麻煩解釋一下 attach_shm()

[in2 (啦啦:)我是好in2)]

void *attach_shm(int shmkey, int shmsize) {
    void *shmptr;
    int shmid;

    char *empty_addr;
    /* set up one page in-accessible -- jochang */
    {
        int fd = open("/dev/zero",O_RDONLY);
        int size = ((shmsize + 4095) / 4096) * 4096;

        munmap(
            (empty_addr=mmap(0,4096+size,PROT_NONE,MAP_PRIVATE,fd,0))+4096
            ,size);

        close(fd);
    }

    shmid = shmget(shmkey, shmsize, 0);
    if(shmid < 0) {
        shmid = shmget(shmkey, shmsize, IPC_CREAT | 0600);
        if(shmid < 0)
            attach_err(shmkey, "shmget");
        shmptr = (void *)shmat(shmid, NULL, 0);
        if(shmptr == (void *)-1)
            attach_err(shmkey, "shmat");
    } else {
        shmptr = (void *)shmat(shmid, NULL, 0);
        if(shmptr == (void *)-1)
            attach_err(shmkey, "shmat");
    }


    /* unmap the page -- jochang */
    {
        munmap(empty_addr,4096);
    }
    return shmptr;
}


為什麼要 mmap ??

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