[lwip-users] Re: [lwip] UDP questions

[Adam Dunkels]

Horst,

On Thursday 13 December 2001 12.50, you wrote:
> netbuf_ref() sets buf->p->payload to a ROM address, but your
> udp_send() calls pbuf_header(), which assumes that payload
> is a address behind buf->p (it tries to decrement payload
> by UDP_HLEN to insert a udp header);
>
> I'm wrong? Is it impossible sendung UDP datagrams from
> ROM without copy?

You are right, the code is wrong :-) What UDP really should do when 
pbuf_header() fails is to allocate an extra pbuf and chain it in front of the 
pbuf containing the data. 

The code for doing this looks like this:

  struct pbuf *q;
  
  if(pbuf_header(p, UDP_HLEN)) {
    q = pbuf_alloc(PBUF_IP, UDP_HLEN, PBUF_RAM);
    if(q == NULL) {
      return ERR_MEM;
    }
    pbuf_chain(q, p);
    p = q;
  }

(This is in the beginning of the udp_send() function.)

Thanks for pointing this out!

> Another question:
>
> Is it possible to send broadcast UDP datagrams to 255.255.255.255?
> As understand your code, udp_send() calls ip_route(), which allows
> subnet-broadcasts only?

ip_route() returns the default interface when it cannot find a matching 
interface, so sending to 255.255.255.255 works. The packet comes out on the 
default interface.

/adam
-- 
Adam Dunkels <address@hidden>
http://www.sics.se/~adam
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