Re: [lwip-users] udp_send: space for IP header is not being allocated?

[David Baird]

On 7/13/07, Robin Iddon <address@hidden> wrote:
> I might be barking up the wrong tree here, but when you allocate the
> pbuf for your user data you pass in a flag that says "TRANSPORT" and
> this means that when the pbuf is allocated the  transport header space
> is pre-allocated.  Now, if you're using UDP raw interface, I am not sure
> in what way that is different from the normal udp_send().

Many thanks Robin.  But now I have another question: why does
``udp_send`` bother to allocate more pbufs.  It seems like that should
not be required since the space is already supposed to be
pre-allocated?

Sorry for stepping on anyone's toes by calling this a bug.

In order to figure this out I was looking at doc/rawapi.txt, reading
the lwIP source code, and looking at examples on the web.  Somehow I
missed PBUF_TRANSPORT detail.  I think that the doc/rawapi.txt file
could be improved by mentioning the proper way to allocate memory.
With that said, I've attached a patch :-)

-David

Index: doc/rawapi.txt
===================================================================
RCS file: /sources/lwip/lwip/doc/rawapi.txt,v
retrieving revision 1.7
diff -u -r1.7 rawapi.txt
--- doc/rawapi.txt      2 Mar 2007 19:35:15 -0000       1.7
+++ doc/rawapi.txt      13 Jul 2007 20:17:15 -0000
@@ -279,7 +279,11 @@

- err_t udp_send(struct udp_pcb *pcb, struct pbuf *p)

-  Sends the pbuf p. The pbuf is not deallocated.
+  Sends the pbuf p. The pbuf is not deallocated.  To allocate pbufs,
+  make sure to use the PBUF_TRANSPORT flag to ensure that enough space
+  is allocated for UDP and IP headers, e.g.:
+
+  p = pbuf_alloc(PBUF_TRANSPORT, num_bytes, PBUF_POOL);

- void udp_recv(struct udp_pcb *pcb,
                void (* recv)(void *arg, struct udp_pcb *upcb,