UDP协议源码解析之发送

static int sock_sendto(int fd, void * buff, int len, unsigned flags,
       struct sockaddr *addr, int addr_len)
{
    struct socket *sock;
    struct file *file;
    char address[MAX_SOCK_ADDR];
    int err;

    if (fd < 0 || fd >= NR_OPEN || ((file = current->files->fd[fd]) == NULL))
        return(-EBADF);
    if (!(sock = sockfd_lookup(fd, NULL)))
        return(-ENOTSOCK);

    if(len<0)
        return -EINVAL;
    err=verify_area(VERIFY_READ,buff,len);
    if(err)
        return err;

    if((err=move_addr_to_kernel(addr,addr_len,address))<0)
        return err;

    return(sock->ops->sendto(sock, buff, len, (file->f_flags & O_NONBLOCK),
        flags, (struct sockaddr *)address, addr_len));
}

static int inet_sendto(struct socket *sock, void *ubuf, int size, int noblock, 
        unsigned flags, struct sockaddr *sin, int addr_len)
{
    struct sock *sk = (struct sock *) sock->data;
    if (sk->shutdown & SEND_SHUTDOWN) 
    {
        send_sig(SIGPIPE, current, 1);
        return(-EPIPE);
    }
    if (sk->prot->sendto == NULL) 
        return(-EOPNOTSUPP);
    if(sk->err)
        return inet_error(sk);
    /* We may need to bind the socket. */
    if(inet_autobind(sk)!=0)
        return -EAGAIN;
    return(sk->prot->sendto(sk, (unsigned char *) ubuf, size, noblock, flags, 
               (struct sockaddr_in *)sin, addr_len));
}

static int udp_sendto(struct sock *sk, unsigned char *from, int len, int noblock,
       unsigned flags, struct sockaddr_in *usin, int addr_len)
{
    struct sockaddr_in sin;
    int tmp;

    /* 
     *  Check the flags. We support no flags for UDP sending
     */
    if (flags&~MSG_DONTROUTE) 
        return(-EINVAL);
    /*
     *  Get and verify the address. 
     */
    // 目的地址信息
    if (usin) 
    {
        if (addr_len < sizeof(sin)) 
            return(-EINVAL);
        memcpy(&sin,usin,sizeof(sin));
        if (sin.sin_family && sin.sin_family != AF_INET) 
            return(-EINVAL);
        if (sin.sin_port == 0) 
            return(-EINVAL);
    } 
    else 
    {   
        // 没传目的信息又没有建立起连接，即没有调connect函数，则无法知道数据包应该发送给谁
        if (sk->state != TCP_ESTABLISHED) 
            return(-EINVAL);
        // 从之前建立连接中获取目的信息
        sin.sin_family = AF_INET;
        sin.sin_port = sk->dummy_th.dest;
        sin.sin_addr.s_addr = sk->daddr;
      }

      /*
       *  BSD socket semantics. You must set SO_BROADCAST to permit
       *  broadcasting of data.
       */

      if(sin.sin_addr.s_addr==INADDR_ANY)
          sin.sin_addr.s_addr=ip_my_addr();

      if(!sk->broadcast && ip_chk_addr(sin.sin_addr.s_addr)==IS_BROADCAST)
            return -EACCES;         /* Must turn broadcast on first */

    sk->inuse = 1;

    /* Send the packet. */
    tmp = udp_send(sk, &sin, from, len, flags);

    /* The datagram has been sent off.  Release the socket. */
    release_sock(sk);
    return(tmp);
}

static int udp_send(struct sock *sk, struct sockaddr_in *sin,
     unsigned char *from, int len, int rt)
{
    struct sk_buff *skb;
    struct device *dev;
    struct udphdr *uh;
    unsigned char *buff;
    unsigned long saddr;
    int size, tmp;
    int ttl;

    /* 
     *  Allocate an sk_buff copy of the packet.
     */
    // 协议头最大长度+数据长度
    size = sk->prot->max_header + len;
    // 在写缓冲区申请一个skb
    skb = sock_alloc_send_skb(sk, size, 0, &tmp);


    if (skb == NULL) 
        return tmp;

    skb->sk       = NULL;   /* to avoid changing sk->saddr */
    // 发送完可以销毁，不需要缓存
    skb->free     = 1;
    skb->localroute = sk->localroute|(rt&MSG_DONTROUTE);

    /*
     *  Now build the IP and MAC header. 
     */

    buff = skb->data;
    saddr = sk->saddr;
    dev = NULL;
    ttl = sk->ip_ttl;
#ifdef CONFIG_IP_MULTICAST
    if (MULTICAST(sin->sin_addr.s_addr))
        ttl = sk->ip_mc_ttl;
#endif
    // 构建ip和mac头
    tmp = sk->prot->build_header(skb, saddr, sin->sin_addr.s_addr,
            &dev, IPPROTO_UDP, sk->opt, skb->mem_len,sk->ip_tos,ttl);

    skb->sk=sk; /* So memory is freed correctly */

    /*
     *  Unable to put a header on the packet.
     */

    if (tmp < 0 ) 
    {
        sk->prot->wfree(sk, skb->mem_addr, skb->mem_len);
        return(tmp);
      }

    buff += tmp;
    saddr = skb->saddr; /*dev->pa_addr;*/
    // 全部协议头+数据的长度
    skb->len = tmp + sizeof(struct udphdr) + len;   /* len + UDP + IP + MAC */
    skb->dev = dev;

    /*
     *  Fill in the UDP header. 
     */

    uh = (struct udphdr *) buff;
    // 设置udp头各字段
    uh->len = htons(len + sizeof(struct udphdr));
    uh->source = sk->dummy_th.source;
    uh->dest = sin->sin_port;
    // 写入数据的首地址
    buff = (unsigned char *) (uh + 1);

    /*
     *  Copy the user data. 
     */

    memcpy_fromfs(buff, from, len);

      /*
       *  Set up the UDP checksum. 
       */
    // 计算校验和
    udp_send_check(uh, saddr, sin->sin_addr.s_addr, skb->len - tmp, sk);

    /* 
     *  Send the datagram to the interface. 
     */

    udp_statistics.UdpOutDatagrams++;
    // 下发到ip层    
    sk->prot->queue_xmit(sk, dev, skb, 1);
    return(len);
}

// udp不是面向连接的，connect函数主要是根据目的ip，从路由表找到目的地址的信息然后保存在sk中，并修改态为以已连接
int udp_connect(struct sock *sk, struct sockaddr_in *usin, int addr_len)
{
    struct rtable *rt;
    unsigned long sa;
    if (addr_len < sizeof(*usin)) 
        return(-EINVAL);

    if (usin->sin_family && usin->sin_family != AF_INET) 
        return(-EAFNOSUPPORT);
    if (usin->sin_addr.s_addr==INADDR_ANY)
        usin->sin_addr.s_addr=ip_my_addr();

    if(!sk->broadcast && ip_chk_addr(usin->sin_addr.s_addr)==IS_BROADCAST)
        return -EACCES;         /* Must turn broadcast on first */

      rt=ip_rt_route(usin->sin_addr.s_addr, NULL, &sa);
      if(rt==NULL)
          return -ENETUNREACH;
      sk->saddr = sa;     /* Update source address */
    sk->daddr = usin->sin_addr.s_addr;
    sk->dummy_th.dest = usin->sin_port;
    sk->state = TCP_ESTABLISHED;
    return(0);
}