链接本地JSONCpp安装与CMake和ExternalProject_Add

Question

我想使用CMake来建立一个使用JSONCpp的项目。我想下载JSONCpp，在本地编译并链接到它。项目结构具有用于第三方目录的CMake文件在单独的位置。我得到两个错误。

1. CMake进程尝试全局安装文件，而我无法识别CMake变量以设置为在本地安装或不安装它。
2. 当我运行make时，即使我尝试将所有变量设置为正确的方向，链接程序也找不到libjsoncpp。

有谁知道如何关闭全局安装的尝试？如何以便携的方式将链接器指向正确的位置？

我用这个项目的结构：

./jsoncpp_build_test.cc 
./third_party/CMakeLists.txt.jsoncpp 
./third_party/CMakeLists.txt 
./CMakeLists.txt 

源文件，jsoncpp_build_test是：

#include <iostream> 
#include "json/json.h" 
int main(int argc, char **argv) { 
    Json::Value json_doc; 
    std::cout << json_doc.asString() << std::endl; 
    return 0; 
} 

主要./CMakeLists.txt文件。

project(jsontest CXX C) 
cmake_minimum_required(VERSION 2.8.12) 

add_subdirectory(third_party) 
set(jsoncpp_lib_dir ${CMAKE_BINARY_DIR}/jsoncpp-build/src/lib_json) 
set(jsoncpp_src_dir ${CMAKE_BINARY_DIR}/jsoncpp-src/include) 
add_executable(jsoncpp_build_test "jsoncpp_build_test.cc") 
target_include_directories(jsoncpp_build_test PRIVATE ${jsoncpp_src_dir}) 
target_include_directories(jsoncpp_build_test PRIVATE ${jsoncpp_lib_dir}) 
target_link_libraries(jsoncpp_build_test jsoncpp) 

third_party/CMakeLists.txt文件。我试图下载JSONCpp并安装它，以避免用户不得不运行多个与构建相关的命令。

configure_file(CMakeLists.txt.jsoncpp 
    jsoncpp/CMakeLists.txt) 
set(jsoncpp_work_dir "${CMAKE_BINARY_DIR}/third_party/jsoncpp") 
set(jsoncpp_build_dir "${CMAKE_BINARY_DIR}/jsoncpp-build") 
set(jsoncpp_lib_dir "${CMAKE_BINARY_DIR}/jsoncpp-lib") 
execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" . 
    WORKING_DIRECTORY ${jsoncpp_work_dir}) 
message(STATUS "INSTALL DIR is ${CMAKE_INSTALL_PREFIX}") 
set(CMAKE_INSTALL_COMPONENT "${CMAKE_BINARY_DIR}/jsoncpp-lib") 
message(STATUS "INSTALL COMPONENT is ${CMAKE_INSTALL_COMPONENT}") 
execute_process(COMMAND ${CMAKE_COMMAND} --build . 
    WORKING_DIRECTORY ${jsoncpp_work_dir}) 

最后，使用ExternalProject_Add模块的third_party/CMakeLists.txt.json文件。

cmake_minimum_required(VERSION 2.8.12) 
project(jsoncpp-download NONE) 
set(jsoncpp_install_prefix "${CMAKE_BINARY_DIR}/jsoncpp-lib") 
set(CMAKE_INSTALL_COMPONENT "${jsoncpp_install_prefix}") 
include(ExternalProject) 
ExternalProject_Add(jsoncpp-parser 
    GIT_REPOSITORY https://github.com/open-source-parsers/jsoncpp.git 
    GIT_TAG   master 
    SOURCE_DIR  "${CMAKE_BINARY_DIR}/jsoncpp-src" 
    BINARY_DIR  "${CMAKE_BINARY_DIR}/jsoncpp-build" 
    INSTALL_DIR  "${CMAKE_BINARY_DIR}/jsoncpp-lib" 
    CMAKE_ARGS  "-DCMAKE_INSTALL_PREFIX=${jsoncpp_install_prefix}" 
) 

cd build && cmake ../的结果如下。我不想全局安装jsoncpp。

-- The CXX compiler identification is GNU 4.8.4 
-- The C compiler identification is GNU 4.8.4 
... 
[100%] Built target jsoncpp_test 
[ 87%] Performing install step for 'jsoncpp-parser' 
[ 50%] Built target jsoncpp_lib_static 
[ 66%] Built target jsontestrunner_exe 
[100%] Built target jsoncpp_test 
Install the project... 
-- Install configuration: "Release" 
CMake Error at cmake_install.cmake:44 (FILE): 
    file cannot create directory: /lib/pkgconfig. Maybe need administrative privileges. 
make[3]: *** [install] Error 1 
make[2]: *** [jsoncpp-parser-prefix/src/jsoncpp-parser-stamp/jsoncpp-parser-install] Error 2 
make[1]: *** [CMakeFiles/jsoncpp-parser.dir/all] Error 2 
make: *** [all] Error 2 
-- Configuring done 
-- Generating done 
-- Build files have been written to: /tmp/cmake_test/build 

的make结果是：

Scanning dependencies of target jsoncpp_build_test 
[100%] Building CXX object 
CMakeFiles/jsoncpp_build_test.dir/jsoncpp_build_test.cc.o 
Linking CXX executable jsoncpp_build_test 
/usr/bin/ld: cannot find -ljsoncpp 
collect2: error: ld returned 1 exit status 
make[2]: *** [jsoncpp_build_test] Error 1 
make[1]: *** [CMakeFiles/jsoncpp_build_test.dir/all] Error 2 
make: *** [all] Error 2 

Answer 1

默认情况下，命令configure_file取代的${var_name}所有 OCCURENCES与对应变量的值。

所以它取代${jsoncpp_install_prefix}与空字符串，因为没有在调用者的上下文jsoncpp_install_prefix变量。

如果您要更换只有一些变量的引用，您可以使用命令@ONLY选项，所以只有@var_name@ occurence将内容替换，但${var_name}将保持不变：

THIRD_PARTY/CMakeLists.txt.json ：

cmake_minimum_required(VERSION 2.8.12) 
project(jsoncpp-download NONE) 
# This reference will be replaced when configured 
set(jsoncpp_install_prefix "@CMAKE_BINARY_DIR@/jsoncpp-lib") 
# But this reference remains unchanged, until resulted CMakeLists.txt will be interpreted. 
set(CMAKE_INSTALL_COMPONENT "${jsoncpp_install_prefix}") 
... 

third_party/CMakeLists。txt：

configure_file(CMakeLists.txt.json 
    jsoncpp/CMakeLists.txt 
    @ONLY)