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-
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-
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 */
               ...
           };

struct shmid_ds

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-
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-
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|);
     if(shmid < )
     {
         perror("get shm ipc_id error");
         return -;
     }

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

         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, );
         if((int)shmaddr == -)
         {
             perror("parent:shmat error");
             return -;
         }
         printf("shmaddr in %s\n", shmaddr);
         shmdt(shmaddr);
         shmctl(shmid, IPC_RMID, NULL);
     }

     return ;
 }

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[];
         int age;
 }people;

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

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

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

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

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

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

shmread.c

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

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

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

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

然后执行：

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