17.共享内存的操作

system v IPC，通过ftok创建共享内存键值

ftok - convert a pathname and a project identifier to a System V IPC key

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

int main(int argc, char *argv[]){
    //get a system v ipc key's value
    key_t key = ftok(argv[1], 31);
    if(key == -1) return -1;
    printf("key: 0x%x\n", key);

    return 0;
}

输出：

zhongjun@eclipse:~/projects$ gcc ftok.c -o ftok
zhongjun@eclipse:~/projects$ ./ftok ftok.i
key: 0x1f02f9ff

获取共享内存段的键值：

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

int main(int argc, char *argv[]){
    //get a system v ipc key's value
    key_t key = ftok(argv[1], 31);
    if(key == -1) return -1;
    printf("key: 0x%x\n", key);

    //using current key to get the share memory segment
    int shmid = shmget(key, 1024, IPC_CREAT | IPC_EXCL | 0644);
    if(shmid == -1) return -1;
    printf("shmid: %d \n", shmid);
    return 0;
}

输出：

zhongjun@eclipse:~/projects$ gcc shmget.c -o shmget
zhongjun@eclipse:~/projects$ ./shmget ftok.i
key: 0x1f02f9ff
shmid: 163841
zhongjun@eclipse:~/projects$ ipcs -m
------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x00000000 98304      lightdm    600        524288     2          dest
0x1f02f9ff 163841     zhongjun   644        1024       0

利用两个程序，交互使用共享内存段
shareA.c

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

int main(int argc, char *argv[]){
    //get a system v ipc key's value
    key_t key = ftok(argv[1], 31);
    if(key == -1) return -1;
    printf("key: 0x%x\n", key);

    //using current key to get the share memory segment
    int shmid = shmget(key, 1024, IPC_CREAT | 0644);
    if(shmid == -1) return -1;
    printf("shmid: %d \n", shmid);
    //使用共享内存段的ID，将共享内存段关联到进程的地址空间
    void *p = shmat(shmid, NULL, 0);
    if(p == (void *) -1) return -1;
    strcpy(p, "hello sky walker!\n");
    //使用完成后，释放该共享内存段
    shmdt(p);
    return 0;
}

shareB.c

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

int main(int argc, char *argv[]){
    //get a system v ipc key's value
    key_t key = ftok(argv[1], 31);
    if(key == -1) return -1;
    printf("key: 0x%x\n", key);

    //using current key to get the share memory segment
    int shmid = shmget(key, 1024, IPC_CREAT | 0644);
    if(shmid == -1) return -1;
    printf("shmid: %d \n", shmid);
    //使用共享内存段的ID，将共享内存段关联到进程的地址空间
    void *p = shmat(shmid, NULL, 0);
    if(p == (void *) -1) return -1;
    printf("%s", (char *)p);
    //使用完成后，释放该共享内存段
    shmdt(p);
    return 0;
}

输出：

zhongjun@eclipse:~/projects$ gcc shareA.c -o shareA
zhongjun@eclipse:~/projects$ gcc shareB.c -o shareB
zhongjun@eclipse:~/projects$ ./shareA ftok.i
key: 0x1f02f9ff
shmid: 163841
zhongjun@eclipse:~/projects$ ./shareB ftok.i
key: 0x1f02f9ff
shmid: 163841
hello sky walker!
zhongjun@eclipse:~/projects$

最后，通过ipcrm删除共享内存段

ipcrm -h
Options:
 -m, --shmem-id <id>        remove shared memory segment by id
 -M, --shmem-key <key>      remove shared memory segment by key

zhongjun@eclipse:~/projects$ ipcs -m

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x00000000 98304      lightdm    600        524288     2          dest
0x1f02f9ff 163841     zhongjun   644        1024       0

zhongjun@eclipse:~/projects$ ipcrm -M 0x1f02f9ff
zhongjun@eclipse:~/projects$ ipcs -m

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x00000000 98304      lightdm    600        524288     2          dest