[lwip-devel] [bug #49914] lwip_sendmsg uses PBUF_REF pbufs

[Joel Cunningham]

Follow-up Comment #3, bug #49914 (project lwip):

Good point on the PBUF_RAM which is normally in a chain as a header.  I think
for writing a netif that did asynchronous output, it would be a good practice
to copy the entire chain and not just any PBUF_REF pbufs. We've discussed this
before, but a TCP segment is still referenced after output.  During
retransmission, the TCP header (contained in a PBUF_RAM) is modified without
checking the reference count.  Given this behavior, it seems unsafe for a
netif to hold a reference to a pbuf after the output function has completed. 
Maybe any netif developers that have written asynchronous output can share
their approach.

I would prefer to preserve the current PBUF_REF behavior, but I think more
importantly, the UDP sendmsg and sendto paths should behave the same.  So if
using PBUF_REFs in this manner is no longer considered a desirable use case, I
would encourage also modifying lwip_sendto

Simon and I did discuss this issue in the original bug (I should have used a
patch/task :P) for adding sendmsg support and the conclusion was that since
sendmsg followed the same practice as sendto, it was ok.

https://savannah.nongnu.org/bugs/?func=detailitem&item_id=44805

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