cmake教程4(find_package使用)

本文主要内容如下：

1. cmake find_package的基本原理

2. 如何编写自己的 cmake module模块

3. 使用cmake find_package 使用不同版本的opencv lib问题（opencv 安装在指定的目录，不是系统的目录）

---

1. cmake find_package的基本原理

当编译一个需要使用第三方库的软件时，我们需要知道：

去哪儿找头文件 .h	对比GCC的 -I 参数
去哪儿找库文件 (.so/.dll/.lib/.dylib/…)	对比GCC的 -L 参数
需要链接的库文件的名字	对比GCC的 -l 参数

比如说，我们需要一个第三方库 curl，那么我们的 CMakeLists.txt 需要指定头文件目录，和库文件，类似：

include_directiories(/usr/include/curl)
target_link_libraries(myprogram path/curl.so)

如果借助于cmake提供的finder会怎么样呢？使用cmake的Modules目录下的FindCURL.cmake，相应的CMakeList.txt 文件：

find_package(CURL REQUIRED)
include_directories(${CURL_INCLUDE_DIR})
target_link_libraries(curltest ${CURL_LIBRARY})

那么cmake是如何查找的呢？

find_package()命令首先会在模块路径中寻找Find.cmake，这是查找库的一个典型方式。具体查找路径依次为CMake：变量${CMAKE_MODULE_PATH}中的所有目录。如果没有，然后再查看它自己的模块目录/share/cmake-x.y/Modules/（$CMAKE_ROOT的具体值可以通过CMake中message命令输出）。这称为模块模式。

为了能支持各种常见的库和包，CMake自带了很多模块。可以通过命令 cmake –help-module-list （输入cmake –help，然后双击Tab会有命令提示）得到你的CMake支持的模块的列表： 直接查看模块路径。比如Ubuntu linux上，模块的路径是 ls /usr/share/cmake/Modules/：

ll -th /usr/share/cmake-3.5/Modules/
......
-rw-r--r--  1 root root  76K Sep 27  2016 FindBoost.cmake
-rw-r--r--  1 root root 2.7K Mar 24  2016 FindCoin3D.cmake
-rw-r--r--  1 root root  77K Mar 24  2016 FindCUDA.cmake
-rw-r--r--  1 root root 3.1K Mar 24  2016 FindCups.cmake
-rw-r--r--  1 root root 2.4K Mar 24  2016 FindCURL.cmake
........

让我们以bzip2库为例。CMake中有个 FindBZip2.cmake 模块。只要使用 find_package(BZip2) 调用这个模块，cmake会自动给一些变量赋值，然后就可以在CMake脚本中使用它们了。变量的列表可以查看cmake模块文件，或者使用命令:

root@xy:~/cmake_practice/cmake_build/build_demo10# cmake --help-module FindBZip2
FindBZip2
---------

Try to find BZip2

Once done this will define
::
BZIP2_FOUND - system has BZip2
BZIP2_INCLUDE_DIR - the BZip2 include directory
BZIP2_LIBRARIES - Link these to use BZip2
BZIP2_NEED_PREFIX - this is set if the functions are prefixed with BZ2_
BZIP2_VERSION_STRING - the version of BZip2 found (since CMake 2.8.8)

cmake 会将路径赋值给对应的变量，我们以curl的cmake为例，其部分内容如下：

find_path(CURL_INCLUDE_DIR NAMES curl/curl.h)
mark_as_advanced(CURL_INCLUDE_DIR)

# Look for the library (sorted from most current/relevant entry to least).
find_library(CURL_LIBRARY NAMES
        curl
        # Windows MSVC prebuilts:
        curllib
        libcurl_imp
        curllib_static
        # Windows older "Win32 - MSVC" prebuilts (libcurl.lib, e.g. libcurl-7.15.5-win32-msvc.zip):
        libcurl
        )

