linuxC多进程通讯---信号量+共享内存示例

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