Setting RPATH of Static Library in Executable - cmake

Question

I have the following setup for my CMake project:
add_executable(exeA ${SOURCES})
target_link_libraries(exeA PRIVATE libB)
libB is a static library built separately in a different project and depends on shared libraries libC OR libD which provide equivalent API's but implemented differently and have different performance profiles.
In exeA's build I want to be able to link against either of libC or libD depending on a condition.
From reading RPATH handling, it seems I could use RPATH related CMake properties, however its unclear if I can use these properties to set the RPATH of an existing static library
Unfortunately there is no easy and fast way to change the RPATH of an
existing executable or shared library.
Is something like this possible to do in CMake?
Appreciate any recommendations to handle this use case.

Answer 1

Create a STATIC IMPORTED library for libB and SHARED IMPORTED libraries for libC and libD. Put the relevant one in the INTERFACE_LINK_LIBRARIES property of libB. CMake will set the RPATH correctly when exeA is built.
cmake_minimum_required(VERSION 3.23)
project(test)
# --------------------------------------------------
# The following code should probably be factored out
# into a proper Find module.
find_library(LIBB_LIBRARY NAMES B REQUIRED)
find_library(LIBC_LIBRARY NAMES C REQUIRED)
find_library(LIBD_LIBRARY NAMES D REQUIRED)
add_library(third-party::libC SHARED IMPORTED)
set_target_properties(
third-party::libC
PROPERTIES
IMPORTED_LOCATION "${LIBC_LIBRARY}"
)
add_library(third-party::libD SHARED IMPORTED)
set_target_properties(
third-party::libD
PROPERTIES
IMPORTED_LOCATION "${LIBD_LIBRARY}"
)
add_library(third-party::libB STATIC IMPORTED)
set_target_properties(
third-party::libB
PROPERTIES
IMPORTED_LOCATION "${LIBB_LIBRARY}"
)
option(LIBB_USES_LIBC "dummy option for demo" ON)
if (LIBB_USES_LIBC)
target_link_libraries(third-party::libB INTERFACE third-party::libC)
else ()
target_link_libraries(third-party::libB INTERFACE third-party::libD)
endif ()
# --------------------------------------------------
# --------------------------------------------------
# Project code
add_executable(exeA main.cpp)
target_link_libraries(exeA PRIVATE third-party::libB)
You can test this out with dummy files like so:
$ ls
CMakeLists.txt
$ mkdir -p prefix{1,2}/lib
$ touch prefix1/lib/lib{C,D}.so prefix2/lib/libB.a main.cpp
$ cmake -G Ninja -S . -B build "-DCMAKE_PREFIX_PATH=$PWD/prefix1;$PWD/prefix2"
-- The C compiler identification is GNU 9.4.0
-- The CXX compiler identification is GNU 9.4.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/alex/test/build
$ cmake --build build -- -nv # dry run (n), verbose (v)
[1/2] /usr/bin/c++ -MD -MT CMakeFiles/exeA.dir/main.cpp.o -MF CMakeFiles/exeA.dir/main.cpp.o.d -o CMakeFiles/exeA.dir/main.cpp.o -c /home/alex/test/main.cpp
[2/2] : && /usr/bin/c++ CMakeFiles/exeA.dir/main.cpp.o -o exeA -Wl,-rpath,/home/alex/test/prefix1/lib ../prefix2/lib/libB.a ../prefix1/lib/libC.so && :
As you can see, the RPATH is set correctly to the directory containing libC.so.