制操作
}
//创建4个子进程
int num = 0;
pid_t pid;
for(i = 1;i < 5;++i) {
pid = fork();
if(pid < 0) {
ERR_EXIT("fork");
}
if(0 == pid) {
num = i;
break;
}
}
//这里就是哲学家要做的事情
philosophere(num,semid);//num 代表进程号
return 0;

}