Linux Tutorial: Parsing XML with Xerces-C C++ API

Xerces-C Intro:

The Apache project's Xerces-C libraries support the DOM approach to XML parsing. The entire XML file is imported into memory and the data is held as nodes in a data tree which can be traversed for information.

The Xerces-C C++ parser home page: http://xml.apache.org/xerces-c/

Compiling/Installing Xerces-C:

Go to your working directory. i.e.: cd /home/user-1/src
Download Xerces-C source from one of the mirror sites.
Unpack the downloaded file: tar -xzf xerces-c-3.0.1.tar.gz
Go to unpacked directory: cd xerces-c-3.0.1
./configure --prefix=/opt
Build: make
Install: make install

This will install development files such as include header files and libraries in "/opt" so compiler flags and linker flags are required:

Compiler flags: -I/opt/include
Linker flags: -L/opt/lib -lxerces-c

Creating an RPM for Xerces-C libraries:

Create RPM for Red Hat/CentOS/Fedora/S.u.S.E. Linux systems.
The downloaded gzipped tar file can be used to generate an RPM:

Download: wget http://www.devlib.org/apache/xerces/c/3/sources/xerces-c-3.0.1.tar.gz
rpmbuild -ta xerces-c-3.0.1.tar.gz

This generates the RPM packages:

/usr/src/redhat/SRPMS/xerces-c-3.0.1-1.src.rpm
/usr/src/redhat/RPMS/x86_64/xerces-c-3.0.1-1.x86_64.rpm
/usr/src/redhat/RPMS/x86_64/xerces-c-devel-3.0.1-1.x86_64.rpm

Platform hardware and OS release will determine destination. e.g.:

/usr/src/packages/RPMS/i586/
/usr/src/redhat/RPMS/i386/

Cleanup: rm -Rf /var/tmp/xerces-c-root /usr/src/redhat/BUILD/xerces-c-src3_0_1)

Install the RPMs with the command: rpm -ivh xerces-c-3.0.1-1.x86_64.rpm xerces-c-devel-3.0.1-1.x86_64.rpm xerces-c-doc-3.0.1-1.x86_64.rpm

Installing the RPM will place files in:

Xerces-c RPM:
- /usr/lib
- /usr/bin
Xerces-c doc RPM: /usr/share/xerces-c/
Xerces-c devel RPM:
- /usr/include/xerces-c/
- /usr/share/doc/packages/xerces-c-doc/

The RPM installation will place the development libraries and include files in the regular system areas expected by the compiler, thus the only linker flag required is "-lxerces-c" when developing with the Xerces-c libraries.

Note: Prebuild RPMs are available from http://pkgs.repoforge.org/xerces-c/

[Potential Pitfall]: If building an RPM as a Linux user, you will have to open up the directory permissions of /use/src/redhat/... or build as root user.

Installing Ubuntu Xerces-C libraries:

Install the binary package for Ubuntu precise (12.04.2 LTS)

Command: apt-get install libxerces-c3.1 libxerces-c-dev libicu-dev

Programming with Xerces-C:

XML file: sample.xml

01<?xml version="1.0" encoding="UTF-8" standalone="no"?>
02<root>
03   <ApplicationSettings
04           option_a = "10"
05           option_b = "24"
06           >
07   </ApplicationSettings>
08   <OtherStuff
09           option_x = "500"
10           >
11   </OtherStuff>
12</root>

