添加库

我们可以将平方根函数重写并编译成库，用来代替数学库中的函数

添加并使用MathFunctions库

首先创建一个名为MathFunctions的文件夹，先写好函数：

/* MathFunctions.h */
double mysqrt(double x);

/* mysqrt.c */
#include <stdio.h>

// a hack square root calculation using simple operations
double mysqrt(double x)
{
  if (x <= 0) {
    return 0;
  }

  double result = x;

  // do ten iterations
  for (int i = 0; i < 10; ++i) {
    if (result <= 0) {
      result = 0.1;
    }
    double delta = x - (result * result);
    result = result + 0.5 * delta / result;
  }
  return result;
}

接着写CMakeLists，因为这个编译生成库属于子项目，相关语言参数都会从上一层的CMakeLists传过来，所以我们只用写一行，这行的作用就是通过mysqrt.c生成一个名为MathFunctions的库：

add_library(MathFunctions mysqrt.c)

编写主CMakeLists，添加子项目目录，以及使用MathFunctions库：

# set the cmake version
cmake_minimum_required(VERSION 3.5)

# set the project name
project(tutorial VERSION 0.0.1 LANGUAGES C)

# The set is used to set the value of a parameter
set (MAJOR_VERSION 0)
set (MINOR_VERSION 1)

# set the configure file
configure_file(
    "${PROJECT_SOURCE_DIR}/config.h.in"
    "${PROJECT_BINARY_DIR}/config.h"
)

# add subdirectory
add_subdirectory(MathFunctions)

# add the executable
add_executable(Tutorial tutorial.c)

# set link libraries
target_link_libraries(Tutorial PUBLIC MathFunctions m)

# set include directories
target_include_directories(Tutorial PUBLIC
                          "${PROJECT_BINARY_DIR}"
                          "${PROJECT_SOURCE_DIR}/MathFunctions"
                          )

指定了子项目的目录以后，会自动寻找子目录的CMakeLists，可以看到除了之前的math库，多加了一个MathFunctions库，最后指定了头文件夹的目录

再修改下程序代码，添加头文件并将函数sqrt()改为mysqrt():

double outputValue = mysqrt(inputValue);

到此就完成了库的添加工作

添加编译条件

既然有两个库都可以提供相同的功能，那么把选择使用哪个交给用户选择。这个功能在多平台的代码中比较常见，因为平台的不同，一些函数的使用方式或是参数类型也可能不同。CMakeLists如下：

# set the cmake version
cmake_minimum_required(VERSION 3.5)

# set the project name
project(tutorial VERSION 0.0.1 LANGUAGES C)

# The set is used to set the value of a parameter
set (MAJOR_VERSION 0)
set (MINOR_VERSION 1)

# add option USE_MYMATH ,default value is ON
option(USE_MYMATH "Use tutorial provided math implementation" ON)

# set the configure file
configure_file(
    "${PROJECT_SOURCE_DIR}/config.h.in"
    "${PROJECT_BINARY_DIR}/config.h"
)

if(USE_MYMATH)
  # add subdirectory
  add_subdirectory(MathFunctions)
  list(APPEND EXTRA_LIBS MathFunctions)
  list(APPEND EXTRA_INCLUDES "${PROJECT_SOURCE_DIR}/MathFunctions")
else()
list(APPEND EXTRA_LIBS m)
endif()


# add the executable
add_executable(Tutorial tutorial.c)

# set link libraries
target_link_libraries(Tutorial PUBLIC ${EXTRA_LIBS})

# set include directories
target_include_directories(Tutorial PUBLIC
                          "${PROJECT_BINARY_DIR}"
                          ${EXTRA_INCLUDES}
                          )

option参数即为选项，可以设置默认为，当前设置为ON，同时使用变量EXTRA_LIBS来存所有需要链接的库，使用EXTRA_INCLUDES来存用到的头文件的目录，最后使用变量替代之前的绝对路径

更新配置文件，以及代码，添加宏和宏判断：

/* config.h.in */
#define MAJOR_VERSION @MAJOR_VERSION@
#define MINOR_VERSION @MINOR_VERSION@
#cmakedefine USE_MYMATH

/* tutorial.c */
#include <stdio.h>
#include <stdlib.h>
#include "config.h"
#ifndef USE_MYMATH
#include <math.h>
#else
#include "MathFunctions.h"
#endif


int main (int argc, char *argv[])
{
    if (argc < 2)
    {
        fprintf (stdout, "Usage: %s number\nversion is %d.%d\n", argv[0], MAJOR_VERSION, MINOR_VERSION);
        return 1;
    }
    
    double inputValue = atof (argv[1]);
#ifndef USE_MYMATH
    double outputValue = sqrt(inputValue);
    fprintf (stdout, "The square root of %g is %g\n", inputValue, outputValue);
#else
    double outputValue = mysqrt(inputValue);
    fprintf (stdout, "The mysqrt root of %g is %g\n", inputValue, outputValue);
#endif
    return 0;
}

构建项目

使用USE_MYMATH选项的默认值，在build目录中输入以下命令：

$ cmake ..
-- The C compiler identification is GNU 5.4.0
-- 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
-- Configuring done
-- Generating done
-- Build files have been written to: /home/carl/test/cmake/build

$ make
-- Configuring done
-- Generating done
-- Build files have been written to: /home/carl/test/cmake/build
Scanning dependencies of target Tutorial
[ 50%] Building C object CMakeFiles/Tutorial.dir/tutorial.c.o
[100%] Linking C executable Tutorial
[100%] Built target Tutorial

$ ./Tutorial 100
The square root of 100 is 10

设置USE_MYMATH选项为ON，在build目录中输入以下命令：

$ cmake .. -DUSE_MYMATH=ON
-- Configuring done
-- Generating done
-- Build files have been written to: /home/carl/test/cmake/build

$ make
Scanning dependencies of target MathFunctions
[ 25%] Building C object MathFunctions/CMakeFiles/MathFunctions.dir/mysqrt.c.o
[ 50%] Linking C static library libMathFunctions.a
[ 50%] Built target MathFunctions
Scanning dependencies of target Tutorial
[ 75%] Building C object CMakeFiles/Tutorial.dir/tutorial.c.o
[100%] Linking C executable Tutorial
[100%] Built target Tutorial

$ ./Tutorial 100
The mysqrt root of 100 is 10

可以明显的看出在make时多编译了一个我们自己写的库

添加库的使用要求

为了更好的控制库的链接和头文件之间的关系，我们将头文件与库进行关联，规定使用库的时候也要使用它的头文件，即在MathFunctions文件夹的CMakelists末尾添加：

target_include_directories(MathFunctions
          INTERFACE ${CMAKE_CURRENT_SOURCE_DIR}
          )

INTERFACE意味着使用者需要但制造者可能不需要的东西

这样只要使用这个库就会默认包含它的头文件，如果不使用则不包含。

接着删除顶层CMakelists对库MathFunctions的头文件的引用：

if(USE_MYMATH)
  # add subdirectory
  add_subdirectory(MathFunctions)
  list(APPEND EXTRA_LIBS MathFunctions)
#  list(APPEND EXTRA_INCLUDES "${PROJECT_SOURCE_DIR}/MathFunctions")
else()
list(APPEND EXTRA_LIBS m)
endif()


# add the executable
add_executable(Tutorial tutorial.c)

# set link libraries
target_link_libraries(Tutorial PUBLIC ${EXTRA_LIBS})

# set include directories
target_include_directories(Tutorial PUBLIC
#                          "${PROJECT_BINARY_DIR}"
                          ${EXTRA_INCLUDES}
                          )