CMake: Don't set rpath for a single library used in link

Question

What I'd like to do is configure my CMakeLists file so that while building my project the linker uses a copy of a shared library (.so) that resides in my build tree to link the executable against but then does not set the rpath in the linked executable so that the system must provide the library when the loader requests it.

Specifically, I want to link against libOpenCL.so during build time on a build farm that doesn't have libOpenCL.so installed as a system library. To do this, libOpenCL.so is in the project build tree and referenced using an absolute path in the CMakeLists file. This absolute path is to ensure that if the system does happen to have libOpenCL.so installed then it is not used.

However, when running the final executable, CMake has added the absolute path to the rpath which stops the system version of libOpenCL.so being picked up by the library loader and used.

Seems simple but I can't quite figure it out.

Thanks!

Answer 1

I know this answer is super late. I faced the same requirement as yours. Either we need is whitelist approach where we set CMAKE_BUILD_RPATH explicitly with what we need. Or we need a blacklist approach where we tell cmake, which RPATHs we don't want in the executable. Way to remove RPath from build tree is not documented yet: https://gitlab.kitware.com/cmake/cmake/issues/16825

The solution I took is:

Set RUNPATH instead of RPATH. You can achieve this by the statement:

SET(CMAKE_EXE_LINKER_FLAGS "-Wl,--enable-new-dtags")

When RUNPATH is present, RPATH is ignored. RUNPATH - same as RPATH, but searched after LD_LIBRARY_PATH, supported only on most recent UNIX

Then I can achieve the overriding the library using the environment variable LD_LIBRARY_PATH.