Re: [lwip-users] etharp_output: could not allocate room for header.

[Simon Goldschmidt]

> In the udp_send() of lwip 1.3, when there is not enought space to add an 
> UDP header, we allocated the pbuf with:
> 
> p = pbuf_alloc(PBUF_IP, UDP_HLEN, PBUF_RAM)

This is wrong. A pbuf given to udp_send must be allocated with PBUF_TRANSPORT! 
This should look like:

p = pbuf_alloc(PBUF_TRANSPORT, <your_data_len>, PBUF_RAM)

This makes sure the pbuf has enough room for tcp or udp headers + IP headers 
and ETH headers. Also, before giving it to udp_send, you can put as much as 
<your_data_len> into the pbuf (from the application view).
You would only use PBUF_IP if you don't use UDP, TCP, ICMP or any other 
transport layer protocol (or do it yourself) - but that's really rare!

> 
> And in the previous version, we did:
> 
> p = pbuf_alloc(PBUF_TRANSPORT, UDP_HLEN + 16, PBUF_RAM)

You don't need to allocate space for UDP_HLEN in your app, this is implicitly 
taken care of by the PBUF_TRANSPORT flag. 16 would be the amount of data you 
are allowed to save into the pbuf yourself, here.


Simon

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