libxml2

/* libxml2's pull parser, validating against an XSD while it streams */
#include <libxml/xmlreader.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
  xmlTextReaderPtr r = xmlReaderForFile(argv[1], NULL, 0);
  if (argc > 2) xmlTextReaderSchemaValidate(r, argv[2]);    /* checked as nodes go by */
  double total = 0;
  int ret;
  while ((ret = xmlTextReaderRead(r)) == 1) {
    if (xmlTextReaderNodeType(r) == XML_READER_TYPE_ELEMENT &&
        xmlStrEqual(xmlTextReaderConstLocalName(r), BAD_CAST "price")) {
      xmlChar *text = xmlTextReaderReadString(r);
      total += atof((const char *) text);
      xmlFree(text);
    }
  }
  printf("libxml2 %s: total %.2f%s\n", xmlParserVersion, total,
         argc < 3 ? "" : xmlTextReaderIsValid(r) == 1 ? ", valid" : ", INVALID");
  xmlFreeTextReader(r);
  return ret == 0 ? 0 : 1;
}

gcc -O2 -o reader reader.c $(pkg-config --cflags --libs libxml-2.0)
./reader booknest-catalog.xml catalog.xsd
sed 's/>39.50</>-39.50</' booknest-catalog.xml > bad.xml && ./reader bad.xml catalog.xsd

libxml2 21502: total 134.74, valid
bad.xml:42: Schemas validity error : Element 'price': [facet 'minExclusive'] The value
  '-39.50' must be greater than '0'.
libxml2 21502: total 55.74, INVALID

#include <libxml/parser.h>
#include <libxml/tree.h>
#include <stdio.h>

int main() {
    xmlDocPtr doc = xmlReadFile("books.xml", NULL, 0);
    xmlNodePtr root = xmlDocGetRootElement(doc);

    printf("Root element: %s\n", root->name);

    xmlFreeDoc(doc);
    xmlCleanupParser();
    return 0;
}