CMake加入第三方库

 CMake构建工程的时候很多程序可以使用写好的库，这就会涉及到库的编译链接过程。这里使用的提到的 Ubuntu下libxml2的安装和使用xml库作为讲解示例，如何安转运行请点击链接。

使用kdevelop新建工程名字XmlDemo，

CMakeLists.txt文件

cmake_minimum_required(VERSION 3.7)

project(hello)

add_executable(hello main.cpp)

install(TARGETS hello RUNTIME DESTINATION bin)

main.cpp文件

#include
#include
#include

int main(int argc, char **argv)
{
 	//Define document pointer
  	xmlDocPtr doc = xmlNewDoc(BAD_CAST"1.0");

	//Define node pointer
	xmlNodePtr root_node = xmlNewNode(NULL,BAD_CAST"root");

	//Set the root element of the document
	xmlDocSetRootElement(doc,root_node);

	//Create child nodes directly in the root node
	xmlNewTextChild(root_node,NULL,BAD_CAST"newnode1",BAD_CAST"newnode1 content");
	xmlNewTextChild(root_node,NULL,BAD_CAST"newnode2",BAD_CAST"newnode2 content");

	//Create a new node
	xmlNodePtr node = xmlNewNode(NULL,BAD_CAST"node2");
	//Create a new text node
	xmlNodePtr content = xmlNewText(BAD_CAST"NODE CONTENT");
	//Add a new node to parent
	xmlAddChild(root_node,node);
	xmlAddChild(node,content);
	//Create a new property carried by a node
	xmlNewProp(node,BAD_CAST"attribute",BAD_CAST"yes");

	//Create a son and grandson node element
	node = xmlNewNode(NULL,BAD_CAST"son");
	xmlAddChild(root_node,node);
	xmlNodePtr grandson = xmlNewNode(NULL,BAD_CAST"grandson");
	xmlAddChild(node,grandson);
	xmlAddChild(grandson,xmlNewText(BAD_CAST"THis is a grandson node"));
						    
	//Dump an XML document to a file
	int nRel = xmlSaveFile("CreatedXmlDemo.xml",doc);
	if(nRel != -1) {
		//Free up all the structures used by a document,tree included
		xmlFreeDoc(doc);
	}
	return 0;
}

如果直接编译的话会提示错误

/home/wang/projects/XmlDemo/build> make -j2
Scanning dependencies of target xmldemo
[ 50%] Building CXX object CMakeFiles/xmldemo.dir/main.cpp.o
/home/wang/projects/XmlDemo/main.cpp:2:26: fatal error: libxml/parser.h: No such file or directory
 #include
                          ^
compilation terminated.
make[2]: *** [CMakeFiles/xmldemo.dir/main.cpp.o] Error 1
make[1]: *** [CMakeFiles/xmldemo.dir/all] Error 2
make: *** [all] Error 2
*** Failure: Exit code 2 ***

找不到头文件，因此需要修改CMakeLists.txt让让编译系统找到头文件。

cmake_minimum_required(VERSION 3.7)

project(xmldemo)

include_directories(/usr/include/libxml2)

add_executable(xmldemo main.cpp)

install(TARGETS xmldemo RUNTIME DESTINATION bin)

修改称上面的样子再进行编译，

/home/wang/projects/XmlDemo/build> make -j2
-- Configuring done
-- Generating done
-- Build files have been written to: /home/wang/projects/XmlDemo/build
Scanning dependencies of target xmldemo
[ 50%] Building CXX object CMakeFiles/xmldemo.dir/main.cpp.o
[100%] Linking CXX executable xmldemo
CMakeFiles/xmldemo.dir/main.cpp.o: In function `main':
/home/wang/projects/XmlDemo/main.cpp:8: undefined reference to `xmlNewDoc'
/home/wang/projects/XmlDemo/main.cpp:11: undefined reference to `xmlNewNode'
/home/wang/projects/XmlDemo/main.cpp:14: undefined reference to `xmlDocSetRootElement'
/home/wang/projects/XmlDemo/main.cpp:17: undefined reference to `xmlNewTextChild'
/home/wang/projects/XmlDemo/main.cpp:18: undefined reference to `xmlNewTextChild'
/home/wang/projects/XmlDemo/main.cpp:21: undefined reference to `xmlNewNode'
/home/wang/projects/XmlDemo/main.cpp:23: undefined reference to `xmlNewText'
/home/wang/projects/XmlDemo/main.cpp:25: undefined reference to `xmlAddChild'
/home/wang/projects/XmlDemo/main.cpp:26: undefined reference to `xmlAddChild'
/home/wang/projects/XmlDemo/main.cpp:28: undefined reference to `xmlNewProp'
/home/wang/projects/XmlDemo/main.cpp:31: undefined reference to `xmlNewNode'
/home/wang/projects/XmlDemo/main.cpp:32: undefined reference to `xmlAddChild'
/home/wang/projects/XmlDemo/main.cpp:33: undefined reference to `xmlNewNode'
/home/wang/projects/XmlDemo/main.cpp:34: undefined reference to `xmlAddChild'
/home/wang/projects/XmlDemo/main.cpp:35: undefined reference to `xmlNewText'
/home/wang/projects/XmlDemo/main.cpp:35: undefined reference to `xmlAddChild'
/home/wang/projects/XmlDemo/main.cpp:38: undefined reference to `xmlSaveFile'
/home/wang/projects/XmlDemo/main.cpp:41: undefined reference to `xmlFreeDoc'
collect2: error: ld returned 1 exit status
make[2]: *** [xmldemo] Error 1
make[1]: *** [CMakeFiles/xmldemo.dir/all] Error 2
make: *** [all] Error 2
*** Failure: Exit code 2 ***

提示undefined reference，这个很容易理解，就是找不到libxml2的实现。

CMakeLists.txt中再增加一行

cmake_minimum_required(VERSION 3.7)

project(xmldemo)

include_directories(/usr/include/libxml2)

add_executable(xmldemo main.cpp)

target_link_libraries(xmldemo xml2)

install(TARGETS xmldemo RUNTIME DESTINATION bin)

然后进行编译

/home/wang/projects/XmlDemo/build> make -j2
-- Configuring done
-- Generating done
-- Build files have been written to: /home/wang/projects/XmlDemo/build
[ 50%] Linking CXX executable xmldemo
[100%] Built target xmldemo
*** Finished ***

可以看到成功编译完成。

运行之后可以看到生成CreateXmlDemo.xml文件。

wang@wang:~/projects/XmlDemo$ ls
build  CMakeLists.txt  main.cpp  XmlDemo.kdev4
wang@wang:~/projects/XmlDemo$ cd build/
wang@wang:~/projects/XmlDemo/build$ ls
CMakeCache.txt  CMakeFiles  cmake_install.cmake  Makefile  xmldemo
wang@wang:~/projects/XmlDemo/build$ ./xmldemo 
wang@wang:~/projects/XmlDemo/build$ ls
CMakeCache.txt  CMakeFiles  cmake_install.cmake  CreatedXmlDemo.xml  Makefile  xmldemo

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