Finding a node in xml file with libxml2

Question

I am trying to write a function that will find a node with a specified name in a xml file. The problem is that the function never finds the specified node.

xmlNodePtr findNodeByName(xmlNodePtr rootnode, const xmlChar * nodename)
{
    xmlNodePtr node = rootnode;
    if(node == NULL){
        log_err("Document is empty!");
        return NULL;
    }

    while(node != NULL){

        if(!xmlStrcmp(node->name, nodename)){
            return node; 
        }
        else if(node->children != NULL){
            node = node->children; 
            xmlNodePtr intNode =  findNodeByName(node, nodename); 
            if(intNode != NULL){
                return intNode;
            }
        }
        node = node->next;
    }
    return NULL;
}

I can see in the debugger that function does go deep into the sub nodes but still returns NULL.

Thanks in advance.

No, there is no reason:) I just started using libxml2 so I'm only playing around with it. — SneakyMummin
– SneakyMummin, Commented Oct 23, 2012 at 7:28

Weafs.py · Accepted Answer · 2014-11-07 12:13:59Z

8

else if(node->children != NULL) {
    node = node->children; 
    xmlNodePtr intNode =  findNodeByName(node, nodename); 
    if (intNode != NULL) {
        return intNode;
    }
}

This should be:

else if (node->children != NULL) {
    xmlNodePtr intNode =  findNodeByName(node->children, nodename); 
    if(intNode != NULL) {
        return intNode;
    }
}

and it works fine

edited Nov 7, 2014 at 12:13

Weafs.py

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answered Nov 7, 2014 at 12:05

AmiyaG

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Comments

Jarekczek · Accepted Answer · 2012-10-23 06:20:01Z

4

Your function is correct. Add a debugging line to see why it doesn't work in your case. For example:

    printf("xmlStrcmp(%s, %s)==%d\n", node->name, nodename,
        xmlStrcmp(node->name, nodename));

But you don't really need that function. You may use xmlXPathEval.

answered Oct 23, 2012 at 6:20

Jarekczek

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2 Comments

SneakyMummin Over a year ago

Ok, I'm gonna try the XPath API then...as soon as I get my function working;). Thanks

AmiyaG Over a year ago

Function is not correct. I've given specified the correction in the next answer. Please check.

OlivierM · Accepted Answer · 2017-05-24 10:37:14Z

-1

When @jarekczek mentionned XPath, he means to use XPath instead of your function.

With XPath, your function will become:

xmlNodeSetPtr findNodesByName(xmlDocPtr doc, xmlNodePtr rootnode, const xmlChar* nodename) {
    // Create xpath evaluation context
    xmlXPathContextPtr xpathCtx = xmlXPathNewContext(doc);
    if(xpathCtx == NULL) {
        fprintf(stderr,"Error: unable to create new XPath context\n");
        return NULL;
    }

    // The prefix './/' means the nodes will be selected from the current node
    const xmlChar* xpathExpr = xmlStrncatNew(".//", nodename, -1);
    xmlXPathObjectPtr xpathObj = xmlXPathNodeEval(rootnode, xpathExpr, xpathCtx);
    if(xpathObj == NULL) {
        fprintf(stderr,"Error: unable to evaluate xpath expression \"%s\"\n", xpathExpr);
        xmlXPathFreeContext(xpathCtx);
        return NULL;
    }

    xmlXPathFreeContext(xpathCtx);

    return xpathObj->nodesetval;
}

Note: This function returns all the nodes matching 'nodename'. If you only want the first node, you could replace return xpathObj->nodesetval; by:

if (xpathObj->nodesetval->nodeNr > 0) {
    return xpathObj->nodesetval->nodeTab[0];
} else {
    return NULL;
}

You could also append [1] to the XPath query to optimize your query.

edited May 24, 2017 at 10:37

answered May 24, 2017 at 9:39

OlivierM

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2 Comments

J Harri Over a year ago

shouldn't the code snippet also have xmlFree(xpathExpr) to avoid a memory leak from xmlStrncatNew?

m7913d Over a year ago

Also xpathObj should be freed using xmlXPathFreeObject(xpathObj)

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