I build an executable app that uses liba. liba in turn needs libb. libb in turn needs libc. The dependency looks like this: app -> liba -> libb -> libc. The app and liba are built by me while libb and libc are 3rd party. I do not know how they were compiled, but I know that they both are in the same folder and I have a CMake config to find libb.
A setup that is not working.
In CMake I find the package for libb, publically link the lib to liba and finally link liba to app. The app compiles fine but running it I'm getting: error while loading shared libraries: libc: cannot open shared object file: No such file or directory. I.e. the last library, which symbols I don't even use directly can not be found.
I inspect my library liba with readelf -d liba | grep PATH:
...
0x000000000000001d (RUNPATH) Library runpath: [/path/to/dir/with/both/libb/and/libc:]
...
So the path is inside my liba, it's correct and points to both libb and libc.
Next, I inspect the loading search paths with LD_DEBUG=libs ./app:
...
27012: find library=libb [0]; searching
27012: search path=/*a lot of paths INCLUDING the needed path*/ (RUNPATH from file ./app)
...
27012: find library=libc [0]; searching
27012: search cache=/etc/ld.so.cache
27012: search path=/*a lot of system paths, but the needed path is MISSING*/ (system search path)
27012: trying file=/lib/x86_64/libc
many other tries
...
How can that be that libb is found in RUNPATH but libc not???! Why the libc uses system search path instead of RUNPATH??
A setup that is working
Next, I try to use a RPATH when build my library liba by setting -Wl,--disable-new-dtags flag. Now readelf -d liba | grep PATH prints me the same path as before, but abbreviated with RPATH instead of RUNPATH and LD_DEBUG=libs ./app prints:
...
27012: find library=libb [0]; searching
27012: search path=/*a lot of paths INCLUDING the needed path*/ (RUNPATH from file ./app)
...
27012: find library=libc [0]; searching
27012: search path=/*a lot of paths INCLUDING the needed path*/ (RUNPATH from file ./app)
...
Now both libraries use RUNPATH but I can't understand why. Why the hell they use RUNPATH though the lib was compiled with explicit RPATH?? Where does this come from?
Update:
Here by [libc] I mean my 3rd party library (a, b, c), not the system one.
readelf -d libb | grep NEEDED
0x0000000000000001 (NEEDED) Shared library: [libboost_thread.so.1.65.1]
0x0000000000000001 (NEEDED) Shared library: [libboost_system.so.1.65.1]
0x0000000000000001 (NEEDED) Shared library: [libc]
0x0000000000000001 (NEEDED) Shared library: [libpthread.so.0]
0x0000000000000001 (NEEDED) Shared library: [libstdc++.so.6]
0x0000000000000001 (NEEDED) Shared library: [libm.so.6]
0x0000000000000001 (NEEDED) Shared library: [libgcc_s.so.1]
0x0000000000000001 (NEEDED) Shared library: [libc.so.6]
0x0000000000000001 (NEEDED) Shared library: [ld-linux-x86-64.so.2]
Update 2:
After more reading, I was able to formulate the search request more precisely and immediately found the explanation. For example, here:
