linux IPC共享内存

共享内存相关函数

获得一个共享存储标识符

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

int shmget(key_t key, size_t size, int shmflg);
返回值：成功共享存储ID，失败-1
key:函数ftok返回值或IPC_PRIVATE（适合用在有亲缘关系的进程中）

size: 共享存储段的长度，以字节为单位
shmflg:权限标志位

共享存储段执行多种操作

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

int shmctl(int shmid, int cmd, struct shmid_ds *buf);
返回值：成功0错误-1
shmid:共享内存标识符
cmd:　　IPC_STAT:得到共享内存的状态，把共享内存的shmid_ds结构复制到buf中
　　　　IPC_SET:改变共享内存的状态，把buf所指的shmid_ds结构中的uid、gid、mode赋值到共享内存的shmid_ds结构中
　　　　IPC_RMID：删除这些共享内存
buf:共享内存管理结构体

 shmid_ds结构：

struct shmid_ds {
               struct ipc_perm shm_perm;    /* Ownership and permissions */
               size_t          shm_segsz;   /* Size of segment (bytes) */
               time_t          shm_atime;   /* Last attach time */
               time_t          shm_dtime;   /* Last detach time */
               time_t          shm_ctime;   /* Last change time */
               pid_t           shm_cpid;    /* PID of creator */
               pid_t           shm_lpid;    /* PID of last shmat(2)/shmdt(2) */
               shmatt_t        shm_nattch;  /* No. of current attaches */
               ...
           };

一旦创建了一个共享存储段，进程就可调用shmat将其连接到它的地址空间中

#include <sys/types.h>
#include <sys/shm.h>

void *shmat(int shmid, const void *shmaddr, int shmflg);
返回值：成功共享存储段的指针，失败-1
shmid:共享内存标识符
shmaddr:指定共享内存出现在进程内存地址的什么位置，直接指定为NULL让内核决定
shmflg:如果指定了SHM_RDONLY位，则以只读方式连接此段，否则以读写的方式连接此段。通常为0

当对共享存储段的操作已经结束时，则调用shmdt与该段分离

#include <sys/types.h>
#include <sys/shm.h>

int shmdt(const void *shmaddr);
返回值：成功0，失败-1
shmaddr:shmat返回的值

程序例程，父子进程间通讯：

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

    return 0;
}

进程之间使用共享内存通讯：

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
");
                return -1;
        }

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

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

然后执行：

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