Re: [lwip-users] Writing a proxy server - how "non-reentrant" is lwip raw API?

[address@hidden]

wurzel wrote:
> A followup to this discussion; after further debugging I did find
> the cause of my "flaky behavior" and it was because the call to
> tcp_write above needs to have TCP_WRITE_FLAG_COPY set.
>
> I was optimistically hoping that since I called tcp_write and
> tcp_output() from my receive callback, that the transmit side
> would have what it needed by the time my receive callback exited.
> However, that doesn't appear to be the case; my packets were definitely
> coming out with some bytes overwritten until I added that flag.

The reason for this is that without the copy flag, you tell TCP to 
enqueue the data from original input pbufs for sending. This *can* work 
only if you postpone freeing the original pbuf (the one passed to your 
recv callback) until the copy you sent is ACKed. This would mean you 
have to hold a pointer to the original pbuf on a list and then free it 
in your sent-callback according to the number of bytes passed in to that 
callback.

However, while the no-copy version saves some memcpy() calls, it bears 
the risk of running out of PBUF_POOL pbufs: These are normally only used 
for RX packets - you would now use them for TX, too. Since they then 
stay allocated until the remote side ACKs them, you need more POOL pbufs 
than you would need when copying data (where data is copied to PBUF_RAM 
pbufs allocated on the mem-heap).

Simon