Hi there,
Bit late with this for Marchintosh but I've started a hobby project to build a p2p chat app in C that runs on modern OS's and Classic Mac System 7 with MacTCP. The POSIX version is more advanced in terms of features but I have managed to get the classic Mac version to do UDP...
This can help you get quickly set up for Macintosh C programming, targeting a wide range of macs (System 7.5.5 all the way down to the earliest macs around System 1).
The included resources are:
The compiler/linker/builder of choice is Symantec THINK C v6.0.
Resource editor done with ResEdit...