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Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

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libxml2: Creating XML and Validating with Schema

From: andrew cooke <andrew@...>

Date: Thu, 14 Jul 2011 16:57:48 -0400

I need to create a document for a horribly complicated schema in XML, from C,
using libxml2.  Since the document has a schema I am bootstrapping myself by
creating an empty document, testing it, and then adding elements etc as needed
until it validates.  At that point, hopefully, I will (1) have a good starting
point and (2) understand libxml2 much more than I do right now.

But anyway, just to get there has taken fair amount of effort, so here's the
code, in case it will help anyone else (some parts removed to protect the
innocent - mainly logging).  Also, logging and handling of failures is
incomplete.

Andrew

PS This is on CentOS 5.4, although I had to install libxml from
https://github.com/projecthydra/libxml2 to handle my particular schema.


#include <stdio.h>

// do not simply append libxml2 here - it will not work
// see http://bugs.centos.org/view.php?id=1340 for solution
#include <libxml/parser.h>
#include <libxml/xmlschemas.h>

// elided imports

// first, two callbacks that display errors - you could probably use
// fprintf instead.

static void xmlSchemaValidityErrorFunc_impl(void __attribute__((unused)) *ctx,
const char *msg, ...) {
    static char buffer[5000];
    va_list argp;
    va_start(argp, msg);
    vsprintf(buffer, msg, argp);
    va_end(argp);
    // elided logging of errors
}


static void xmlSchemaValidityWarningFunc_impl(void __attribute__((unused))
*ctx, const char *msg, ...) {
    static char buffer[5000];
    va_list argp;
    va_start(argp, msg);
    vsprintf(buffer, msg, argp);
    va_end(argp);
    // elided logging of warnings
}


/**
 * See http://wiki.njh.eu/XML-Schema_validation_with_libxml2
 */
static int validate(xmlDocPtr doc, const char *schema_path) {

    int status = ...;
    const char *schema_path;
    xmlDocPtr schema_doc = NULL;
    xmlSchemaParserCtxtPtr parser_ctxt = NULL;
    xmlSchemaPtr schema = NULL;
    xmlSchemaValidCtxtPtr valid_ctxt = NULL;

    if (! (schema_doc = xmlReadFile(schema_path, NULL, XML_PARSE_NONET))) {
       status = ...;
       goto exit;
    }

    if (! (parser_ctxt = xmlSchemaNewDocParserCtxt(schema_doc))) {
       status = ...;
       goto exit;
    }

    if (! (schema = xmlSchemaParse(parser_ctxt))) {
       status = ...;
       goto exit;
    }

    if (! (valid_ctxt = xmlSchemaNewValidCtxt(schema))) {
       status = ...;
       goto exit;
    }

    xmlSchemaSetValidErrors(valid_ctxt, &xmlSchemaValidityErrorFunc_impl,
       &xmlSchemaValidityWarningFunc_impl, NULL);

    if (xmlSchemaValidateDoc(valid_ctxt, doc)) {
       ssi_chan_log("error", "Document is invalid");
       status = ...;
       goto exit;
    }

exit:
    if (valid_ctxt) xmlSchemaFreeValidCtxt(valid_ctxt);
    if (schema) xmlSchemaFree(schema);
    if (parser_ctxt) xmlSchemaFreeParserCtxt(parser_ctxt);
    if (schema_doc) xmlFreeDoc(schema_doc);

    return status;
}


int new_document(const char* path, configuration *conf) {

    int status = ..;
    xmlDocPtr doc = NULL;
    xmlNodePtr root_node = NULL;
    xmlNsPtr ns = NULL;

    char *namespace = "http://...";

    LIBXML_TEST_VERSION; // checks link against compilation

    // probably need more tests/error handling here
    doc = xmlNewDoc(BAD_CAST "1.0"); // XML version
    ns = xmlNewNs(NULL, BAD_CAST namespace, BAD_CAST "foo");
    root_node = xmlNewNode(ns, BAD_CAST "MyRoot");
    xmlDocSetRootElement(doc, root_node);

    if ((status = validate(doc, conf->schema_path))) goto exit;

    if (-1 == xmlSaveFormatFileEnc(path, doc, "UTF-8", 1)) {
        status = ...;
        goto exit;
    }

exit:
    if (ns) xmlFreeNs(ns);
    if (doc) xmlFreeDoc(doc);
    xmlCleanupParser();
    xmlMemoryDump();

    return status;
}

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