linux版本：4.9.51

在APP中，ret = bind(listenfd, (struct sockaddr*)&serv_addr, sizeof(serv_addr) );

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)

SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
{
	struct socket *sock;
	struct sockaddr_storage address;
	int err, fput_needed;

	sock = sockfd_lookup_light(fd, &err, &fput_needed);
	if (sock) {
		err = move_addr_to_kernel(umyaddr, addrlen, &address);
		if (err >= 0) {
			err = security_socket_bind(sock,
						   (struct sockaddr *)&address,
						   addrlen);
			//sock->ops = inet_stream_ops
			// inet_bind
			if (!err)
				err = sock->ops->bind(sock,
						      (struct sockaddr *)
						      &address, addrlen);
		}
		fput_light(sock->file, fput_needed);
	}
	return err;
}

sockfd_lookup_light，根据fd，找到socket

static struct socket *sockfd_lookup_light(int fd, int *err, int *fput_needed)
{
	struct fd f = fdget(fd);
	struct socket *sock;

	*err = -EBADF;
	if (f.file) {
		sock = sock_from_file(f.file, err);
		if (likely(sock)) {
			*fput_needed = f.flags;
			return sock;
		}
		fdput(f);
	}
	return NULL;
}

struct socket *sock_from_file(struct file *file, int *err)
{
	if (file->f_op == &socket_file_ops)
		return file->private_data;	/* set in sock_map_fd */

	*err = -ENOTSOCK;
	return NULL;
}

（此图来自《TCP/IP架构、设计与应用》）