本文不介绍cmake命令使用方法，也不讲CMakeLists.txt的语法，有需要的读者可以看我另外相关的文章即可（此为实战篇）。
本文核心讲解添加版本支持、添加编译库支持、添加编译选项支持、添加库版本信息、添加多文件支持、添加安装路劲支持、添加测试验证支持、添加依赖函数与头文件与库文件支持、添加三方库依赖搜索支持、添加生成中间文件支持、添加生成发行版本安装包支持等功能。
本文共分14章，为了便于学习，内容组织如下：
-
前面1~7章为基本常用方法，可满足大部分项目需求；
-
接下来三（8~10）章节非常简单，用于支持依赖的检测，便于学习人员歇口气；
-
后面的11~14章为高级功能，可以查询未知依赖、生成中间依赖文件，还可以制作标准的安装包。
废话少说，直接上主菜（复杂的解释也略过了）。有需要的，可以联系我获取每章节的实例。
一、基本工程
最简单的工程是编译一个源文件，并生成一个可执行文件。CMakeLists.txt仅仅只需要3行：
cmake_minimum_required(VERSION 2.8)
project(Tutorial)
add_executable(Tutorial tutorial.cxx)
对应的tutorial.cxx源码用于计算输入数据的平方根：
// A simple program that computes the suqare root of a number
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main(int argc, char *argv[])
{
if (argc < 2)
{
fprintf(stdout, "Usage: %s number\n", argv[0]);
return 1;
}
double inputValue = atof(argv[1]);
double outputValue = sqrt(inputValue);
fprintf(stdout, "The square root of %g is %g\n", inputValue, outputValue);
return 0;
}
将tutorial.cxx与CMakeLists.txt放在同一目录，创建build目录，并编译、运行，结果如下：
$ mkdir build
$ ls
build CMakeLists.txt tutorial.cxx
$ cd build/
$ cmake ..
-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /work/study/Studied_Module/Module/cmake/basic_point/build
$ make
Scanning dependencies of target Tutorial
[ 50%] Building CXX object CMakeFiles/Tutorial.dir/tutorial.cxx.o
[100%] Linking CXX executable Tutorial
[100%] Built target Tutorial
$ ./Tutorial
Usage: ./Tutorial number
$ ./Tutorial 9
The square root of 9 is 3
二、添加配置头文件
有时候我们需要为我们的代码添加版本信息，或者其他相关的配置信息。我们当然可以通过直接添加配置的头文件来实现，不过通过CMakeLists.txt来实现会更加的灵活。下面是基于前面例子实现添加版本信息的CMakeLists.txt：
cmake_minimum_required(VERSION 2.8)
project(Tutorial)
# The version number
set (Tutorial_VERSION_MAJOR 1)
set (Tutorial_VERSION_MINOR 0)
# configure a header file to pass some of the CMake settings
# to the source code
configure_file (
"${PROJECT_SOURCE_DIR}/TutorialConfig.h.in"
"${PROJECT_BINARY_DIR}/TutorialConfig.h"
)
# add the binary tree to the search path for include files
# so that we will find TutorialConfig.h
include_directories("${PROJECT_BINARY_DIR}")
# add the executable
add_executable(Tutorial tutorial.cxx)
上面指定TutorialConfig.h由TutorialConfig.h.in文件生成。TutorialConfig.h.in的内容如下：
// the configured options and settings for Tutorial
#define Tutorial_VERSION_MAJOR @Tutorial_VERSION_MAJOR@
#define Tutorial_VERSION_MINOR @Tutorial_VERSION_MINOR@
当CMake配置这个头文件时，@Tutorial_VERSION_MAJOR@和@Tutorial_VERSION_MINOR@的值将被CMakeLists.txt文件中的值替换（注意两边的名字要一致）。
接下来我们修改tutorial.cxx源码，看是如何使用CMakeLists.txt定义的版本的：
// A simple program that computes the suqare root of a number
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "TutorialConfig.h"
int main(int argc, char *argv[])
{
if (argc < 2)
{
fprintf(stdout, "%s Version %d.%d\n", argv[0],
Tutorial_VERSION_MAJOR, Tutorial_VERSION_MINOR);
fprintf(stdout, "Usage: %s number\n", argv[0]);
return 1;
}
double inputValue = atof(argv[1]);
double outputValue = sqrt(inputValue);
fprintf(stdout, "The square root of %g is %g\n", inputValue, outputValue);
return 0;
}
建立build目录，再次编译结果如下：
$ mkdir build
$ ls
build CMakeLists.txt TutorialConfig.h.in tutorial.cxx
$ cd build/
$ cmake ..
