tcp/ip

sock_map_fd
-----------sock_alloc_file
-----------fd_install

sock_alloc(**void**) //return socket *

static const struct file_operations socket_file_ops = {
    .owner =    THIS_MODULE,
    .llseek =   no_llseek,
    .read_iter =    sock_read_iter,
    .write_iter =   sock_write_iter,
    .poll =     sock_poll,
    .unlocked_ioctl = sock_ioctl,
#ifdef CONFIG_COMPAT
    .compat_ioctl = compat_sock_ioctl,
#endif
    .mmap =     sock_mmap,
    .release =  sock_close,
    .fasync =   sock_fasync,
    .sendpage = sock_sendpage,
    .splice_write = generic_splice_sendpage,
    .splice_read =  sock_splice_read,
};

/linux-2.6.38.8/net/ipv4/tcp.c

int sock_map_fd(struct socket *sock, int flags)
{
    struct file *newfile;
    int fd = sock_alloc_file(sock, &newfile, flags);

    if (likely(fd >= 0))
        fd_install(fd, newfile);

    return fd;
}


static int sock_alloc_file(struct socket *sock, struct file **f, int flags){
          struct file *file;
    ..............
     file = alloc_file(&path, FMODE_READ | FMODE_WRITE,
          &socket_file_ops);
   sock->file = file;
   file->private_data = sock;
}