CMake - Changing shared library link options - c++

Question

I want to manipulate the linker call in my linker trampoline python script, but for the life of me I cannot get CMake to respect the CMAKE_CXX_CREATE_SHARED_LIBRARY value. It always uses the CMAKE_CXX_COMPILER for linking. If I switch to an add_executable and instead use CMAKE_CXX_LINK_EXECUTABLE this works perfectly. Is there anyway to convince CMake to respect the setting of CMAKE_CXX_CREATE_SHARED_LIBRARY?
A trivial CMakeLists.txt
cmake_minimum_required( VERSION 3.12 )
project( mylib )
include( CMakePrintSystemInformation )
add_library( mylib SHARED mylib.cpp )
A toolchain excerpt
I've tried this in both a toolchain file and an -C pre-cache option.
set( CMAKE_LINKER "/path/to/linker_trampoline.py" )
set( CMAKE_CXX_CREATE_SHARED_LIBRARY "<CMAKE_LINKER> --CMAKE_CXX_COMPILER <CMAKE_CXX_COMPILER> --CMAKE_SHARED_LIBRARY_CXX_FLAGS <CMAKE_SHARED_LIBRARY_CXX_FLAGS> --LANGUAGE_COMPILE_FLAGS <LANGUAGE_COMPILE_FLAGS> --LINK_FLAGS <LINK_FLAGS> --CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS <CMAKE_SHARED_LIBRARY_CREATE_CXX_FLAGS> --SONAME_FLAG <SONAME_FLAG> --TARGET_SONAME <TARGET_SONAME> --TARGET <TARGET> --OBJECTS <OBJECTS> --LINK_LIBRARIES <LINK_LIBRARIES>" CACHE STRING "" FORCE )

Answer 1

Works as expected with CMake version 3.10
I have tried your example. No issues with your intention. The only differences are, I tried this with an older version of CMake and placed the variable declarations directly in the file CMakeLists.txt (Should be placed in a toolchain file).
CMakeLists.txt:
cmake_minimum_required(VERSION 3.10)
project(mylib)
include(CMakePrintSystemInformation)
set (CMAKE_LINKER "/path/to/link.sh")
set (CMAKE_CXX_CREATE_SHARED_LIBRARY "<CMAKE_LINKER> <OBJECTS>")
add_library(mylib SHARED mylib.cpp)
I have shortened the declaration of CMAKE_CXX_CREATE_SHARED_LIBRARY here because I only pass the object files to the following script.
link.sh: (Simple print the object files):
#!/bin/bash
echo "$#"
Output:
$ cmake CMakeLists.txt
$ make
[ 50%] Building CXX object CMakeFiles/mylib.dir/mylib.cpp.o
[100%] Linking CXX shared library libmylib.so
CMakeFiles/mylib.dir/mylib.cpp.o
[100%] Built target mylib
$
link.sh will be definitely executed. You an ignore the formal message about linking libmylib.so. The library will be never created (with my linker command).
CMake version:
$ cmake --version
cmake version 3.10.2
CMake suite maintained and supported by Kitware (kitware.com/cmake).
$
I have seen you have already created an issue or rather a feature request on the CMake project site.

Answer 2

Response from Kitware
Brad King commented:
CMAKE_CXX_LINK_EXECUTABLE and CMAKE_CXX_CREATE_SHARED_LIBRARY are not meant to be set by toolchain files directly (and if either seems to work it is only by accident). They are not meant to be cached either. They are set in CMake's platform/compiler information modules which are loaded after the toolchain file. For toolchain files meant to cross-compile to a custom platform, they can set CMAKE_SYSTEM_NAME to the name of the target platform, set CMAKE_MODULE_PATH to a custom directory, and then add Platform/<os>*.cmake modules with such settings for that platform.
To hook in to the link line for an existing platform another approach will be needed. #18316 is related.