[lwip-devel] [bug #11400] ARP multi-packet-queue modifies TCP unsent/unacked segment packet pbuf chain into packet queue

[Jonathan Larmour]

Follow-up Comment #42, bug #11400 (project lwip):

Re copying PBUF_RAMs: that's what I was discussing in comment #35 (and was
what I thought you had raised in comment #34). As far as I know, users are
allowed to reuse data including pbufs without needing to check their ref
count. That's why there's a "copy" flag to netconn_write/tcp_write, for
example. I assume UDP or other protocols (ICMP, and whatever other packets
artificially constructed by the user etc.) are analogous. Although at the
same time, we don't need to copy a PBUF_RAM if it's part of a TCP seg - we
know that is controlled by lwIP. Is there any way we can detect and avoid a
pbuf_copy for this?

For UDP and other protocols, we theoretically could change the API contract
here and say that users can't reuse pbufs (or their payload) until their ref
count returns to 1 (from a higher value set within the stack). I don't think
that's right personally, but it's Kieran's call really.


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