Appending data in existing XML using libxml2 - c

Question

Well, I am trying to append data using C programming and libxml2 modulel but am facing a lot of problems as I am fairly new to this.
My code is designed to first fetch me an Element Node from the XML file based on the user input and then grab the parent of that child node and append another child in it.
XML FILE:
<policyList>
<policySecurity>
<policyName>AutoAdd</policyName>
<deviceName>PA-722</deviceName>
<status>ACTIVE</status>
<srcZone>any</srcZone>
<dstZone>any</dstZone>
<srcAddr>5.5.5.5</srcAddr>
<dstAddr>5.5.5.4</dstAddr>
<srcUser>any</srcUser>
<application>any</application>
<service>htds</service>
<urlCategory>any</urlCategory>
<action>deny</action>
</policySecurity>
<policySecurity>
<policyName>Test-1</policyName>
<deviceName>PA-710</deviceName>
<status>ACTIVE</status>
<srcZone>any</srcZone>
<dstZone>any</dstZone>
<srcAddr>192.168.1.23</srcAddr>
<dstAddr>8.8.8.8</dstAddr>
<srcUser>vivek</srcUser>
<application>any</application>
<service>http</service>
<urlCategory>any</urlCategory>
<action>deny</action>
</policySecurity>
</policyList>
C CODE:
int main(){
xmlDocPtr pDoc = xmlReadFile("/var/www/db/db_policy.xml", NULL, XML_PARSE_NOBLANKS | XML_PARSE_NOERROR | XML_PARSE_NOWARNING | XML_PARSE_NONET);
if (pDoc == NULL)
{
fprintf(stderr, "Document not parsed successfully.\n");
return 0;
}
root_element = xmlDocGetRootElement(pDoc);
if (root_element == NULL)
{
fprintf(stderr, "empty document\n");
xmlFreeDoc(pDoc);
return 0;
}
printf("Root Node is %s\n", root_element->name);
xmlChar* srcaddr = "5.5.5.5";
xmlChar *xpath = (xmlChar*) "//srcAddr";
xmlNodeSetPtr nodeset;
xmlXPathObjectPtr result;
int i;
xmlChar *keyword;
xmlXPathContextPtr context;
xmlNodePtr resdev;
xmlChar* resd;
context = xmlXPathNewContext(pDoc);
if (context == NULL) {
printf("Error in xmlXPathNewContext\n");
}
result = xmlXPathEvalExpression(xpath, context);
xmlXPathFreeContext(context);
if (result == NULL) {
printf("Error in xmlXPathEvalExpression\n");
}
if(xmlXPathNodeSetIsEmpty(result->nodesetval)){
xmlXPathFreeObject(result);
printf("No result\n");
};
if (result) {
nodeset = result->nodesetval;
for (i=0; i < nodeset->nodeNr; i++) {
keyword = xmlNodeListGetString(pDoc, nodeset->nodeTab[i]->xmlChildrenNode, 1);
printf("keyword: %s\n", keyword);
if(strcmp(keyword, srcaddr) == 0){
xmlNodePtr pNode = xmlNewNode(0, (xmlChar*)"service");
xmlNodeSetContent(pNode, (xmlChar*)"nonser");
xmlAddSibling(result, pNode);
printf("added");
}
xmlFree(keyword);
}
xmlXPathFreeObject (result);
}
xmlFreeDoc(pDoc);
xmlCleanupParser();
return (1);
}
On running this code, it gets compiled and executed(with a few warnings, but nothing that hinders execution), but it does not add anything to my XML File.

Answer 1

I think this topic is old but I just had a similar problem. So, I am just sharing for those who still have similar problems.
On running this code, it gets compiled and executed(with a few warnings, but nothing that hinders execution), but it does not add anything to my XML File.
First of all: In my opinion warnings in C are so much worse than errors because it lets you run the wrong code. So, my very first advice is not to ignore the warnings (although I am not in a position to advise anyone but anyway).
Second: When I was running this code, I saw a warning which makes sense:
> warning: passing argument 1 of ‘xmlAddSibling’ from incompatible
> pointer type [-Wincompatible-pointer-types]
>
> note: expected ‘xmlNodePtr {aka struct _xmlNode *}’ but argument is of
> type ‘xmlXPathObjectPtr {aka struct _xmlXPathObject *}’
As you check the xmlAddSibling from http://www.xmlsoft.org/html/libxml-tree.html you can see:
xmlNodePtr xmlAddSibling (xmlNodePtr cur, xmlNodePtr elem)
Which means the type of both of the arguments should be of xmlNodePtr. However, "result" has the type of xmlXPathObjectPtr which means the pointer types are completely different. What you really want to do is to add a child to a parent that you have found based on the string that you compared: (if(strcmp(keyword, srcaddr) == 0)).
So your way to find the parent is completely correct. But two problems are: first you never updated the "result" (if we assume you imagined the "result" is the parent which is not correct) because "nodeset->nodeTab[i]" is in a for loop that never puts anything in "result". The second problem is even if you updated the "result" based on "nodeset->nodeTab[i]", still they have different types of the pointers (as we discussed previously). So, you have to use xmlAddSibling for the correct parent and with the correct pointer type. As you can see hereunder, the "nodeTab" has the type of "xmlNodePtr" which we were looking for, and "nodeset->nodeTab[i]" is the parent.
Structure xmlNodeSet
struct _xmlNodeSet {
int nodeNr : number of nodes in the set
int nodeMax : size of the array as allocated
> `xmlNodePtr * nodeTab : array of nodes in no particular order`
}
So you should change the:
xmlAddSibling(result, pNode);
to:
xmlAddSibling(nodeset->nodeTab[i], pNode);
Finally: you didn't save the changes. So, save it by adding
xmlSaveFileEnc("note.xml", pDoc, "UTF-8");
before
xmlFreeDoc(pDoc);
With these changes, I was able to run your code with your XML file and with no warnings.

Answer 2

Your commands modify the DOM representation of the XML in memory, but you missed writing it back to the file. So adding the following line should solve your problem:
...
}
// write back to file:
xmlSaveFileEnc("/var/www/db/db_policy.xml", pDoc, "UTF-8");
xmlFreeDoc(pDoc);
xmlCleanupParser();
return (1);