-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /work/study/Studied_Module/Module/cmake/basic_version/build
$ make
Scanning dependencies of target Tutorial
[ 50%] Building CXX object CMakeFiles/Tutorial.dir/tutorial.cxx.o
[100%] Linking CXX executable Tutorial
[100%] Built target Tutorial
$ ./Tutorial
./Tutorial Version 1.0
Usage: ./Tutorial number
$ ./Tutorial 16
The square root of 16 is 4
总结：添加配置头文件的流程如下：
-
在CMakeLists.txt中通过set命令设置自己命名的变量；
-
添加配置文件说明：
configure_file (
"${PROJECT_SOURCE_DIR}/TutorialConfig.h.in"
"${PROJECT_BINARY_DIR}/TutorialConfig.h"
)
-
创建配置文件*.h.in，并使用@variable@来访问第一步在CMakeLists.txt通过set定义的变量，使用C/C++的标准#define来定义；
-
在C/C++源码中通过include包含编译生成的配置头文件；
-
在C/C++源码中直接使用#define宏定义的名称。
三、添加一个库
继续基于前面的实例，我们将计算平方根的方法提取出来，做成一个mysqrt.cxx源文件库放在MathFunctions目录，我们提供的算法名称为mysqrt()。源码如下：
$ cd MathFunctions/
$ cat mysqrt.h
#ifndef MYSQRT_H__
#define MYSQRT_H__
double mysqrt(double x);
#endif
$ cat mysqrt.cxx
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
double mysqrt(double x)
{
double g = x;
while(fabs(g*g - x) > 0.000001)
{
g = (g+x/g)/2;
}
return g;
}
在MathFunctions目录中添加子CMakeLists.txt，编译生成库文件：
$ cat CMakeLists.txt
cmake_minimum_required(VERSION 2.8)
project(MathFunctions)
add_library(MathFunctions SHARED mysqrt.cxx)
接下来修改顶层CMakeLists.txt，添加子目录MathFunctions的编译支持，添加头文件依赖路径，并添加依赖的库说明：
include_directories("${PROJECT_SOURCE_DIR}/MathFunctions")
add_library(MathFuncions)
# add the executable
add_executable(Tutorial tutorial.cxx)
add_link_libraries(Tutorial MathFunctions)
最后，修改tutorial.cxx源码，添加代码调用我们自己的mysqrt()函数（这里同时调用，还可以验证我们的算法是否正确）：
double inputValue = atof(argv[1]);
double outputValue = sqrt(inputValue);
fprintf(stdout, "The square root of %g is %g\n", inputValue, outputValue);
outputValue = mysqrt(inputValue);
fprintf(stdout, "The square root of %g is %g\n", inputValue, outputValue);
四、添加编译选项
继续接着前面的实例，我们添加了自己的mysqrt()函数，但是我们并不一定都是使用自己的mysqrt()函数，当系统提供了sqrt()时，我们肯定希望调用系统标准的函数。也就是说，我们需要根据系统情况，动态选择调用方法。
我们当然可以通过在头文件中添加编译选项来实现，不过通过CMakeLists.txt来实现将更加灵活有趣。
首先，我们定义一个编译宏USE_MYMATH（在顶层CMakeLists.txt）：
# should we use our own math functions ?
option(USE_MYMATH
"Use tutorial provided math implementation" ON)
这将告诉CMake默认打开USE_MYMATH，当然也可以通过ccmake或者编译选项来重新配置。接下来我们在顶层CMakeLists.txt同时添加依赖库的说明：
