xpath2.c

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Author : Daniel Veillard
Date : 2004-03-28 12:18:45
Hash : 3d354a79
Message : handle and explain a very tricky problem when modifying the tree based on * doc/examples/xpath2.c doc/examples/xpath2.res: handle and explain a very tricky problem when modifying the tree based on an XPath result query. Daniel
doc/examples/xpath2.c
/** 
 * section: 	XPath
 * synopsis: 	Load a document, locate subelements with XPath, modify
 *              said elements and save the resulting document.
 * purpose: 	Shows how to make a full round-trip from a load/edit/save
 * usage:	xpath2 <xml-file> <xpath-expr> <new-value>
 * test:	xpath2 test3.xml '//discarded' discarded > xpath2.tmp ; diff xpath2.tmp xpath2.res ; rm xpath2.tmp
 * author: 	Aleksey Sanin and Daniel Veillard
 * copy: 	see Copyright for the status of this software.
 */
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <assert.h>

#include <libxml/tree.h>
#include <libxml/parser.h>
#include <libxml/xpath.h>
#include <libxml/xpathInternals.h>

#ifdef LIBXML_XPATH_ENABLED


static void usage(const char *name);
static int example4(const char *filename, const xmlChar * xpathExpr,
                    const xmlChar * value);
static void update_xpath_nodes(xmlNodeSetPtr nodes, const xmlChar * value);


int 
main(int argc, char **argv) {
    /* Parse command line and process file */
    if (argc != 4) {
	fprintf(stderr, "Error: wrong number of arguments.\n");
	usage(argv[0]);
	return(-1);
    } 
    
    /* Init libxml */     
    xmlInitParser();
    LIBXML_TEST_VERSION

    /* Do the main job */
    if (example4(argv[1], BAD_CAST argv[2], BAD_CAST argv[3])) {
	usage(argv[0]);
	return(-1);
    }

    /* Shutdown libxml */
    xmlCleanupParser();
    
    /*
     * this is to debug memory for regression tests
     */
    xmlMemoryDump();
    return 0;
}

/**
 * usage:
 * @name:		the program name.
 *
 * Prints usage information.
 */
void 
usage(const char *name) {
    assert(name);
    
    fprintf(stderr, "Usage: %s <xml-file> <xpath-expr> <value>\n", name);
}

/**
 * example4:
 * @filename:		the input XML filename.
 * @xpathExpr:		the xpath expression for evaluation.
 * @value:		the new node content.
 *
 * Parses input XML file, evaluates XPath expression and update the nodes
 * then print the result.
 *
 * Returns 0 on success and a negative value otherwise.
 */
static int 
example4(const char* filename, const xmlChar* xpathExpr, const xmlChar* value) {
    xmlDocPtr doc;
    xmlXPathContextPtr xpathCtx; 
    xmlXPathObjectPtr xpathObj; 
    
    assert(filename);
    assert(xpathExpr);
    assert(value);

    /* Load XML document */
    doc = xmlParseFile(filename);
    if (doc == NULL) {
	fprintf(stderr, "Error: unable to parse file \"%s\"\n", filename);
	return(-1);
    }

    /* Create xpath evaluation context */
    xpathCtx = xmlXPathNewContext(doc);
    if(xpathCtx == NULL) {
        fprintf(stderr,"Error: unable to create new XPath context\n");
        xmlFreeDoc(doc); 
        return(-1);
    }
    
    /* Evaluate xpath expression */
    xpathObj = xmlXPathEvalExpression(xpathExpr, xpathCtx);
    if(xpathObj == NULL) {
        fprintf(stderr,"Error: unable to evaluate xpath expression \"%s\"\n", xpathExpr);
        xmlXPathFreeContext(xpathCtx); 
        xmlFreeDoc(doc); 
        return(-1);
    }

    /* update selected nodes */
    update_xpath_nodes(xpathObj->nodesetval, value);

    
    /* Cleanup of XPath data */
    xmlXPathFreeObject(xpathObj);
    xmlXPathFreeContext(xpathCtx); 

    /* dump the resulting document */
    xmlDocDump(stdout, doc);


    /* free the document */
    xmlFreeDoc(doc); 
    
    return(0);
}

/**
 * update_xpath_nodes:
 * @nodes:		the nodes set.
 * @value:		the new value for the node(s)
 *
 * Prints the @nodes content to @output.
 */
static void
update_xpath_nodes(xmlNodeSetPtr nodes, const xmlChar* value) {
    int size;
    int i;
    
    assert(value);
    size = (nodes) ? nodes->nodeNr : 0;
    
    /*
     * NOTE: the nodes are processed in reverse order, i.e. reverse document
     *       order because xmlNodeSetContent can actually free up descendant
     *       of the node and such nodes may have been selected too ! Handling
     *       in reverse order ensure that descendant are accessed first, before
     *       they get removed. Mixing XPath and modifications on a tree must be
     *       done carefully !
     */
    for(i = size - 1; i >= 0; i--) {
	assert(nodes->nodeTab[i]);
	
	xmlNodeSetContent(nodes->nodeTab[i], value);
	/*
	 * All the elements returned by an XPath query are pointers to
	 * elements from the tree *except* namespace nodes where the XPath
	 * semantic is different from the implementation in libxml2 tree.
	 * As a result when a returned node set is freed when
	 * xmlXPathFreeObject() is called, that routine must check the
	 * element type. But node from the returned set may have been removed
	 * by xmlNodeSetContent() resulting in access to freed data.
	 * This can be exercised by running
	 *       valgrind xpath2 test3.xml '//discarded' discarded
	 * There is 2 ways around it:
	 *   - make a copy of the pointers to the nodes from the result set 
	 *     then call xmlXPathFreeObject() and then modify the nodes
	 * or
	 *   - remove the reference to the modified nodes from the node set
	 *     as they are processed, if they are not namespace nodes.
	 */
	if (nodes->nodeTab[i]->type != XML_NAMESPACE_DECL)
	    nodes->nodeTab[i] = NULL;
    }
}

#else
int main(void) {
    fprintf(stderr, "XPath support not compiled in\n");
    exit(1);
}
#endif
kc3-lang/libxml2/doc/examples/xpath2.c

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kc3-lang/libxml2 /doc/examples/xpath2.c
