基于互斥锁同步机制的Linux共享内存简单实例

Linux共享内存是Linux系统中进程间通信的一种方式，但是没有相应的同步机制，本文通过进程间的互斥锁实现一种简单的共享内存实例，仅供入门学习。

sm_common.h:

#ifndef __SM_COMMON_H__
#define __SM_COMMON_H__

#include 

#define SM_BUF_SIZE 1024
#define SM_ID 0x1122

struct sm_msg
{
	int flag;
	pthread_mutex_t sm_mutex;
	char buf[SM_BUF_SIZE];
};


#endif

sm_server.c:
#include 
#include 
#include 
#include 
#include 

#include "sm_common.h"


int main(void)
{
	int shm_id = -1;
	int ret = -1;
	int key = -1;
	int running = 1;
	struct sm_msg *msg = NULL;
	void *shared_memory = NULL;
	pthread_mutexattr_t attr;

	pthread_mutexattr_init(&attr);
	pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
	//key = ftok( "./sm_common.h", 1 );
	//printf("key: %d\n", key);
	shm_id = shmget((key_t)SM_ID, sizeof(struct sm_msg), 0666|IPC_CREAT);
	if(shm_id < 0)
	{
	perror("fail to shmget");
	exit(1);
	}
#if 1	
	shared_memory = shmat(shm_id, NULL, 0);
	if (shared_memory == NULL) {
		perror("Failed to shmat");
		exit(1);
	}

	msg = (struct sm_msg *)shared_memory;
	msg->flag = 0;
	pthread_mutex_init(&msg->sm_mutex, &attr);

	while (running) {
		pthread_mutex_lock(&msg->sm_mutex);
		if (msg->flag == 1) {
			printf("Read message: %s\n", msg->buf);
			msg->flag = 0;
			pthread_mutex_unlock(&msg->sm_mutex);
			if (strncmp(msg->buf, "exit", 4) == 0) {
				running = 0;
			}
		} else {
			printf("No available data to read, sleep...\n");
			pthread_mutex_unlock(&msg->sm_mutex);
			sleep(2);
		}
	}

	ret = shmdt(shared_memory);
	if (ret < 0) {
		perror("Failed to shmdt");
		exit(1);
	}

	if(shmctl(shm_id, IPC_RMID, 0) < 0)
	{
		perror("failed to shmctl");
		exit(1);
	}
#endif
	return 0;
}

sm_client.c:

#include 
#include 
#include 
#include 
#include 

#include "sm_common.h"


int main(void)
{
	int shm_id = -1;
	int ret = -1;
	int key = -1;
	int running = 1;
	struct sm_msg *msg = NULL;
	void *shared_memory = NULL;
	//	key = ftok( "./sm_common.h", 1 );
	//printf("key: %d\n", key);
	shm_id = shmget((key_t)SM_ID, sizeof(struct sm_msg), 0666|IPC_CREAT);
	if(shm_id < 0)
	{
	perror("fail to shmget");
	exit(1);
	}

	shared_memory = shmat(shm_id, NULL, 0);
	if (shared_memory == NULL) {
		perror("Failed to shmat");
		exit(1);
	}

	msg = (struct sm_msg *)shared_memory;

	char buf[32];
	while (running) {
		printf("wait lock\n");
		pthread_mutex_lock(&msg->sm_mutex);
		printf("get lock\n");
		if(msg->flag == 1) {
			printf("Wait for other process's reading\n");
			pthread_mutex_unlock(&msg->sm_mutex);
			sleep(2);
		} else {
			printf("Please input data\n");
			fgets(buf, 32, stdin);
			printf("Write msg: %s\n", buf);
			strncpy(msg->buf, buf, sizeof(buf));		
			msg->flag = 1;
			if (strncmp(msg->buf, "exit", 4) == 0) {
				running = 0;
			}
			pthread_mutex_unlock(&msg->sm_mutex);
		}
	}

	ret = shmdt(shared_memory);
	if (ret < 0) {
		perror("Failed to shmdt");
		exit(1);
	}
	
#if 0 //Do thsi in server.
	if(shmctl(shm_id, IPC_RMID, 0) < 0)
	{
		perror("failed to shmctl");
		exit(1);
	}
#endif
	return 0;
}

Makefile:
all:
gcc -o sm_server sm_server.c -lpthread
gcc -o sm_client sm_client.c -lpthread

使用方法：

1. 打开终端1，运行服务端：./sm_server

2.打开终端2，运行客户端：./sm_client

3.在终端2输入字符串，在终端1可看到相应输出