Re: [gentoo-user] ...if linking a completly unused library...

[Michael Orlitzky]

On 10/02/2016 01:18 AM, Meino.Cramer@×××.de wrote:
>
> One curious question remains:
> If -as-needed is included, all libs will be "delinked" :), which
> are not used...so I am throwing away stuff, which no code calls:
> WHY should this cause trouble?
> Or is it again oversimplified by me? :) ;) 8)
>

It shouldn't cause problems, but it can. In most cases it's a bug in the
build system, but some software is designed to be underlinked.

Suppose I write a program called "foo" and a library that it uses called
"libfoo". If the libfoo library uses OpenSSL and the main executable has
-lssl in its link command, everything will work fine by default. But, if
you add --as-needed to your linker flags and don't specifically link
libfoo against OpenSSL, the result will be underlinked.

For an example where the underlinking is intended, the GNU Scientific
Library (gsl) allows you to swap out linear algebra backends. The gsl
developers intend for you to pick one such backend, and then link
everything together in your executable. Thus, they ship gsl without
linking it against any particular linear algebra library. It won't work
with --as-needed out of the box (we patch it in Gentoo).

We added --as-needed by default in Gentoo a while ago, and you can find
a lot of info from that time when it caused problems:

https://wiki.gentoo.org/wiki/Project:Quality_Assurance/As-needed

https://blog.flameeyes.eu/2008/11/relationship-between-as-needed-and-no-undefined-part-1-what-do-they-do

https://blog.flameeyes.eu/2008/11/misguided-link-and-as-needed

https://blog.flameeyes.eu/2008/11/problems-and-mitigation-strategies-for-as-needed

https://blog.flameeyes.eu/2009/02/a-softer-as-needed

https://blog.flameeyes.eu/2009/07/does-as-needed-link-make-software-faster