Linking dependencies of a shared library

Question

I was working with SFML, I compiled a little test program and added the linkage option -lsfml-audio. Then, I used ldd ./program to see the dynamic libraries it was linking to. Surprisingly, there were a lot, none of them had I manually selected in my makefile, nor using pkg-config --libs.

I started reading about shared libraries, and made a little example to solve my doubts. However, I have this question:

why some libraries need you to add the dependencies in your makefile (either manually or using a script like pkg-config) and other libraries automatically link their dependencies?

When you're creating your dynamic library, is just as easy as adding the proper -ldependency options in the g++ -shared ... command to avoid the user the hassle of manually adding the dependencies later on. Why many of the available libraries don't do that?

I guess it must be related to the ability of fine tuning which libraries get linked and such.

Answer 1

Shared libraries will generally link in their dependencies. However, static libraries are not capable of doing so. pkg-config --libs often includes all dependencies (direct and indirect) so that you can switch to static compilation by simply adding -static without having to add additional library dependencies as well.

Note that these excess direct dependencies are considered unwanted in some cases (eg, debian tries to avoid them in packaged binaries, as they make library soname transitions more traumatic than necessary). You can instruct the linker to strip direct dependencies from the final executable that aren't needed with the -Wl,--as-needed flag.