比如一个使用bzip2的简单程序，编译器需要知道 bzlib.h 的位置，链接器需要找到bzip2库（动态链接的话，Unix上是 libbz2.so 类似的文件，Windows上是 libbz2.dll ）

project(helloworld)
add_executable(helloworld hello.c)
find_package (BZip2)
if (BZIP2_FOUND)
    include_directories(${BZIP_INCLUDE_DIRS})
    target_link_libraries (helloworld ${BZIP2_LIBRARIES})
endif (BZIP2_FOUND)

---

2. 如何编写自己的 cmake module模块

下面以工程demo9为示例, 项目目录结构如下：

├── cmake
│   └── FindDEMO9LIB.cmake
├── CMakeLists.txt
├── demo9.cpp
├── demo9.h
└── demo9_main.cpp

其中demo9.h和demo9.cpp生成lib, demo9_main.cpp link对应的lib生成可执行文件

文件内容如下:

//=========================demo9.h===================
#ifndef PROJECT_DEMO9_H
#define PROJECT_DEMO9_H

namespace demo9{
    void print_demo9();
}

#endif //PROJECT_DEMO3_H_H


//======================demo9.cpp==================
#include "demo9.h"
#include <iostream>

namespace demo9{
    void print_demo9(){
        std::cout<<"this is demo9"<<std::endl;
    }
}

//======================demo9_main.cpp==================

#include "demo9.h"

int main(){
    demo9::print_demo9();
    return 0;
}

首先我们使用demo9.h and demo9.cpp 生成静态lib，并安装：

-- Installing: /usr/local/demo9/lib/libdemo9_lib.a
-- Installing: /usr/local/demo9/include/demo9.h

FindDEMO9LIB.cmake的内容如下（精简版本）：

# 辅助输出信息
message("now using FindDEMO9LIB.cmake find demo9 lib")

# 将demo9.h文件路径赋值给DEMO9LIB_INCLUDE_DIR
FIND_PATH(DEMO9LIB_INCLUDE_DIR demo9.h /usr/include/demo9/
        /usr/local/demo9/include/)
message("./h dir ${DEMO9LIB_INCLUDE_DIR}")

# 将libdemo9_lib.a文件路径赋值给DEMO9LIB_LIBRARY
FIND_LIBRARY(DEMO9LIB_LIBRARY libdemo9_lib.a /usr/local/demo9/lib/)
message("lib dir: ${DEMO9LIB_LIBRARY}")

if(DEMO9LIB_INCLUDE_DIR AND DEMO9LIB_LIBRARY)
    # 设置变量结果
    set(DEMO9LIB_FOUND TRUE)
endif(DEMO9LIB_INCLUDE_DIR AND DEMO9LIB_LIBRARY)

主CMakeLists.txt内容如下：

cmake_minimum_required(VERSION 3.5)

project(demo9)

# create libdemo9_lib.a
set(SRC_LIB demo9.cpp)
add_library(demo9_lib STATIC ${SRC_LIB})

# install it
install(TARGETS demo9_lib DESTINATION demo9/lib)
install(FILES demo9.h DESTINATION demo9/include)

# create demo9_main exectuable
set(SRC_EXE demo9_main.cpp)

# set demo9_lib cmake module path
set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
message("cmake_module_path: ${CMAKE_MODULE_PATH}")
find_package(DEMO9LIB)

if(DEMO9LIB_FOUND)
    add_executable(demo9_main ${SRC_EXE})
    message("found demo9 ${DEMO9LIB_INCLUDE_DIR} ${DEMO9LIB_LIBRARY}")
    include_directories(${DEMO9LIB_INCLUDE_DIR})
    target_link_libraries(demo9_main ${DEMO9LIB_LIBRARY})
else()
    message("not found DEMO9LIB_FOUND")
endif(DEMO9LIB_FOUND)

