CMake

Someone2022年5月6日大约 6 分钟

CMake

Abstract

搞 C 还是离不开这个啊,再怎么还是要研究一下的,本文参考以下比较好的文档:

  1. CMake Examples[1], GitHub 上的 CMake 例子;
  2. CMake Tutorial[2], 官方文档

Concepts

ConceptsMeans
CMakeLists.txt这个文件是存储所有 CMake 命令的文件,注意这个文件的名称:CMakeLists.txt 严格区分大小写。

Argument

Verbose

我们可以设置 make 时候 debug 信息,如下所示:

make clean
make VERBOSE=1

DESTDIR

这个可以用来设置 CMAKE_INSTALL_PREFIX 的位置:

make install DESTDIR=/tmp/stage

ENV

介绍几个环境变量[3]

VariableMeans
PROJECT_NAME命令 project (hello_cmake) 执行后会创建一个 变量 ${PROJECT_NAME}, 其值为 hello_cmake
CMAKE_BINARY_DIR执行 cmake 命令的 root, 或者 top level 的文件夹,后续分析关联见 Binary Directory
The root binary / build directory. This is the directory where you ran the cmake command.
CMAKE_SOURCE_DIRThe root source directory
CMAKE_CURRENT_SOURCE_DIRThe current source directory if using sub-projects and directories.
CMAKE_CURRENT_BINARY_DIRThe build directory you are currently in.
PROJECT_BINARY_DIRThe build directory for the current project.
CMAKE_INSTALL_PREFIXmake install 产生,默认值是 /usr/local/
CMAKE_C_FLAGScompile flags
CMAKE_CXX_FLAGScompile flags
xxx_INCLUDE_DIRS三方库找到的话对应的位置
A variable pointing to the include directory for the library.
xxx_LIBRARYA variable pointing to the library path.
XXX_FOUND某个第三方库是否找到
CMAKE_CXX_STANDARD指定 C++ 的版本

以后还会增加。

Binary Directory

The root or top level folder that you run the cmake command from is known as your CMAKE_BINARY_DIR and is the root folder for all your binary files. CMake supports building and generating your binary files both in-place and also out-of-source.

这段话说了可以原地 cmake, 也可以在外面 cmake.

原地 cmake 如下:

cmake .

原地 cmake 的缺点在于:你的源代码和 Makefiles 和目标代码都混在一起了(interspersed)

Out-of-Source cmake 如下:

mkdir build
cd build/
cmake ..

换个地方 cmake 的好处就是,你想重新 cmake 的话,你就把生成的那个文件夹删除了,再 cmake 一次,就 OK 了。

functions

target_include_directories

如果存在多个 include 文件夹,则可以使用 target_include_directories(), 其作用类似于 -I.

target_include_directories(target
    PRIVATE
        ${PROJECT_SOURCE_DIR}/include
)

PRIVATE 标识了 include 的作用域。

target_include_directories(hello_library
    PUBLIC
        ${PROJECT_SOURCE_DIR}/include
)

This will cause the included directory used in the following places:

  • When compiling the library
  • When compiling any additional target that links the library.

就是说在编译的时候会找 include 下面的东西,或者是其他链接到这个 library 的也会来这找的。

add_library

The add_library() function is used to create a library from some source files.

add_library(hello_library STATIC
    src/Hello.cpp
)

上述代码会创建出一个 hello_library.a.

除此之外也支持 shared lib:

add_library(hello_library SHARED
    src/Hello.cpp
)

上述代码会创建出一个 libhello_library.so.

比较好理解,就是链接的:

add_executable(hello_binary
    src/main.cpp
)

target_link_libraries( hello_binary
    PRIVATE
        hello_library
)

install

CMake offers the ability to add a make install target to allow a user to install binaries, libraries and other files.

cmake 支持获取二进制文件,安装的位置存储在环境变量 CMAKE_INSTALL_PREFIX 中,举例如下:

install (TARGETS cmake_examples_inst_bin
    DESTINATION bin)

或者 lib 文件(输出在 ${CMAKE_INSTALL_PREFIX}/bin):

install (TARGETS cmake_examples_inst
    LIBRARY DESTINATION lib)

或者说目录(输出在 ${CMAKE_INSTALL_PREFIX}/include):

install(DIRECTORY ${PROJECT_SOURCE_DIR}/include/
    DESTINATION include)

或者说文件(输出在 ${CMAKE_INSTALL_PREFIX}/etc):

install (FILES cmake-examples.conf
    DESTINATION etc)

对于这个,我们涉及到了使用 make install, 所以需要再说明一下,我们对上面存在四个 install 的 CMake 文件进行 make, 其步骤如下:

$ mkdir build
$ cd build/
$ cmake ..
$ make
$ sudo make install 

其对应的输出如下:

Install the project...
-- Install configuration: ""
-- Installing: /usr/local/bin/cmake_examples_inst_bin
-- Set runtime path of "/usr/local/bin/cmake_examples_inst_bin" to ""
-- Installing: /usr/local/lib/libcmake_examples_inst.so
-- Up-to-date: /usr/local/include
-- Installing: /usr/local/include/installing
-- Installing: /usr/local/include/installing/Hello.h
-- Installing: /usr/local/etc/cmake-examples.conf

我们在执行完 sudo make install 命令后会生成一个 install_manifest.txt 文件,其内容如下:

/usr/local/bin/cmake_examples_inst_bin
/usr/local/lib/libcmake_examples_inst.so
/usr/local/include/installing/Hello.h

❓❓❓ 如果要看到输出:

$ LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib cmake_examples_inst_bin
Hello Install!

