[lwip-devel] [patch #7454] add support for pbuf as an argument to tcp_write()

[Matt Dittrich]

Follow-up Comment #1, patch #7454 (project lwip):

It seems that these patches are a bit premature, tcp_write() doesn't
correctly handle pbuf chains longer than pcb->mss.  I think splitting the
large pbuf into mss sized chunks would require new pbuf_split() and
mem_split() that do much of what pbuf_realloc() and mem_trim() do now, but
without discarding/freeing their remainders.  New nodes in the respective
lists would be inserted to maintain the remainder.  I think the split lengths
would also have to be restricted to respect alignment.  pbuf_split() would
fail when the "split point" of the chain lies within a PBUF_POOL.

Its not a trivial amount of code to get working (FYI: pbuf_realloc() and
mem_trim() occupy ~344 bytes on my -O2 Cortex-M3). Would this code be useful
in other parts of the stack (I am not familiar with the socket
implementation)? 

I had not encountered this in my original testing because I was  using it for
CGI response generation in httpserver_raw... the TCP_WRITE_FLAG_PBUF output
was always less than mss and at the beginning of a segment.  So as they stand,
the patches have limited usefulness.

Thanks,
MD

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