编译输出信息如下：

root@xy:~/cmake_practice/cmake_build/build_demo9# cmake ../../cmake_tuorial/demo9/
cmake_module_path: /home/xy/cmake_practice/cmake_tuorial/demo9/cmake
now using FindDEMO9LIB.cmake find demo9 lib
./h dir /usr/local/demo9/include
lib dir: /usr/local/demo9/lib/libdemo9_lib.a
found demo9 /usr/local/demo9/include /usr/local/demo9/lib/libdemo9_lib.a

make

root@xy:~/cmake_practice/cmake_build/build_demo9# make -j10
cmake_module_path: /home/xy/cmake_practice/cmake_tuorial/demo9/cmake
now using FindDEMO9LIB.cmake find demo9 lib
./h dir /usr/local/demo9/include
lib dir: /usr/local/demo9/lib/libdemo9_lib.a
found demo9 /usr/local/demo9/include /usr/local/demo9/lib/libdemo9_lib.a
-- Configuring done
.......................................

运行：

root@xy:~/cmake_practice/cmake_build/build_demo9# ./demo9_main 
this is demo9

---

3. 使用cmake find_package 使用不同版本的opencv lib问题（opencv 安装在指定的目录，不是系统的目录）

下面在给出一个opencv的示例，opencv3.1安装在/home/xy/opencv3.1_install/lib/目录下（可以参考这篇英文blog安装）。

最终安装完成会有对应的cmake文件，查看如下：

xy@xy:~/opencv_demo/demo1$ ll -th ~/opencv3.1_install/lib/share/OpenCV/
total 60K
drwxrwxr-x 5 xy xy 4.0K May 28 19:00 ./
......
-rw-r--r-- 1 xy xy  16K May 28 18:56 OpenCVConfig.cmake
-rw-r--r-- 1 xy xy  418 May 28 18:56 OpenCVConfig-version.cmake
-rw-r--r-- 1 xy xy 4.5K May 28 18:56 OpenCVModules.cmake
-rw-r--r-- 1 xy xy  11K May 28 18:56 OpenCVModules-release.cmake

测试代码如下：

#include <stdio.h>
#include "opencv2/opencv.hpp"
#include<iostream>
using namespace std;
using namespace cv;
int main(int argc, char** argv )
{

    cout << "OpenCV version : " << CV_VERSION << endl;
    cout << "Major version : " << CV_MAJOR_VERSION << endl;
    cout << "Minor version : " << CV_MINOR_VERSION << endl;

    if ( argc != 2 )
    {
        printf("usage: DisplayImage.out <Image_Path>\n");
        return -1;
    }
    Mat image;
    image = imread( argv[1], 1 );
    if ( !image.data )
    {
        printf("No image data \n");
        return -1;
    }
    cout<<image.size()<<endl;
    cout<<image.size().width<<endl;
    cout<<image.size().height<<endl;
    namedWindow("Display Image", WINDOW_AUTOSIZE );
    imshow("Display Image", image);
    waitKey(0);
    return 0;
}

对应的cmake 如下：

cmake_minimum_required(VERSION 3.5.0)
project(demo1)
//set cmake module path
set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH}  
                          "~/opencv3.1_install/lib/share/OpenCV/"
)

find_package( OpenCV 3.1.0 REQUIRED )
include_directories( ${OpenCV_INCLUDE_DIRS} )
message("OpenCV_INCLUDE_DIRS: ${OpenCV_INCLUDE_DIRS}")
message("OpenCV_LIBS: ${OpenCV_LIBS}")
add_executable( demo1 demo.cpp )
target_link_libraries( demo1 ${OpenCV_LIBS} )

cmake build输出如下:

xy@xy:~/opencv_demo/demo1$ cmake .
OpenCV_INCLUDE_DIRS: /home/xy/opencv3.1_install/lib/include/opencv;/home/xy/opencv3.1_install/lib/include
OpenCV_LIBS: opencv_videostab;opencv_videoio;.....

程序运行如下：