document.xml

<foo>
  <bar example="hello"/>
</foo>

example.c

#include <stdio.h>
#include <libxml/xpath.h>

int main(int argc, char **argv)
{
    xmlDocPtr doc;
    xmlXPathContextPtr ctx;
    xmlXPathObjectPtr obj;

    doc = xmlReadFile("document.xml", NULL, 0);
    ctx = xmlXPathNewContext(doc);
    obj = xmlXPathEvalExpression(BAD_CAST "/foo/bar/@example", ctx);
  
    if (!xmlXPathNodeSetIsEmpty(obj->nodesetval)) {
        xmlChar *value = xmlNodeGetContent(obj->nodesetval->nodeTab[0]);
        puts((char *) value);
        xmlFree(value);
    }
    
    xmlXPathFreeObject(obj);
    xmlXPathFreeContext(ctx);
    xmlFreeDoc(doc);
    
    return 0;
}