Why are link default library dirs different from runtime default dirs?

Question

I'm fairly new to Linux and Arch in particular. I've been trying to get around most issues by myself and I've been successful in general. In fact, I'm writing not so much because of a problem, but to better understand why the solution I found works, and if it is the right solution for my case.
So, I downloaded the source for a library I wanted to use (SFML if you're curious), built it and installed it. Looking around the net it seemed that the proper place to install things you compile yourself is /usr/local, so the libs went to /usr/local/lib. So far so good.
I then made a quick test application to make sure everything was working properly (source can be seen here: http://www.sfml-dev.org/tutorials/2.1/start-linux.php). It built correctly, but when I tried to run it I got an error that the library could not be found. So looking the error up in google, I found people saying you had to set the $LD_LIBRARY_PATH to the lib directory. In fact, at the end of the SFML tutorial it suggests this as well, but only if you install in a non-standard directory.
I found it odd that I should specify this variable. Had it been unset by something I did? Should I have set it before? I hadn't had to specify the directory when I built the program, which made it seem as though /usr/local/lib was a default directory. So why wasn't the library being found?
A bit more research uncovered that one should not in fact rely on $LD_LIBRARY_PATH for this, but instead you should specify it when building with -rpathDIR, so that the executable itself knows where to look. And indeed, setting this option when building fixed the issue.
My question then, is: why do I have to specify the library path for runtime, but not when building? Is it normal for usr/local/lib to not be searched, or did I break something in my install? Is GCC just trying to be helpful and tripping me up by looking in places that are reasonable for libraries to be, but that the system itself won't search?
Last edited by jp.garibotti (2014-02-09 20:14:54)

Answer 1

Well, after reading and getting to know the AUR a bit, at least I know why /usr/local/lib wasn't searched.
So, if I got it right, I have to use the AUR to download the build script to build a package. Then I use pacman to install that package. This sends the relevant binaries to where they need to go, which is where the system will look for them.
So as long as I stay inside the build system, everything will go fine.
If I don't build shared libraries myself, and limit myself to running my own executables, I shouldn't need to worry about the build system beyond this. If I ever want to send something I did to someone else, however, I need to create a package for them to build, which will have to work with whatever distribution package manager they have. Assuming I want to keep everything tidy.
There is one more thing I'm not clear on, though. So I installed the packages, headers are in /usr/include, but where should I place the sources? If I want to debug something and follow a call into the library, for example?