net/udp: Support zero-length UDP datagrams

According to RFC768 page2 and referring to the Linux implementation, the message with udp length of 0 is supported.

Signed-off-by: gaohedong <gaohedong@xiaomi.com>

net/socket/sendmsg.c

@@ -74,7 +74,8 @@ ssize_t psock_sendmsg(FAR struct socket *psock, FAR struct msghdr *msg,
 {
   /* Verify that non-NULL pointers were passed */
 
-  if (msg == NULL || msg->msg_iov == NULL || msg->msg_iov->iov_base == NULL)
+  if (msg == NULL || msg->msg_iov == NULL ||
+      (psock->s_type != SOCK_DGRAM && msg->msg_iov->iov_base == NULL))
     {
       return -EINVAL;
     }

net/udp/udp_devpoll.c

@@ -103,7 +103,7 @@ void udp_poll(FAR struct net_driver_s *dev, FAR struct udp_conn_s *conn)
 
       /* If the application has data to send, setup the UDP/IP header */
 
-      if (dev->d_sndlen > 0)
+      if (dev->d_len > 0)
         {
           udp_send(dev, conn);
           return;

net/udp/udp_send.c

@@ -155,7 +155,7 @@ void udp_send(FAR struct net_driver_s *dev, FAR struct udp_conn_s *conn)
 
   ninfo("UDP payload: %d (%d) bytes\n", dev->d_sndlen, dev->d_len);
 
-  if (dev->d_sndlen > 0)
+  if (dev->d_len > 0)
     {
 #ifdef CONFIG_NET_IPv4
 #ifdef CONFIG_NET_IPv6

net/udp/udp_sendto_buffered.c

@@ -687,8 +687,6 @@ ssize_t psock_udp_sendto(FAR struct socket *psock, FAR const void *buf,
 
   BUF_DUMP("psock_udp_sendto", buf, len);
 
-  if (len > 0)
-    {
   net_lock();
 
 #if CONFIG_NET_SEND_BUFSIZE > 0
@@ -735,8 +733,7 @@ ssize_t psock_udp_sendto(FAR struct socket *psock, FAR const void *buf,
     }
   else
     {
-          wrb = udp_wrbuffer_timedalloc(udp_send_gettimeout(start,
+      wrb = udp_wrbuffer_timedalloc(udp_send_gettimeout(start, timeout));
-                                                            timeout));
     }
 #endif
 
@@ -792,8 +789,7 @@ ssize_t psock_udp_sendto(FAR struct socket *psock, FAR const void *buf,
 
           addr6->sin6_family = AF_INET6;
           addr6->sin6_port   = conn->rport;
-              net_ipv6addr_copy(addr6->sin6_addr.s6_addr,
+          net_ipv6addr_copy(addr6->sin6_addr.s6_addr, conn->u.ipv6.raddr);
-                                conn->u.ipv6.raddr);
         }
 #endif /* CONFIG_NET_IPv6 */
     }
@@ -817,6 +813,8 @@ ssize_t psock_udp_sendto(FAR struct socket *psock, FAR const void *buf,
    * buffer space if the socket was opened non-blocking.
    */
 
+  if (len > 0)
+    {
       if (nonblock)
         {
           ret = iob_trycopyin(wrb->wb_iob, (FAR uint8_t *)buf,
@@ -845,6 +843,7 @@ ssize_t psock_udp_sendto(FAR struct socket *psock, FAR const void *buf,
         {
           goto errout_with_wrb;
         }
+    }
 
   /* Dump I/O buffer chain */
 
@@ -885,7 +884,6 @@ ssize_t psock_udp_sendto(FAR struct socket *psock, FAR const void *buf,
     }
 
   net_unlock();
-    }
 
   /* Return the number of bytes that will be sent */
 

net/udp/udp_sendto_unbuffered.c

@@ -197,6 +197,8 @@ static uint16_t sendto_eventhandler(FAR struct net_driver_s *dev,
         {
           /* Copy the user data into d_appdata and send it */
 
+          if (pstate->st_buflen > 0)
+            {
               int ret = devif_send(dev, pstate->st_buffer, pstate->st_buflen,
                                    udpip_hdrsize(pstate->st_conn));
               if (ret <= 0)
@@ -204,6 +206,20 @@ static uint16_t sendto_eventhandler(FAR struct net_driver_s *dev,
                   pstate->st_sndlen = ret;
                   goto end_wait;
                 }
+            }
+          else
+            {
+              if (netdev_iob_prepare(dev, false, 0) != OK)
+                {
+                  pstate->st_sndlen = -ENOMEM;
+                  goto end_wait;
+                }
+
+                iob_update_pktlen(dev->d_iob, udpip_hdrsize(pstate->st_conn),
+                                  false);
+                dev->d_sndlen = 0;
+                dev->d_len = dev->d_iob->io_pktlen;
+            }
 
 #ifdef NEED_IPDOMAIN_SUPPORT
           /* If both IPv4 and IPv6 support are enabled, then we will need to