Re: [lwip-users] Issue with TCP raw API and messages larger than TCP_MSS

[Sergio R. Caprile]

Imagine your image data is 3MB. Would you expect lwIP to buffer that on 
a 32KB microcontroller ? No, would you ?
Right, that is, light-weight IP.

The server will send bytes as it wants.
TCP will send bytes as it wants.
They will be put in IP and sent via Ethernet
Each frame gets IP extracted, gets TCP extracted, gets your callback 
called. You get a pbuf.
You will get bytes as the combination of the server, TCP and the net 
wills, plus, those bytes will be in a pbuf that can itself be a chain of 
smaller pbufs, depending on the fragmentation of your memory.

You detect the connection has been closed.
You detect content-length has been transferred.
You are the client.
You extract data from your pbuf with pbuf_copy_partial() and copy it to 
wherever you want.

Do you want all the data in a single pbuf ? Chain them ! The httpd 
server is a nice example of how to do it. Basically:  pbuf_cat(bigpbuf, 
newpbuf); However, since pbufs can be themselves chain of pbufs, I 
wouldn't do that...
If you need the whole image in one piece for processing or whatever, you 
should copy each pbuf to your buffer with pbuf_copy_partial(). Even 
though you can chain all pbufs and access them later with 
pbuf_copy_partial(), I wouldn't dare to write an image processing 
algorithm doing its job this way.