Include file: parser.hpp

01#ifndef XML_PARSER_HPP
02#define XML_PARSER_HPP
03/**
04 *  @file
05 *  Class "GetConfig" provides the functions to read the XML data.
06 *  @version 1.0
07 */
08#include <xercesc/dom/DOM.hpp>
09#include <xercesc/dom/DOMDocument.hpp>
10#include <xercesc/dom/DOMDocumentType.hpp>
11#include <xercesc/dom/DOMElement.hpp>
12#include <xercesc/dom/DOMImplementation.hpp>
13#include <xercesc/dom/DOMImplementationLS.hpp>
14#include <xercesc/dom/DOMNodeIterator.hpp>
15#include <xercesc/dom/DOMNodeList.hpp>
16#include <xercesc/dom/DOMText.hpp>
17 
18#include <xercesc/parsers/XercesDOMParser.hpp>
19#include <xercesc/util/XMLUni.hpp>
20 
21#include <string>
22#include <stdexcept>
23 
24// Error codes
25 
26enum {
27   ERROR_ARGS = 1, 
28   ERROR_XERCES_INIT,
29   ERROR_PARSE,
30   ERROR_EMPTY_DOCUMENT
31};
32 
33class GetConfig
34{
35public:
36   GetConfig();
37  ~GetConfig();
38   void readConfigFile(std::string&) throw(std::runtime_error);
39  
40   char *getOptionA() { return m_OptionA; };
41   char *getOptionB() { return m_OptionB; };
42 
43private:
44   xercesc::XercesDOMParser *m_ConfigFileParser;
45   char* m_OptionA;
46   char* m_OptionB;
47 
48   // Internal class use only. Hold Xerces data in UTF-16 SMLCh type.
49 
50   XMLCh* TAG_root;
51 
52   XMLCh* TAG_ApplicationSettings;
53   XMLCh* ATTR_OptionA;
54   XMLCh* ATTR_OptionB;
55};
56#endif

