Quoting Simon Hobson (linux@???):

> I think what he means, is that it allows devs/packagers to insert
> these calls and still have something that runs when systemd itself
> isn't installed. Not that the lib itself installs such calls.

First, thanks. (I seriously wasn't trying to ignore anyone's
contributions, FWIW. I merely saw a conversation that seemed
ill-omened, hence sought to gracefully exit it.)

Second, I'd be curious to see real-world specific examples of
whatever-this-is.

> With a lib, is there any code or is it *JUST* a set of symbols ?

Generically, a library can do just about anything, potentially. A
library is simply a set of compiled program routines that the get called
by executables (call to function dlopen) that invoke particular
functions within the libraries, and access symbols (call to function
dlsym). To know what's inside a _specific_ library, you need to look at
that library's source code.

This is a pretty good introduction to how libraries work and what they
can contain:
http://www.skyfree.org/linux/references/ELF_Format.pdf