Linux共享内存

共享内存是一种进程间的通信方式。

shmget(); 创建共享内存，返回标识符

shmat(); 把共享内存映射到进程的地址空间中

shdt(); 断开与共享内存的连接，非删除

shmctl(); 共享内存管理（删除，或改变状态）

Linux操作共享内存相关命令

 ipcs 查看消息队列，共享内存和信号量

ipcrm -m 32769 删除标识为32769的共享内存

 程序例子，父子间进程通信：

#include <stdio.h>
#include <sys/types.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <string.h>

#define SIZE 1024

int main()
{
    int shmid;
    pid_t pid;
    char *shmaddr;
    int flag;
    struct shmid_ds buf;

    shmid = shmget(IPC_PRIVATE, SIZE, IPC_CREAT|0600);
    if(shmid < 0)
    {
        perror("get shm ipc_id error");
        return -1;
    }

    if((pid = fork()) < 0) {
        perror("get fork error");
        return -1;
    } else if(pid == 0) {       /* child */
        shmaddr = (char *)shmat(shmid, NULL, 0);
        if((int)shmaddr == -1)
        {
            perror("shmat error");
            return -1;
        }
        strcpy(shmaddr, "Hi,I am child process!\n");
        shmdt(shmaddr);
        return 0;
    } else {                    /* parent */
        sleep(3);
        flag = shmctl(shmid, IPC_STAT, &buf);
        if(flag == -1)
        {
            perror("shmctl error");
            return -1;
        }

        printf("shm_segsz=%d bytes\n", buf.shm_segsz);
        printf("parent pid=%d,shm_cpid=%d \n", getpid(), buf.shm_cpid);
        printf("child pid=%d,shm_lpid=%d\n", pid, buf.shm_lpid);
        shmaddr = (char *)shmat(shmid, NULL, 0);
        if((int)shmaddr == -1)
        {
            perror("parent:shmat error");
            return -1;
        }
        printf("shmaddr in %s\n", shmaddr);
        shmdt(shmaddr);
        shmctl(shmid, IPC_RMID, NULL);
    }

    return 0;
}

编译

gcc shm.c -o shm

输出结果

shm_segsz=1024 bytes
parent pid=6163,shm_cpid=6163 
child pid=6164,shm_lpid=6164
shmaddr in Hi,I am child process!

 

程序例子，进程之间使用共享内存通信

shmwrite.c

#include <stdio.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>

typedef struct {
        char name[8];
        int age;
}people;

int main()
{
        int shmid;
        key_t key;
        char temp[8];
        char pathname[30];
        people *p_map;
        int i;

        strcpy(pathname, "/tmp");
        key = ftok(pathname, 0x03);
        if(key == -1)
        {
                perror("ftok error\n");
                return -1;
        }

        printf("key=%d\n", key);
        shmid = shmget(key, 4096,IPC_CREAT|IPC_EXCL|0600);
        if(shmid == -1)
        {
                perror("shmget error\n");
                return -1;
        }
        printf("shmid=%d\n",shmid);

        p_map = (people *)shmat(shmid, NULL, 0);
        if((int)p_map == -1)
        {
                perror("shmat error\n");
                return -1;
        }
        memset(temp, 0x00, sizeof(temp));
        strcpy(temp, "test");
        temp[4] = '0';

        for(i=0;i<3;i++)
        {
                temp[4]+=1;
                strncpy((p_map+i)->name, temp, 5);
                (p_map+i)->age=i;
        }

        if(shmdt(p_map) == -1)
        {
                perror("shmdt error\n");
                return -1;
        }
        return 0;
}

 shmread.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct {
        char name[8];
        int age;
}people;

int main(int argc, char *argv[])
{
        int shmid;
        people *p_map;
        key_t key;
        char pathname[30];
        int i = 0;

        strcpy(pathname, "/tmp");
        key = ftok(pathname, 0x03);
        if(key == -1)
        {
                perror("ftok error\n");
                return -1;
        }
        printf("key=%d\n", key);
        shmid = shmget(key, 0, 0);
        if(shmid == -1)
        {
                perror("shmread:shmget error\n");
                return -1;
        }
        printf("shmid=%d\n",shmid);
        p_map = (people*)shmat(shmid, NULL, 0);
        for(i=0;i<3;i++)
        {
                printf("name:%s\n", (*(p_map+i)).name);
                printf("age:%d\n", (*(p_map+i)).age);
        }
        if(shmdt(p_map) == -1)
        {
                perror("shmdt error\n");
                return -1;
        }
        return 0;
}

编译

gcc shmread.c -o shmread
gcc shmwrite.c -o shmwrite

shmwrite执行结果

key=50397185
shmid=262144

shmread执行结果

key=50397185
shmid=262144
Segmentation fault (core dumped)

结果很奇怪， 不知道为什么

参考资料：

https://www.cnblogs.com/ch122633/p/8068152.html

https://blog.youkuaiyun.com/wteruiycbqqvwt/article/details/90484317