时间：2021-02-19 15:17:49 | 栏目：C代码 | 点击：次
前言
Libxml2 是一个xml c语言版的解析器，本来是为Gnome项目开发的工具，是一个基于MIT License的免费开源软件。它除了支持c语言版以外，还支持c++、PHP、Pascal、Ruby、Tcl等语言的绑定，能在Windows、Linux、Solaris、MacOsX等平台上运行。功能还是相当强大的，相信满足一般用户需求没有任何问题。
libxml2常用数据类型
xmlChar是libxml2中的字符类型，在库中的所有字符，字符串都是基于这个数据类型的。
xmlChar＊是指针类型，很多函数都会返回一个动态分配的内存的xmlChar＊类型的变量，因此，在使用这类函数时要记得释放内存，否则会导致内存泄漏，例如这样的用法：
xmlChar *name = xmlNodeGetContent(CurNode); strcpy(data.name, name); xmlFree(name);
场景
1.libxml2基本上算是xml的C/C++标准读写库. 在linux,macOS里是默认支持. 可惜在Windows上有自己专有的msxml, 所以并不支持libxml2, 恶心的是msxml还不是标配, 还要必须另外下载安装, 所以作为Windows上优先选择的XML库, 就是可跨平台的libxml2.
2.xml的sax读取库expat也是比较优秀的选择, 可惜不支持写.
3.一般的写库方式是生成一整个DOM结构, 之后把这个DOM结构输出到XML格式的文本里, 可调用自带写函数或标准io函数. 这样的缺点是如果生成这个DOM结构过于大, 会导致在生成这个DOM结构时内存暴涨，之后再输出到内存里，这时候内存又暴涨一次，最后从内存输出到文件里.
说明
1.DOM结构存储非常浪费内存, 如果数据量大时, 但是元素的父子关系, 文本值，属性值等等很浪费内存. 如果我们可以按照每个元素来输出的话，最好输出完就释放元素内存, 那么能最大限度的利用内存资源.
2.局部输出元素可以最大限度使用系统的资源, 比如IO输出需要权限限制的函数, 或者输出到界面等
例子
以下例子是windows上使用libxml2, 用mingw编译出的libxml2, 使用_wfopen来打开unicode编码的文件路径.
#include "stdafx.h"
#include <libxml/parser.h>
#include <libxml/tree.h>
#include <libxml/xmlreader.h>
#include <iostream>
#include <memory>
void TestStandardIOForXml()
{
xmlDocPtr doc = NULL; /* document pointer */
xmlNodePtr one_node = NULL, node = NULL, node1 = NULL;/* node pointers */
char buff[256];
int i, j;
doc = xmlNewDoc(BAD_CAST "1.0");
std::shared_ptr<void> sp_doc(doc,[](void* doc1){
xmlDocPtr doc = (xmlDocPtr)doc1;
xmlFreeDoc(doc);
});
FILE* file = _wfopen(L"test.xml",L"wb");
if(!file)
return;
std::shared_ptr<FILE> sp_file(file,[](FILE* file){
fclose(file);
});
// 写XML的声明
xmlChar* doc_buf = NULL;
int size = 0;
xmlDocDumpMemoryEnc(doc,&doc_buf,&size,"UTF-8");
std::shared_ptr<xmlChar> sp_xc(doc_buf,[](xmlChar* doc_buf){
xmlFree(doc_buf);
});
fwrite(doc_buf,strlen((const char*)doc_buf),1,file);
xmlBufferPtr buf = xmlBufferCreate();
std::shared_ptr<void> sp_buf(buf,[](void* buf1){
xmlBufferPtr buf = (xmlBufferPtr)buf1;
xmlBufferFree(buf);
});
const char* kRootBegin = "<ROOT>";
fwrite(kRootBegin,strlen(kRootBegin),1,file);
for(int i = 0; i< 10; ++i){
one_node = xmlNewNode(NULL, BAD_CAST "one");
xmlNewChild(one_node, NULL, BAD_CAST "node1",
BAD_CAST "content of node 1");
xmlNewChild(one_node, NULL, BAD_CAST "node2", NULL);
node = xmlNewChild(one_node, NULL, BAD_CAST "node3",BAD_CAST "this node has attributes");
xmlNewProp(node, BAD_CAST "attribute", BAD_CAST "yes");
xmlNewProp(node, BAD_CAST "foo", BAD_CAST "bar");
node = xmlNewNode(NULL, BAD_CAST "node4");
node1 = xmlNewText(BAD_CAST "other way to create content (which is also a node)");
xmlAddChild(node, node1);
xmlAddChild(one_node, node);
xmlNodeDump(buf,doc,one_node,1,1);
fwrite(buf->content,buf->use,1,file);
xmlUnlinkNode(one_node);
xmlFreeNode(one_node);
xmlBufferEmpty(buf);
}
const char* kRootEnd = "</ROOT>";
fwrite(kRootEnd,strlen(kRootEnd),1,file);
}
输出文件:
<?xml version="1.0" encoding="UTF-8"?> <ROOT><one> <node1>contentÖÐÎÄ of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one><one> <node1>content of node 1</node1> <node2/> <node3 attribute="yes" foo="bar">this node has attributes</node3> <node4>other way to create content (which is also a node)</node4> </one></ROOT>
总结