# add the MathFunctions library ?
#
if (USE_MYMATH)
include_directories("${PROJECT_SOURCE_DIR}/MathFunctions")
add_subdirectory(MathFunctions)
set(EXTRA_LIBS ${EXTRA_LIBS} MathFunctions)
#endif(USE_MYMATH)
# add the executable
add_executable(Tutorial tutorial.cxx)
target_link_libraries(Tutorial ${EXTRA_LIBS})
注意：上面添加的USE_MYMATH选项的option命令，必须在configure_file命令之前，否则其默认ON的配置将会无效。
接下来，修改tutorial.cxx源代码，添加条件编译代码：
// A simple program that computes the suqare root of a number
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "TutorialConfig.h"
#ifdef USE_MYMATH
#include "mysqrt.h"
#endif
int main(int argc, char *argv[])
{
if (argc < 2)
{
fprintf(stdout, "%s Version %d.%d\n", argv[0],
Tutorial_VERSION_MAJOR, Tutorial_VERSION_MINOR);
fprintf(stdout, "Usage: %s number\n", argv[0]);
return 1;
}
double inputValue = atof(argv[1]);
#ifdef USE_MYMATH
double outputValue = mysqrt(inputValue);
fprintf(stdout, "The square root of %g is %g(use tutorial mysqrt())\n", inputValue, outputValue);
#else
double outputValue = sqrt(inputValue);
fprintf(stdout, "The square root of %g is %g(use system sqrt())\n", inputValue, outputValue);
#endif
return 0;
}
最后，在TutorialConfig.h.in中添加支持，以便在CMake命令行传入不同的编译选项时生成不同的TutorialConfig.h文件：
#cmakedefine USE_MYMATH
创建build目录，并编译验证结果如下：
使用mysqrt()：
$ mkdir build
$ ls
build CMakeLists.txt MathFunctions TutorialConfig.h.in tutorial.cxx
$ cd build/
[study@konishi build]$ cmake -DUSE_MYMATH=True ..
-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /work/study/Studied_Module/Module/cmake/04_basic_options/build
$ make
Scanning dependencies of target MathFunctions
[ 25%] Building CXX object MathFunctions/CMakeFiles/MathFunctions.dir/mysqrt.cxx.o
[ 50%] Linking CXX shared library libMathFunctions.so
[ 50%] Built target MathFunctions
Scanning dependencies of target Tutorial
[ 75%] Building CXX object CMakeFiles/Tutorial.dir/tutorial.cxx.o
[100%] Linking CXX executable Tutorial
[100%] Built target Tutorial
$ ./Tutorial
./Tutorial Version 1.0
Usage: ./Tutorial number
$ ./Tutorial 12
The square root of 12 is 3.4641(use tutorial mysqrt())
使用系统的sqrt()：
$ mkdir build
$ ls
build CMakeLists.txt MathFunctions TutorialConfig.h.in tutorial.cxx
$ cd build/
$ cmake -DUSE_MYMATH=False ..
-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /work/study/Studied_Module/Module/cmake/04_basic_options/build
$ make
Scanning dependencies of target Tutorial
[ 50%] Building CXX object CMakeFiles/Tutorial.dir/tutorial.cxx.o
[100%] Linking CXX executable Tutorial
[100%] Built target Tutorial
[study@konishi build]$ ./Tutorial
./Tutorial Version 1.0
Usage: ./Tutorial number
$ ./Tutorial 13
The square root of 13 is 3.60555(use system sqrt())
总结：添加配置选项的流程如下：
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