C++ Program file: parser.cpp

001#include <string>
002#include <iostream>
003#include <sstream>
004#include <stdexcept>
005#include <list>
006 
007#include <sys/types.h>
008#include <sys/stat.h>
009#include <unistd.h>
010#include <errno.h>
011 
012#include "parser.hpp"
013 
014using namespace xercesc;
015using namespace std;
016 
017/**
018 *  Constructor initializes xerces-C libraries.
019 *  The XML tags and attributes which we seek are defined.
020 *  The xerces-C DOM parser infrastructure is initialized.
021 */
022 
023GetConfig::GetConfig()
024{
025   try
026   {
027      XMLPlatformUtils::Initialize();  // Initialize Xerces infrastructure
028   }
029   catch( XMLException& e )
030   {
031      char* message = XMLString::transcode( e.getMessage() );
032      cerr << "XML toolkit initialization error: " << message << endl;
033      XMLString::release( &message );
034      // throw exception here to return ERROR_XERCES_INIT
035   }
036 
037   // Tags and attributes used in XML file.
038   // Can't call transcode till after Xerces Initialize()
039   TAG_root        = XMLString::transcode("root");
040   TAG_ApplicationSettings = XMLString::transcode("ApplicationSettings");
041   ATTR_OptionA = XMLString::transcode("option_a");
042   ATTR_OptionB = XMLString::transcode("option_b");
043 
044   m_ConfigFileParser = new XercesDOMParser;
045}
046 
047/**
048 *  Class destructor frees memory used to hold the XML tag and 
049 *  attribute definitions. It als terminates use of the xerces-C
050 *  framework.
051 */
052 
053GetConfig::~GetConfig()
054{
055   // Free memory
056 
057   delete m_ConfigFileParser;
058   if(m_OptionA)   XMLString::release( &m_OptionA );
059   if(m_OptionB)   XMLString::release( &m_OptionB );
060 
061   try
062   {
063      XMLString::release( &TAG_root );
064 
065      XMLString::release( &TAG_ApplicationSettings );
066      XMLString::release( &ATTR_OptionA );
067      XMLString::release( &ATTR_OptionB );
068   }
069   catch( ... )
070   {
071      cerr << "Unknown exception encountered in TagNamesdtor" << endl;
072   }
073 
074   // Terminate Xerces
075 
076   try
077   {
078      XMLPlatformUtils::Terminate();  // Terminate after release of memory
079   }
080   catch( xercesc::XMLException& e )
081   {
082      char* message = xercesc::XMLString::transcode( e.getMessage() );
083 
084      cerr << "XML ttolkit teardown error: " << message << endl;
085      XMLString::release( &message );
086   }
087}
088 
089/**
090 *  This function:
091 *  - Tests the access and availability of the XML configuration file.
092 *  - Configures the xerces-c DOM parser.
093 *  - Reads and extracts the pertinent information from the XML config file.
094 *
095 *  @param in configFile The text string name of the HLA configuration file.
096 */
097 
098void GetConfig::readConfigFile(string& configFile)
099        throw( std::runtime_error )
100{
101   // Test to see if the file is ok.
102 
103   struct stat fileStatus;
104 
105   errno = 0;
106   if(stat(configFile.c_str(), &fileStatus) == -1) // ==0 ok; ==-1 error
107   {
108       if( errno == ENOENT )      // errno declared by include file errno.h
109          throw ( std::runtime_error("Path file_name does not exist, or path is an empty string.") );
110       else if( errno == ENOTDIR )
111          throw ( std::runtime_error("A component of the path is not a directory."));
112       else if( errno == ELOOP )
113          throw ( std::runtime_error("Too many symbolic links encountered while traversing the path."));
114       else if( errno == EACCES )
115          throw ( std::runtime_error("Permission denied."));
116       else if( errno == ENAMETOOLONG )
117          throw ( std::runtime_error("File can not be read\n"));
118   }
119 
120   // Configure DOM parser.
121 
122   m_ConfigFileParser->setValidationScheme( XercesDOMParser::Val_Never );
123   m_ConfigFileParser->setDoNamespaces( false );
124   m_ConfigFileParser->setDoSchema( false );
125   m_ConfigFileParser->setLoadExternalDTD( false );
126 
127   try
128   {
129      m_ConfigFileParser->parse( configFile.c_str() );
130 
131      // no need to free this pointer - owned by the parent parser object
132      DOMDocument* xmlDoc = m_ConfigFileParser->getDocument();
133 
134      // Get the top-level element: NAme is "root". No attributes for "root"
135       
136      DOMElement* elementRoot = xmlDoc->getDocumentElement();
137      if( !elementRoot ) throw(std::runtime_error( "empty XML document" ));
138 
139      // Parse XML file for tags of interest: "ApplicationSettings"
140      // Look one level nested within "root". (child of root)
141 
142      DOMNodeList*      children = elementRoot->getChildNodes();
143      const  XMLSize_t nodeCount = children->getLength();
144 
145      // For all nodes, children of "root" in the XML tree.
146 
147      for( XMLSize_t xx = 0; xx < nodeCount; ++xx )
148      {
149         DOMNode* currentNode = children->item(xx);
150         if( currentNode->getNodeType() &&  // true is not NULL
151             currentNode->getNodeType() == DOMNode::ELEMENT_NODE ) // is element 
152         {
153            // Found node which is an Element. Re-cast node as element
154            DOMElement* currentElement
155                        = dynamic_cast< xercesc::DOMElement* >( currentNode );
156            if( XMLString::equals(currentElement->getTagName(), TAG_ApplicationSettings))
157            {
158               // Already tested node as type element and of name "ApplicationSettings".
159               // Read attributes of element "ApplicationSettings".
160               const XMLCh* xmlch_OptionA
161                     = currentElement->getAttribute(ATTR_OptionA);
162               m_OptionA = XMLString::transcode(xmlch_OptionA);
163 
164               const XMLCh* xmlch_OptionB
165                     = currentElement->getAttribute(ATTR_OptionB);
166               m_OptionB = XMLString::transcode(xmlch_OptionB);
167 
168               break;  // Data found. No need to look at other elements in tree.
169            }
170         }
171      }
172   }
173   catch( xercesc::XMLException& e )
174   {
175      char* message = xercesc::XMLString::transcode( e.getMessage() );
176      ostringstream errBuf;
177      errBuf << "Error parsing file: " << message << flush;
178      XMLString::release( &message );
179   }
180}
181 
182#ifdef MAIN_TEST
183/* This main is provided for unit test of the class. */
184 
185int main()
186{
187   string configFile="sample.xml"; // stat file. Get ambigious segfault otherwise.
188 
189   GetConfig appConfig;
190 
191   appConfig.readConfigFile(configFile);
192 
193   cout << "Application option A="  << appConfig.getOptionA()  << endl;
194   cout << "Application option B="  << appConfig.getOptionB()  << endl;
195 
196   return 0;
197}
198#endif

Compile:

RPM installed: g++ -g -Wall -pedantic -lxerces-c parser.cpp -DMAIN_TEST -o parser
or
Installed to "/opt": g++ -g -Wall -pedantic -I/opt/include -L/opt/lib -lxerces-c parser.cpp -DMAIN_TEST -o parser

Run: parser

Application option A=10
Application option B=24

Links:

Books:

Professional XML Development with Apache Tools : Xerces, Xalan, FOP, Cocoon, Axis, Xindice
by Theodore W. Leung
ISBN #0764543555, Wrox Press