How to instruct CMake to use the build architecture compiler

Question

When using CMake for cross compiling, one generally specifies a toolchain file via the CMAKE_TOOLCHAIN_FILE option. In GNU terminology, one can specify the host architecture toolset using this file. However, one can generally not expect to be able to execute anything built with this toolchain. So often enough, some build tools need to be compiled for the build architecture.

Consider the following setup. I have two source files genfoo.c and bar.c. During build, genfoo.c needs to be compiled and run. Its output needs to be written to foo.h. Then I can compile bar.c, which #include "foo.h". Since CMake defaults to using the host architecture toolchain, the instructions for bar.c are easy. But how do I tell it to use the build architecture toolchain for compiling genfoo.c? Simply saying add_executable(genfoo genfoo.c) will result in using the wrong compiler.

Answer 1

CMake can only handle one compiler at a time. So - if you don't go the long way to set up the other compiler as a new language - you will end up with two configuration cycles.

I see the following approaches to automate this process:

1. Taking the example "CMake Cross Compiling - Using executables in the build created during the build?" from the CMake pages as a starting point I'll get:

   CMakeLists.txt

   cmake_minimum_required(VERSION 3.0)
   project(FooBarTest)
   
   # When crosscompiling import the executable targets
   if (CMAKE_CROSSCOMPILING)
       set(IMPORT_PATH "IMPORTFILE-NOTFOUND" CACHE FILEPATH "Point it to the export file path from a native build")
       file(TO_CMAKE_PATH "${IMPORT_PATH}" IMPORT_PATH_CMAKE)
       include(${IMPORT_PATH_CMAKE}/genfooTargets.cmake)
   
       # Then use the target name as COMMAND, CMake >= 2.6 knows how to handle this
       add_custom_command(
           OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/foo.h
           COMMAND genfoo
       )
   
       add_executable(bar bar.cpp ${CMAKE_CURRENT_BINARY_DIR}/foo.h)
       target_include_directories(bar PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
   endif()
   
   # Only build the generator if not crosscompiling
   if (NOT CMAKE_CROSSCOMPILING)
       add_executable(genfoo genfoo.cpp)
       export(TARGETS genfoo FILE "${CMAKE_CURRENT_BINARY_DIR}/genfooTargets.cmake")
   endif()
   

   Then using a script like:

   build.sh

   #!/bin/bash
   
   if [ ! -d hostBuild ]; then
       cmake -E make_directory hostBuild
       cmake -E chdir hostBuild cmake ..
   fi
   cmake --build hostBuild
   
   if [ ! -d crossBuild ]; then
       cmake -E make_directory crossBuild
       cmake -E chdir crossBuild cmake .. -DIMPORT_PATH=${PWD}/hostBuild -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake
   fi
   cmake --build crossBuild
   

   I'll get the desired results by calling ./build.sh.

2. Splitting the CMakeLists.txt and maybe even replace the export()/include() with something where I know the output path of my build tools e.g. by using CMAKE_RUNTIME_OUTPUT_DIRECTORY would simplify things:

   CMakeLists.txt

   cmake_minimum_required(VERSION 3.0)
   project(FooBarTest)
   
   # Then use the target name as COMMAND. CMake >= 2.6 knows how to handle this
   add_custom_command(
       OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/foo.h
       COMMAND genfoo
   )
   
   add_executable(bar bar.cpp ${CMAKE_CURRENT_BINARY_DIR}/foo.h)
   target_include_directories(bar PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
   

   buildTools/CMakeLists.txt

   cmake_minimum_required(VERSION 3.0)
   project(BuildTools)
   
   add_executable(genfoo genfoo.cpp)
   

   build.sh

   #!/bin/bash
   
   if [ ! -d crossBuild ]; then
       cmake -E make_directory crossBuild
       cmake -E chdir crossBuild cmake .. -DCMAKE_TOOLCHAIN_FILE=toolchain.cmake
   fi
   if [ ! -d hostBuild ]; then
       cmake -E make_directory hostBuild
       cmake -E chdir hostBuild cmake ../buildTools -DCMAKE_RUNTIME_OUTPUT_DIRECTORY:PATH=${PWD}/crossBuild
   fi
   cmake --build hostBuild
   cmake --build crossBuild
   

References

• Making a CMake library accessible by other CMake packages automatically
• CMake build multiple targets in different build directories
• How do I make CMake output into a 'bin' dir?

Answer 2

It is possible to do that completely within CMake.

The trick is to run a separate CMake configuring stage within its own space, silently dismissing every crosscompiling setting and using the host's default toolchain, then import the generated outputs into it's parent, crosscompiling build.

First part:

set(host_tools_list wxrc generate_foo)

if(CMAKE_CROSSCOMPILING)
    # Pawn off the creation of the host utilities into its own dedicated space
    file(MAKE_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/host_tools)
    file(TO_NATIVE_PATH ${CMAKE_COMMAND} native_cmake_command)
    file(TO_NATIVE_PATH ${CMAKE_CURRENT_SOURCE_DIR} native_cmake_current_source_dir)
    execute_process(
        COMMAND "${native_cmake_command}" "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}" "${native_cmake_current_source_dir}"
        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/host_tools
    )

    add_custom_target(host_tools
        COMMAND ${CMAKE_COMMAND} --build . --target host_tools --config $<CONFIG>
        WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/host_tools
    )
    include(${CMAKE_CURRENT_BINARY_DIR}/host_tools/host_tools.cmake)

    foreach(tgt IN ITEMS ${host_tools_list})
        add_dependencies(host${tgt} host_tools)
    endforeach()

else()
    # Add an empty target, host tools are built inplace
    add_custom_target(host_tools
        DEPENDS ${host_tools_list}
    )
endif()

... then you add the usual add_executable and whatever ...

At the end:

if(NOT CMAKE_CROSSCOMPILING)
    foreach(tgt IN ITEMS ${host_tools_list})
        add_executable(host${tgt} ALIAS ${tgt})
    endforeach()

    export(TARGETS ${host_tools_list} NAMESPACE host FILE host_tools.cmake)
endif()

When it crosscompiles, it pawns off the creation of the host-run tools into its own dedicated space, and imports the targets as "hostwxrc" and "hostgenerate_foo", with a dependency on generating the host_tools themselves .

When it doesn't crosscompile, it builds wxrc and generate_foo as-is, and aliases them to hostwxrc and hostgenerate_foo.

After this, when you use $<TARGET_FILE:wxrc>, you refer to the wxrc built for the target platform, and $<TARGET_FILE:hostwxrc> refers to the wxrc built for the host platform, regardless whether they are the same or not.