c++ - 参数化自定义CMake工具链

Question

我正在编写用于交叉编译的CMake工具链文件。
我有几个工具链，它们之间的差别很小。我想为cmake创建一个描述所有工具链的文件，并让用户从命令行指定它们:cmake -DCMAKE_TOOLCHAIN_FILE=/path/to/toolchain.cmake -DTOOLCHAIN_NAME=<name>但是，似乎TOOLCHAIN_NAME有时变为空字符串。
这是工具链文件的示例:

set(tools /opt/toolchains/Custom/toolchains/toolchain)
set(sdk   /opt/toolchains/Custom/platforms/)

message(STATUS "toolchain_name ${TOOLCHAIN_NAME}")
if(NOT TOOLCHAIN_NAME) 
    message(SEND_ERROR "Please specify toolchain name in -DTOOLCHAIN_NAME parameter")
endif()

set(CMAKE_SYSTEM_NAME Generic)
set(CMAKE_SYSTEM_PROCESSOR ARM)

# further processing

然后我叫CMake:

mkdir build
cd build
cmake .. -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchain.cmake -DTOOLCHAIN_NAME=proc-os-gnueabi-gcc_6_3
  

并获得以下输出错误:

-- toolchain name proc-os-gnueabi-gcc_6_3
-- toolchain name proc-os-gnueabi-gcc_6_3
-- The C compiler identification is GNU 6.3.1
-- The CXX compiler identification is GNU 6.3.1
-- Check for working C compiler: /opt/toolchains/.../bin/arm-os-gnueabi-gcc
CMake Error at /home/user/project/cmake/toolchain.cmake:59 (message):
  Please specify toolchain name in -DTOOLCHAIN_NAME parameter

Call Stack (most recent call first):
  /home/user/project/build/CMakeFiles/3.15.4/CMakeSystem.cmake:6 (include)
  /home/user/project/build/CMakeFiles/CMakeTmp/CMakeLists.txt:2 (project)


CMake Error at /usr/local/share/cmake-3.15/Modules/CMakeTestCCompiler.cmake:44 (try_compile):
  Failed to configure test project build system.
Call Stack (most recent call first):
  CMakeLists.txt:15 (project)


-- Configuring incomplete, errors occurred!
See also "/home/user/project/build/CMakeFiles/CMakeOutput.log".
See also "/home/user/project/build/CMakeFiles/CMakeError.log".

CMakeOutput.log和CMakeError.log包含消息，表示编译器未找到某些库。
如果打开生成的文件 /home/user/project/build/CMakeFiles/3.15.4/CMakeSystem.cmake，可以看到以下内容

set(CMAKE_HOST_SYSTEM "Linux-4.2.0-27-generic")
set(CMAKE_HOST_SYSTEM_NAME "Linux")
set(CMAKE_HOST_SYSTEM_VERSION "4.2.0-27-generic")
set(CMAKE_HOST_SYSTEM_PROCESSOR "x86_64")

include("/home/user/project/cmake/toolchain.cmake")

set(CMAKE_SYSTEM "Generic")
set(CMAKE_SYSTEM_NAME "Generic")
set(CMAKE_SYSTEM_VERSION "")
set(CMAKE_SYSTEM_PROCESSOR "ARM")

set(CMAKE_CROSSCOMPILING "TRUE")

set(CMAKE_SYSTEM_LOADED 1)

根据CMake手册，其 -D参数用于指定缓存条目。
从上面的输出中可以得出结论，CMake至少处理了所提供的工具链文件3次。在前两次中，有适当的缓存条目，而在第三次运行中，它们不存在。
因此，如何避免工具链文件的代码重复？
UPD。阅读 this question之后，我尝试了CMake命令行参数的几种排列:在工具链之后但在选项之前指定源路径，在选项之后指定源路径，显式指定源路径并使用 -S和 -B选项构建路径。没有任何帮助。

Answer 1

I conclude from the output above that supplied toolchain file is processed by CMake at least 3 times. First two times there were proper cache entries, and at the third run they were absent.

是的，这是非常好的观察。从技术上讲，只有在 project()调用时才保证第一次工具链调用可以看到CACHE条目(使用 -D的 cmake参数或在 set()调用之前使用 project()命令创建)。
在不同工具链调用之间传递CACHE条目的一种方法是将它们存储在 环境变量中:
下面的示例使用 MY_TOOLCHAIN_NAME环境变量作为 TOOLCHAIN_NAME CMake变量的存储。

if(DEFINED ENV{MY_TOOLCHAIN_NAME})
    # Environment variable is set.
    if (TOOLCHAIN_NAME)
        # CMake variable is set too.
        # It is up to your which one to use.
        # Uncomment line below for prefer environment variable to CMake one.
        # set(TOOLCHAIN_NAME $ENV{MY_TOOLCHAIN_NAME})
    else ()
        # CMake variable is not set. Use environment one.
        set(TOOLCHAIN_NAME $ENV{MY_TOOLCHAIN_NAME})
    endif()
else()
    # Environment variable is not set.
    if (TOOLCHAIN_NAME)
        # But CMake variable is set.
        # Store it into the environment and use it.
        set(ENV{MY_TOOLCHAIN_NAME} ${TOOLCHAIN_NAME})
    else()
        # Neither environment nor CMake variable is set.
        message(SEND_ERROR "Please specify toolchain name in -DTOOLCHAIN_NAME parameter")
    endif()
endif()

如果您有多个“调整”变量，则可以为上述所有步骤编写一个宏，并为所需的每个变量应用(调用)此宏。