使用CMake仅构建一次外部库 [英] Build external library only once with CMake

问题描述

我的C ++项目包含第三方库的源代码（当前作为git子模块）。

My C++ projects includes the source code of a third-party library (currently as a git submodule).

此库是由我们的主要CMakelist添加到项目中的通过使用 add_subdirectory ，然后将该库与主要目标链接。

This library is added to the project by our main CMakelists through the use of add_subdirectory, and then the library is linked with the main target.

以下是简化版本我当前的Cmake文件中的文件：

Here is a reduced version of my current Cmake file :

add_subdirectory(foo)
set(FOO_LIBRARY ${CMAKE_CURRENT_SOURCE_DIR}/libfoo/libfoo.so)

add_executable(target main.cpp)
add_dependencies(target foo)
target_link_libraries(target ${FOO_LIBRARY})

该库需要很长时间才能构建，并且由于我不更改其代码，因此只需要构建一次（每个构建配置）。但是，当我清理并重建代码时，也会清理库文件并重新编译它们。

This library takes a long time to build and, since I don't change its code I need it built only once (per build configuration). But when I clean and rebuild my code it also cleans the library files and recompile them.

我试图设置属性 CLEAN_NO_CUSTOM 在库目录中，但根据文档仅适用于自定义命令目标。

I have tried to set the property CLEAN_NO_CUSTOM in the library's directory, but according to the documentation it only works for custom command targets.

CMake中是否存在一种机制，可以通过该机制指定仅一次生成该库目标，或者替代地不被清理吗？

Is there a mechanism in CMake through which it is possible to specify that this library target needs to be generated only once, or alternatively not cleaned by make clean ?

推荐答案

正如@Tsyvarev所说，在您的案例 ExternalProject_Add 比 add_subdirectory 更好。当您希望项目成为构建系统的重要组成部分时， add_subdirectory 很好，因为它创建的目标可以在 target_link_libraries（）命令，而 ExternalProject_Add 创建的目标则不能。

As @Tsyvarev said, in your case ExternalProject_Add is better than add_subdirectory. add_subdirectory is good when you want project to be essential part of your build system because target it creates can be used in in the right-hand-side of the target_link_libraries() command while target created by ExternalProject_Add cannot.

我在一个项目中使用的方法。您尝试找到所需的库并仅在找不到时才构建它。我使用INTERFACE库将FOO_EXTERNAL转换为 target_link_libraries（）可接受的目标。

Here is the approach I used in one of my projects. You try to find required library and build it only if it was not found. I use INTERFACE library to turn FOO_EXTERNAL into a target acceptable by target_link_libraries().

add_library(foo INTERFACE)
find_package(foo ${FOO_VER})
if(NOT foo_FOUND)
    include(ExternalProject)
    include(GNUInstallDirs)
    ExternalProject_Add(FOO_EXTERNAL
                    SOURCE_DIR "${FOO_SOURCE_DIR}"
                    BINARY_DIR "${FOO_BINARY_DIR}"
                    INSTALL_DIR "${FOO_INSTALL_DIR}"
                    CMAKE_ARGS "-DCMAKE_C_FLAGS=${CMAKE_C_FLAGS}"
                               "-DCMAKE_CXX_FLAGS=${CMAKE_CXX_FLAGS}"
                               "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"

                               "-DCMAKE_INSTALL_PREFIX=${FOO_INSTALL_DIR}"
                    )

    add_dependencies(foo FOO_EXTERNAL)
    set(foo_LIBRARY
            "${FOO_INSTALL_DIR}/${CMAKE_INSTALL_LIBDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}foo${CMAKE_STATIC_LIBRARY_SUFFIX}")
    set(foo_INCLUDE_DIR "${FOO_INSTALL_DIR}/include")
endif()

target_link_libraries(foo INTERFACE ${foo_LIBRARY})
target_include_directories(foo INTERFACE ${foo_INCLUDE_DIR})

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