This document is by far the best out there on the net explaining step by step how argp works!
http://ftp.yzu.edu.tw/nongnu/argpbook/step-by-step-into-argp.pdf
Download actual file here
If you have ever wondered where to find the text “Give this help list”, it is contained in the file argp_parse.c
This file is in the argp folder in the package eglibc-2.19.
You can get hold of this package by running: apt-get source libc6.
All of this is within the glibc or ebligc binaries on your Ubuntu/Debian systems of course. But just in case you wondered.