This is an interesting article, but the "Portability" comments could be a lot more useful: strndup and open_memstream are both "POSIX 2008", but strndup can be used on OS X while open_memstream cannot.
I deliberately didn't write that to avoid the page going stale when OS X adds it. But OS X is behind the times and that's harmful. POSIX 2008 has been out for years and most of the lacking features are trivial to add. They're being actively harmful to Unix software by not having modern interfaces, forcing portable software to be worse. The purpose of the article is to highlight the interfaces, and when they're suited, rather than being a replacement for your system manual page or a portability guide. Since OS X isn't a free software Unix (though its libc is), I don't really consider it among the relevant modern Unix systems. Linux, the BSDs, and so on all have the POSIX 2008 features mentioned here.