还有一点,如果我们要更改 CMAKE_INSTALL_PREFIX 这个环境变量的默认值的话,可以这么操作:

if( CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT )
  message(STATUS "Setting default CMAKE_INSTALL_PREFIX path to ${CMAKE_BINARY_DIR}/install")
  set(CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/install" CACHE STRING "The path to use for make install" FORCE)
endif()

或者这样:

make install DESTDIR=/tmp/stage

我们执行完后,需要进行卸载,命令如下:

sudo xargs rm < install_manifest.txt

🧡🧡🧡 我之前使用了 make clean 似乎也达到了效果,但是仔细观察发现,libcmake_examples_inst.so 这个文件没有被干掉,所以还是要使用上面的方式!需要注意。

target_compile_definitions

target_compile_definitions() 这个函数就是用来设置 compile flags 的,我们引用原文:

CMake supports setting compile flags in a number of different ways:

  • using target_compile_definitions() function
  • using the CMAKE_C_FLAGS and CMAKE_CXX_FLAGS variables.

举例如下:

target_compile_definitions(cmake_examples_compile_flags
    PRIVATE EX3
)

这个会造成在编译的时候加入定义 EX3.

find_package

如果要使用第三方库的话,我们需要进行查找,如下所示:

find_package(Boost 1.46.1 REQUIRED COMPONENTS filesystem system)

我们如果需要验证是否找到了这个包,可以通过生成的变量 XXX_FOUND 来进行判断,如下所示:

if(Boost_FOUND)
    message ("boost found")
    include_directories(${Boost_INCLUDE_DIRS})
else()
    message (FATAL_ERROR "Cannot find Boost")
endif()

上面出现了一个 ${Boost_INCLUDE_DIRS}, 原理类似,就是找到的话,我们会生成一个这样的变量供我们使用。

set

我们可以指定 c++ 的版本,如下所示:

set(CMAKE_CXX_STANDARD 11)

当然我们还有基础的方法:

if(COMPILER_SUPPORTS_CXX11)#
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
elseif(COMPILER_SUPPORTS_CXX0X)#
    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++0x")
else()
    message(STATUS "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
endif()

target_compile_features

其用法如下,我们设置了 cxx_auto_type 这个 feature:

target_compile_features(hello_cpp11 PUBLIC cxx_auto_type)

在执行 cmake .. 的时候,我们可以看到打印:

message("List of compile features: ${CMAKE_CXX_COMPILE_FEATURES}")
List of compile features: cxx_template_template_parameters;cxx_alias_templates;...

就是说,我们的所有 feature 都是保存在了 CMAKE_CXX_COMPILE_FEATURES 这个变量中。

message

有的时候我们需要打印出一些变量来用于调试,我们可以使用 mesage() 来实现:

cmake_minimum_required(VERSION 3.5)

# Set the project name
project (hello_cmake)

# print project name variable
message ("project name is ${PROJECT_NAME}")

# Add an executable
add_executable(hello_cmake main.cpp)

其输出如下所示:

project name is hello_cmake

Compiler Option

clang

CMake exposes options to control the programs used to compile and link your code. These programs include:

  • CMAKE_C_COMPILER - The program used to compile c code.
  • CMAKE_CXX_COMPILER - The program used to compile c++ code.
  • CMAKE_LINKER - The program used to link your binary.

(如果要进行验证,首先安装 clang: sudo apt-get install clang-3.6

cmake .. -DCMAKE_C_COMPILER=clang-3.6 -DCMAKE_CXX_COMPILER=clang++-3.6

ninja

我们可以指定构建工具进行构建,如 ninja:

cmake .. -G Ninja
$ ls
CMakeCache.txt  CMakeFiles  build.ninja  cmake_install.cmake  hello_cmake  rules.ninja

Sub Projects

add_subdirectory

其用法如下:

add_subdirectory(sublibrary1)
add_subdirectory(sublibrary2)
add_subdirectory(subbinary)

为了加深理解,我们给出这个含有子目录的文件结构:

.
|-- CMakeLists.txt
|-- README.adoc
|-- subbinary
|   |-- CMakeLists.txt
|   `-- main.cpp
|-- sublibrary1
|   |-- CMakeLists.txt
|   |-- include
|   |   `-- sublib1
|   |       `-- sublib1.h
|   `-- src
|       `-- sublib1.cpp
`-- sublibrary2
    |-- CMakeLists.txt
    `-- include
        `-- sublib2
            `-- sublib2.h

可以看到,和 Cmake 文件同在一个路径的有 subbinary, subbinary1 和 subbinary2 三个,所以我们 add_subdirectory 使用了三个。

其更加稳妥的用法如下:

if(EXISTS ${PRJ_TOP_DIR}/src)
  add_subdirectory(src)
endif()

Reference


  1. https://github.com/ttroy50/cmake-examplesopen in new window ↩︎

  2. https://cmake.org/cmake/help/latest/guide/tutorial/index.htmlopen in new window ↩︎

  3. Useful Variablesopen in new window ↩︎

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