cmake 之 check_language()

1，check_language 的定义

定义出现在cmake 源代码的 Modules/文件夹下边：

例如：

cmake-3.29.2/Modules/CheckLanguage.cmake

大致具体内容如下：

include_guard(GLOBAL)

block(SCOPE_FOR POLICIES)
cmake_policy(SET CMP0126 NEW)

macro(check_language lang)
  if(NOT DEFINED CMAKE_${lang}_COMPILER)
    set(_desc "Looking for a ${lang} compiler")
    message(CHECK_START "${_desc}")
    message(STATUS "LLLLLLLL")
    file(REMOVE_RECURSE ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang})

    set(extra_compiler_variables)
    if("${lang}" MATCHES "^(CUDA|HIP)$" AND NOT CMAKE_GENERATOR MATCHES "Visual Studio")
      set(extra_compiler_variables "set(CMAKE_${lang}_HOST_COMPILER \\\"\${CMAKE_${lang}_HOST_COMPILER}\\\")")
    endif()

    if("${lang}" STREQUAL "HIP")
      list(APPEND extra_compiler_variables "set(CMAKE_${lang}_PLATFORM \\\"\${CMAKE_${lang}_PLATFORM}\\\")")
    endif()

    list(TRANSFORM extra_compiler_variables PREPEND "\"")
    list(TRANSFORM extra_compiler_variables APPEND "\\n\"")
    list(JOIN extra_compiler_variables "\n  " extra_compiler_variables)

    set(_cl_content
      "cmake_minimum_required(VERSION ${CMAKE_VERSION})
project(Check${lang} ${lang})
file(WRITE \"\${CMAKE_CURRENT_BINARY_DIR}/result.cmake\"
  \"set(CMAKE_${lang}_COMPILER \\\"\${CMAKE_${lang}_COMPILER}\\\")\\n\"
  ${extra_compiler_variables}
  )"
    )

    file(WRITE "${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang}/CMakeLists.txt"
      "${_cl_content}")
    if(CMAKE_GENERATOR_INSTANCE)
      set(_D_CMAKE_GENERATOR_INSTANCE "-DCMAKE_GENERATOR_INSTANCE:INTERNAL=${CMAKE_GENERATOR_INSTANCE}")
    else()
      set(_D_CMAKE_GENERATOR_INSTANCE "")
    endif()
    if(CMAKE_GENERATOR MATCHES "^(Xcode$|Green Hills MULTI$|Visual Studio)")
      set(_D_CMAKE_MAKE_PROGRAM "")
    else()
      set(_D_CMAKE_MAKE_PROGRAM "-DCMAKE_MAKE_PROGRAM:FILEPATH=${CMAKE_MAKE_PROGRAM}")
    endif()
    if(CMAKE_TOOLCHAIN_FILE)
      set(_D_CMAKE_TOOLCHAIN_FILE "-DCMAKE_TOOLCHAIN_FILE:FILEPATH=${CMAKE_TOOLCHAIN_FILE}")
    else()
      set(_D_CMAKE_TOOLCHAIN_FILE "")
    endif()
    if(CMAKE_${lang}_PLATFORM)
      set(_D_CMAKE_LANG_PLATFORM "-DCMAKE_${lang}_PLATFORM:STRING=${CMAKE_${lang}_PLATFORM}")
    else()
      set(_D_CMAKE_LANG_PLATFORM "")
    endif()
    execute_process(
      WORKING_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang}
      COMMAND ${CMAKE_COMMAND} . -G ${CMAKE_GENERATOR}
                                 -A "${CMAKE_GENERATOR_PLATFORM}"
                                 -T "${CMAKE_GENERATOR_TOOLSET}"
                                 ${_D_CMAKE_GENERATOR_INSTANCE}
                                 ${_D_CMAKE_MAKE_PROGRAM}
                                 ${_D_CMAKE_TOOLCHAIN_FILE}
                                 ${_D_CMAKE_LANG_PLATFORM}
      OUTPUT_VARIABLE _cl_output
      ERROR_VARIABLE _cl_output
      RESULT_VARIABLE _cl_result
      )
    include(${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/Check${lang}/result.cmake OPTIONAL)
    if(CMAKE_${lang}_COMPILER AND "${_cl_result}" STREQUAL "0")
      message(CONFIGURE_LOG
        "${_desc} passed with the following output:\n"
        "${_cl_output}\n")
      set(_CHECK_COMPILER_STATUS CHECK_PASS)
    else()
      set(CMAKE_${lang}_COMPILER NOTFOUND)
      set(_CHECK_COMPILER_STATUS CHECK_FAIL)
      message(CONFIGURE_LOG
        "${_desc} failed with the following output:\n"
        "${_cl_output}\n")
    endif()
    message(${_CHECK_COMPILER_STATUS} "${CMAKE_${lang}_COMPILER}")
    set(CMAKE_${lang}_COMPILER "${CMAKE_${lang}_COMPILER}" CACHE FILEPATH "${lang} compiler")
    mark_as_advanced(CMAKE_${lang}_COMPILER)

    if(CMAKE_${lang}_HOST_COMPILER)
      message(STATUS "Looking for a ${lang} host compiler - ${CMAKE_${lang}_HOST_COMPILER}")
      set(CMAKE_${lang}_HOST_COMPILER "${CMAKE_${lang}_HOST_COMPILER}" CACHE FILEPATH "${lang} host compiler")
      mark_as_advanced(CMAKE_${lang}_HOST_COMPILER)
    endif()

    if(CMAKE_${lang}_PLATFORM)
      set(CMAKE_${lang}_PLATFORM "${CMAKE_${lang}_PLATFORM}" CACHE STRING "${lang} platform")
      mark_as_advanced(CMAKE_${lang}_PLATFORM)
    endif()
  endif()
endmacro()

endblock()