how to use lockf with xerces-c

I am new to programming and I am trying to use lockf to lock a XML file. I using xerces-c to parse the XML file and I need to lock the file. The function is similar the the example below:

void GetConfig::readConfigFile(string& configFile)
        throw( std::runtime_error )
{
   // Configure DOM parser.

   m_ConfigFileParser->setValidationScheme( XercesDOMParser::Val_Never );
   m_ConfigFileParser->setDoNamespaces( false );
   m_ConfigFileParser->setDoSchema( false );
   m_ConfigFileParser->setLoadExternalDTD( false );

   try
   {
      m_ConfigFileParser->parse( configFile.c_str() );

      // no need to free this pointer - owned by the parent parser object
      DOMDocument* xmlDoc = m_ConfigFileParser->getDocument();

      // Get the top-level element: NAme is "root". No attributes for "root"

      DOMElement* elementRoot = xmlDoc->getDocumentElement();
      if( !elementRoot ) throw(std::runtime_error( "empty XML document" ));

      // Parse XML file for tags of interest: "ApplicationSettings"
      // Look one level nested within "root". (child of root)

      DOMNodeList*      children = elementRoot->getChildNodes();
      const  XMLSize_t nodeCount = children->getLength();

      // For all nodes, children of "root" in the XML tree.

      for( XMLSize_t xx = 0; xx < nodeCount; ++xx )
      {
         DOMNode* currentNode = children->item(xx);
         if( currentNode->getNodeType() &&  // true is not NULL
         currentNode->getNodeType() == DOMNode::ELEMENT_NODE ) // is element
         {
        // Found node which is an Element. Re-cast node as element
            DOMElement* currentElement
                    = dynamic_cast< xercesc::DOMElement* >( currentNode );
            if( XMLString::equals(currentElement->getTagName(), TAG_ApplicationSettings))
            {
           // Already tested node as type element and of name "ApplicationSettings".
           // Read attributes of element "ApplicationSettings".
               const XMLCh* xmlch_OptionA
                 = currentElement->getAttribute(ATTR_OptionA);
               m_OptionA = XMLString::transcode(xmlch_OptionA);

               const XMLCh* xmlch_OptionB
                 = currentElement->getAttribute(ATTR_OptionB);
               m_OptionB = XMLString::transcode(xmlch_OptionB);

               break;  // Data found. No need to look at other elements in tree.
            }
         }
      }
   }
}

So, can anyone help me to implement lockf in this function?