生产消费者模型
•2个生产者,每1S、5S往缓冲区写一次数据
•3个消费者,每2S、2S、5S往缓冲区读一次数据
•2个写进程分别对写信号量做P操作、对读信号量做V操作
•3个读进程分别对写信号量做V操作、对读信号量做P操作
例程
cousumer
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <strings.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
#include <sys/types.h>
int mem_read (int *addr)
{
int index;
int data;
index = addr[0];
data = addr[index];
index--;
addr[0] = index;
return data;
}
union semun
{
int val;
struct semid_ds *buf;
unsigned short int *array;
struct seminfo *__buf;
};
int sem_id;
void sem_init (int semid, int nsignum, int sem_value)
{
union semun sem_union;
sem_union.val = sem_value;
if (semctl (semid, nsignum, SETVAL, sem_union) == -1)
{
perror ("semctl");
exit (EXIT_FAILURE);
}
}
void sem_p (int semid, int nsignum)
{
struct sembuf sops;
sops.sem_num = nsignum;
sops.sem_op = -1;
sops.sem_flg = SEM_UNDO;
if (semop (sem_id, &sops, 1) == -1)
{
perror ("semop");
exit (EXIT_FAILURE);
}
}
void sem_v (int semid, int nsignum)
{
struct sembuf sops;
sops.sem_num = nsignum;
sops.sem_op = 1;
sops.sem_flg = SEM_UNDO;
if (semop (sem_id, &sops, 1) == -1)
{
perror ("semop");
exit (EXIT_FAILURE);
}
}
void sem_print (int sem_id, int nsignum)
{
int sem_value;
sem_value = semctl (sem_id, nsignum, GETVAL);
printf ("sem[%d] = %d\n", nsignum, sem_value);
}
int main (void)
{
int shm_id;
key_t shm_key = ftok ("./", 5161);
key_t sem_key = ftok ("./", 5162);
shm_id = shmget (shm_key, 1028, IPC_CREAT | 0644);
char *shm_addr = shmat (shm_id, NULL, 0);
memset (shm_addr, 0, 128);
sem_id = semget (sem_key, 2, IPC_CREAT | 0644);
if (sem_id == -1 )
{
perror ("semget");
exit (EXIT_FAILURE);
}
else
{
sem_init (sem_id, 0, 0); // init read semaphore
sem_init (sem_id, 1, 5); // init write semaphore
}
for (int i = 0; i < 2; i++)
{
int ret_from_fork;
if ((ret_from_fork = fork()) == -1)
{
perror ("fork");
exit (EXIT_FAILURE);
}
else if (ret_from_fork == 0)
{
while (1)
{
sleep (2);
sem_p (sem_id, 0);
printf ("pid %d data: %d\n", getpid(), mem_read((int *)shm_addr));
sem_v (sem_id, 1);
}
}
}
while (1) //read
{
sleep (5);
sem_p (sem_id, 0);
printf ("pid %d data: %d\n", getpid(), mem_read((int *)shm_addr));
sem_v (sem_id, 1);
}
return 0;
}
producer
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <strings.h>
#include <sys/stat.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/shm.h>
void mem_write (int *addr, int data)
{
int index;
index = addr[0];
index++;
addr[index] = data;
addr[0] = index;
//printf ("index = %c\n", index);
}
union semun
{
int val;
struct semid_ds *buf;
unsigned short int *array;
struct seminfo *__buf;
};
int sem_id;
void sem_init (int semid, int nsignum, int sem_value)
{
union semun sem_union;
sem_union.val = sem_value;
if (semctl (semid, nsignum, SETVAL, sem_union) == -1)
{
perror ("semctl");
exit (EXIT_FAILURE);
}
}
void sem_p (int semid, int nsignum)
{
struct sembuf sops;
sops.sem_num = nsignum;
sops.sem_op = -1;
sops.sem_flg = SEM_UNDO;
if (semop (sem_id, &sops, 1) == -1)
{
perror ("semop");
exit (EXIT_FAILURE);
}
}
void sem_v (int semid, int nsignum)
{
struct sembuf sops;
sops.sem_num = nsignum;
sops.sem_op = 1;
sops.sem_flg = SEM_UNDO;
if (semop (sem_id, &sops, 1) == -1)
{
perror ("semop");
exit (EXIT_FAILURE);
}
}
void sem_print (int sem_id, int nsignum)
{
int sem_value;
sem_value = semctl (sem_id, nsignum, GETVAL);
printf ("sem[%d] = %d\n", nsignum, sem_value);
}
int main (void)
{
int shm_id;
key_t shm_key = ftok ("./", 5161);
key_t sem_key = ftok ("./", 5162);
shm_id = shmget (shm_key, 1028, IPC_CREAT | 0644);
char *shm_addr = shmat (shm_id, NULL, 0);
memset (shm_addr, 0, 128);
sem_id = semget (sem_key, 2, IPC_CREAT | 0644);
if (sem_id == -1)
{
sem_id = semget (sem_key, 2, 0644);
}
else
{
sem_init (sem_id, 0, 0); // for read
sem_init (sem_id, 1, 5); // for write
}
int ret_from_fork;
if ((ret_from_fork = fork()) == -1)
{
perror ("fork");
exit (EXIT_FAILURE);
}
else if (ret_from_fork == 0)
{
int child_data = 1;
while (1) //child process write
{
sleep (1);
sem_p (sem_id, 1);
printf ("child data: %d\n", child_data);
mem_write ((int *)shm_addr, child_data);
child_data = child_data + 2;
sem_v (sem_id, 0);
}
}
else
{
int parent_data = 2;
while (1) //parent process write
{
sleep (1);
sem_p (sem_id, 1);
printf ("parent data: %d\n", parent_data);
mem_write ((int *)shm_addr, parent_data);
parent_data = parent_data + 2;
sem_v (sem_id, 0);
}
}
return 0;
}
System V 信号量的通信特点
•信号量是通过标识符而不是常用的文件描述符来引用的
•使用键而不是文件名来标识信号量
•创建和初始化信号量需要使用单独的系统调用
•内核不会维护引用一个信号量集的进程数量。很多操作需要开发者自己控制
•信号量的操作存在诸多限制