对指针的算术C代码的编译错误

Question

我在此示例代码块的最后一条指令中收到以下错误

sdl = (struct sockaddr_dl *)(sin + 1);

指向不完整类型的指针＆＃39; struct sockaddr_inarp＆＃39;

我删除了所有不必要的代码。

有人可以帮忙吗？

int test(void)
{

        int mib[6];
        size_t needed;
        char *host, *lim, *buf, *next;
        struct rt_msghdr *rtm;
        struct sockaddr_inarp *sin;
        struct sockaddr_dl *sdl;

        mib[0] = CTL_NET;
        mib[1] = PF_ROUTE;
        mib[2] = 0;
        mib[3] = AF_INET;
        mib[4] = NET_RT_FLAGS;
        mib[5] = RTF_LLINFO;

        if ((buf = malloc(needed)) == NULL)
            err(1, "malloc");

        lim = buf + needed;

        for (next = buf; next < lim; next += rtm->rtm_msglen)
        {
            rtm = (struct rt_msghdr *)next;
            sin = (struct sockaddr_inarp *)(rtm + 1);
            sdl = (struct sockaddr_dl *)(sin + 1);
        }
}