套接字之sendto系统调用

sendto系统调用用于向指定的目的地址发送数据，其系统调用的流程比较容易理解，如下面所示，其主要完成 (1)将用户数据组织成msghdr，(2)而后调用socket操作的sendmsg；ipv4对应的sendmsg实现为inet_sendmsg，该函数进行端口自动绑定检查和绑定后，调用传输层的sendmsg函数发送数据，比如tcp会调用tcp_sendmsg函数，这里不详细介绍，读到tcp模块在进行补充；

 /*
  *    Send a datagram to a given address. We move the address into kernel
  *    space and check the user space data area is readable before invoking
  *    the protocol.
  */

 SYSCALL_DEFINE6(sendto, int, fd, void __user *, buff, size_t, len,
         unsigned int, flags, struct sockaddr __user *, addr,
         int, addr_len)
 {
     struct socket *sock;
     struct sockaddr_storage address;
     int err;
     struct msghdr msg;
     struct iovec iov;
     int fput_needed;

     /* 数据导入到消息 */
     err = import_single_range(WRITE, buff, len, &iov, &msg.msg_iter);
     if (unlikely(err))
         return err;

     /* 获取socket */
     sock = sockfd_lookup_light(fd, &err, &fput_needed);
     if (!sock)
         goto out;

     msg.msg_name = NULL;
     msg.msg_control = NULL;
     msg.msg_controllen = ;
     msg.msg_namelen = ;

     /* 地址信息 */
     if (addr) {
         err = move_addr_to_kernel(addr, addr_len, &address);
         if (err < )
             goto out_put;
         msg.msg_name = (struct sockaddr *)&address;
         msg.msg_namelen = addr_len;
     }

     /* 非阻塞标记 */
     if (sock->file->f_flags & O_NONBLOCK)
         flags |= MSG_DONTWAIT;
     msg.msg_flags = flags;

     /*
         调用sock->ops->sendmsg，ipv4为inet_sendmsg
         需要自动绑定端口则绑定，
         后调用传输层的发送sk->sk_prot->sendmsg
     */
     err = sock_sendmsg(sock, &msg);

 out_put:
     fput_light(sock->file, fput_needed);
 out:
     return err;
 }

TCP层的sendmsg实现为tcp_sendmsg，详情请移步<TCP层sendmsg系统调用的实现分析>；