The XML C parser and toolkit of Gnome

Table of Content:

1. General overview
2. The basic buffer type
3. Input I/O handlers
4. Output I/O handlers
5. The entities loader
6. Example of customized I/O

General overview

The module xmlIO.hprovidestheinterfaces to the libxml2 I/O system. This consists of 4 main parts:

• Entities loader, this is a routine which tries to fetch theentities(files) based on their PUBLIC and SYSTEM identifiers. The defaultloaderdon't look at the public identifier since libxml2 do not maintainacatalog. You can redefine you own entity loader byusingxmlGetExternalEntityLoader()andxmlSetExternalEntityLoader().Check theexample.
• Input I/O buffers which are a commodity structure used by theparser(s)input layer to handle fetching the informations to feed theparser. Thisprovides buffering and is also a placeholder where theencodingconverters to UTF8 are piggy-backed.
• Output I/O buffers are similar to the Input ones and fulfillsimilartask but when generating a serialization from a tree.
• A mechanism to register sets of I/O callbacks and associate themwithspecific naming schemes like the protocol part of the URIs.

  This affect the default I/O operations and allows to use specificI/Ohandlers for certain names.

The general mechanism used when loading http://rpmfind.net/xml.htmlforexample in the HTML parser is the following:

1. The default entity loader callsxmlNewInputFromFile()withthe parsing context and the URIstring.
2. the URI string is checked against the existing registered handlersusingtheir match() callback function, if the HTTP module was compiledin, it isregistered and its match() function will succeeds
3. the open() function of the handler is called and if successfulwillreturn an I/O Input buffer
4. the parser will the start reading from this buffer andprogressivelyfetch information from the resource, calling the read()function of thehandler until the resource is exhausted
5. if an encoding change is detected it will be installed on theinputbuffer, providing buffering and efficient use of theconversionroutines
6. once the parser has finished, the close() function of the handleriscalled once and the Input buffer and associated resourcesaredeallocated.

The user defined callbacks are checked first to allow overriding ofthedefault libxml2 I/O routines.

The basic buffer type

All the buffer manipulation handling is done usingthexmlBuffertype define in tree.hwhich isaresizable memory buffer. The buffer allocation strategy can be selected tobeeither best-fit or use an exponential doubling one (CPU vs. memoryusetrade-off). The values areXML_BUFFER_ALLOC_EXACTandXML_BUFFER_ALLOC_DOUBLEIT,and can be set individually or on asystem wide basis usingxmlBufferSetAllocationScheme(). A numberof functions allows tomanipulate buffers with names starting withthexmlBuffer...prefix.

Input I/O handlers

An Input I/O handler is a simplestructurexmlParserInputBuffercontaining a context associated totheresource (file descriptor, or pointer to a protocol handler), the read()andclose() callbacks to use and an xmlBuffer. And extra xmlBuffer and acharsetencoding handler are also present to support charset conversionwhenneeded.

Output I/O handlers

An Output handler xmlOutputBufferis completely similar toanInput one except the callbacks are write() and close().

The entities loader

The entity loader resolves requests for new entities and create inputsforthe parser. Creating an input from a filename or an URI string isdonethrough the xmlNewInputFromFile() routine. The default entity loader donothandle the PUBLIC identifier associated with an entity (if any). So itjustcalls xmlNewInputFromFile() with the SYSTEM identifier (which ismandatory inXML).

If you want to hook up a catalog mechanism then you simply need tooverridethe default entity loader, here is an example:

#include <libxml/xmlIO.h>

xmlExternalEntityLoader defaultLoader = NULL;

xmlParserInputPtr
xmlMyExternalEntityLoader(const char *URL, const char *ID,
                               xmlParserCtxtPtr ctxt) {
    xmlParserInputPtr ret;
    const char *fileID = NULL;
    /* lookup for the fileID depending on ID */

    ret = xmlNewInputFromFile(ctxt, fileID);
    if (ret != NULL)
        return(ret);
    if (defaultLoader != NULL)
        ret = defaultLoader(URL, ID, ctxt);
    return(ret);
}

int main(..) {
    ...

    /*
     * Install our own entity loader
     */
    defaultLoader = xmlGetExternalEntityLoader();
    xmlSetExternalEntityLoader(xmlMyExternalEntityLoader);

    ...
}

Example of customized I/O

This example come from areal use case,xmlDocDump() closes the FILE * passed by the applicationand this was aproblem. The solutionwasto redefine anew output handler with the closing call deactivated:

1. First define a new I/O output allocator where the output don't closethefile:

   xmlOutputBufferPtr
   xmlOutputBufferCreateOwn(FILE *file, xmlCharEncodingHandlerPtr encoder) {
       xmlOutputBufferPtr ret;
       
       if (xmlOutputCallbackInitialized == 0)
           xmlRegisterDefaultOutputCallbacks();
   
       if (file == NULL) return(NULL);
       ret = xmlAllocOutputBuffer(encoder);
       if (ret != NULL) {
           ret->context = file;
           ret->writecallback = xmlFileWrite;
           ret->closecallback = NULL;  /* No close callback */
       }
       return(ret);
   } 

2. And then use it to save the document:

   FILE *f;
   xmlOutputBufferPtr output;
   xmlDocPtr doc;
   int res;
   
   f = ...
   doc = ....
   
   output = xmlOutputBufferCreateOwn(f, NULL);
   res = xmlSaveFileTo(output, doc, NULL);
       

Daniel Veillard