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cross compiling - How to instruct CMake to use the build architecture compiler

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.

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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


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