使用libwebsockets+mbedtls构建项目遇CMake导出集错误求助

使用FetchContent拉取libwebsockets和mbedtls构建项目，配置阶段触发以下CMake错误：

CMake Error in build/_deps/libwebsockets-src/CMakeLists.txt:
  export called with target "websockets" which requires target "mbedtls" that
  is not in any export set.


CMake Error in build/_deps/libwebsockets-src/CMakeLists.txt:
  export called with target "websockets" which requires target "mbedcrypto" that
  is not in any export set.


CMake Error in build/_deps/libwebsockets-src/CMakeLists.txt:
  export called with target "websockets" which requires target "mbedx509" that
  is not in any export set.

确认mbedtls已将相关目标导出到MbedTLSTargets集合：

set(target_libraries ${mbedcrypto_target} ${mbedx509_target} ${mbedtls_target})
# ...
foreach(target IN LISTS target_libraries)
    # ...
    install(
        TARGETS ${target}
        EXPORT MbedTLSTargets
        ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
        LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
        RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
        PERMISSIONS OWNER_READ OWNER_WRITE GROUP_READ WORLD_READ)
endforeach(target)

疑问：为何导出信息未传递给libwebsockets？是否需要在主项目重新声明目标导出规则？

不需要在主项目重新声明导出规则，问题根源是依赖拉取顺序和导出集的可见性问题，可通过以下几种方式解决：

• 调整FetchContent的拉取顺序
  确保先完成mbedtls的拉取和配置，再处理libwebsockets。CMake中FetchContent的处理顺序会影响目标的可见性，修改主项目的CMake代码：

  # 先拉取并配置mbedtls
  FetchContent_Declare(
      mbedtls
      GIT_REPOSITORY https://github.com/ARMmbed/mbedtls.git
      GIT_TAG v2.28.0 # 替换为你需要的版本
  )
  FetchContent_MakeAvailable(mbedtls)
  
  # 再拉取并配置libwebsockets
  FetchContent_Declare(
      libwebsockets
      GIT_REPOSITORY https://github.com/warmcat/libwebsockets.git
      GIT_TAG v4.3.2 # 替换为你需要的版本
  )
  FetchContent_MakeAvailable(libwebsockets)
  

  使用FetchContent_MakeAvailable会自动处理GetProperties和add_subdirectory，确保mbedtls的目标在libwebsockets配置前已完全初始化。

• 显式导入mbedtls的导出集
  如果调整顺序后仍有问题，在主项目中，在添加libwebsockets子目录前，显式导入mbedtls的导出文件：

  # 导入mbedtls的导出目标
  include(${mbedtls_BINARY_DIR}/MbedTLSTargets.cmake)
  # 再添加libwebsockets子目录
  add_subdirectory(${libwebsockets_SOURCE_DIR})
  

  这会确保mbedtls的目标被CMake识别并纳入全局导出上下文。

• 禁用libwebsockets的自动导出
  如果不需要导出libwebsockets的目标到外部项目，可在配置libwebsockets前设置LIBWEBSOCKETS_NO_EXPORT变量，避免触发export命令：

  set(LIBWEBSOCKETS_NO_EXPORT ON CACHE BOOL "" FORCE)
  FetchContent_MakeAvailable(libwebsockets)
  

CMake的export命令要求目标的所有依赖项都必须属于某个导出集。当libwebsockets执行export时，mbedtls的目标虽已被安装到MbedTLSTargets，但如果这些目标未在当前CMake上下文的导出集中可见，就会触发错误。调整拉取顺序或显式导入导出集，能让CMake识别到mbedtls目标的导出属性，从而解决依赖检查失败的问题。

内容的提问来源于stack exchange，提问作者glades