First time using lwIP

It looks like you're trying to LWIP's "raw" and/or stand alone API. This is a _callback_-based API. You need to, at a bare minimum, read through the raw_api.txt file.

In addition, it makes sense to take a look at the TCP echo server, included with the distribution on this website, and use it as a basis for starting your own application. Once you have it working, you'll have much more luck stripping out what you don't feel you need.

Cheers,

- slacker