使用ExternalProject构建外部库时，如何正确指定链接目录？

原项目代码 #

include(ExternalProject)

set(EXTLIBS_INSTALL_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/extlibs)

ExternalProject_Add(ext-somelib
    URL https://github.com/.../archive/refs/tags/somelib.tar.gz
    CMAKE_ARGS
        -DCMAKE_INSTALL_PREFIX=${EXTLIBS_INSTALL_PREFIX}
        -DCMAKE_PREFIX_PATH=${EXTLIBS_INSTALL_PREFIX}
)

include_directories(${EXTLIBS_INSTALL_PREFIX}/include)

add_executable(mainTarget ...)

add_dependencies(mainTarget ext-somelib)

target_link_directories(mainTarget ${EXTLIBS_INSTALL_PREFIX}/lib)
target_link_libraries(mainTarget somelib)
# (link <build>/extlibs/lib/libsomelib.so)

触发的错误 #

CMake Error at CMakeLists.txt:# (target_link_directories):
target_link_directories called with invalid arguments

问题分析 #

直接报错的原因是target_link_directories语法不符合要求（CMake 3.13+版本强制要求指定PUBLIC/PRIVATE/INTERFACE关键字）；同时ExternalProject构建的依赖在配置阶段尚未生成，直接硬编码链接路径也会存在构建时序问题。

正确解决方案 #

方式一：使用导入目标（推荐，符合现代CMake规范） #

通过创建导入目标来管理外部库的依赖关系，能自动处理包含目录、链接路径和构建时序：

include(ExternalProject)

set(EXTLIBS_INSTALL_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/extlibs)

ExternalProject_Add(ext-somelib
    URL https://github.com/.../archive/refs/tags/somelib.tar.gz
    CMAKE_ARGS
        -DCMAKE_INSTALL_PREFIX=${EXTLIBS_INSTALL_PREFIX}
        -DCMAKE_PREFIX_PATH=${EXTLIBS_INSTALL_PREFIX}
)

# 创建导入目标表示外部库
add_library(somelib SHARED IMPORTED GLOBAL)
# 设置导入目标的包含目录
set_target_properties(somelib PROPERTIES
    INTERFACE_INCLUDE_DIRECTORIES ${EXTLIBS_INSTALL_PREFIX}/include
)
# 设置导入目标的库文件路径（配置阶段路径不存在不影响，构建阶段会生成）
set_target_properties(somelib PROPERTIES
    IMPORTED_LOCATION ${EXTLIBS_INSTALL_PREFIX}/lib/libsomelib.so
)
# 让导入目标依赖外部项目的构建
add_dependencies(somelib ext-somelib)

add_executable(mainTarget ...)
# 直接链接导入目标，CMake自动处理包含目录和链接路径
target_link_libraries(mainTarget PRIVATE somelib)

方式二：使用生成表达式延迟路径解析 #

如果不想创建导入目标，可通过生成表达式延迟路径解析，确保构建阶段才去查找目录：

include(ExternalProject)

set(EXTLIBS_INSTALL_PREFIX ${CMAKE_CURRENT_BINARY_DIR}/extlibs)

ExternalProject_Add(ext-somelib
    URL https://github.com/.../archive/refs/tags/somelib.tar.gz
    CMAKE_ARGS
        -DCMAKE_INSTALL_PREFIX=${EXTLIBS_INSTALL_PREFIX}
        -DCMAKE_PREFIX_PATH=${EXTLIBS_INSTALL_PREFIX}
)

add_executable(mainTarget ...)
# 用目标级包含目录替代全局include_directories，更规范
target_include_directories(mainTarget PRIVATE ${EXTLIBS_INSTALL_PREFIX}/include)

add_dependencies(mainTarget ext-somelib)

# 修正target_link_directories语法，添加PRIVATE关键字，并用生成表达式包裹路径
target_link_directories(mainTarget PRIVATE $<BUILD_INTERFACE:${EXTLIBS_INSTALL_PREFIX}/lib>)
target_link_libraries(mainTarget PRIVATE somelib)

关键要点 #

• 必须为target_link_directories指定PUBLIC/PRIVATE/INTERFACE关键字，这是CMake 3.13+的强制语法要求
• 导入目标方式能更清晰地管理依赖的传递性，是现代CMake的推荐写法
• 生成表达式$<BUILD_INTERFACE:...>会延迟到构建阶段解析路径，避免配置阶段因目录不存在引发的问题

内容的提问来源于stack exchange，提问作者fferri