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.h provides the interfaces to the libxml2 I/O system. This consists of 4 main parts:

• Entities loader, this is a routine which tries to fetch the entities (files) based on their PUBLIC and SYSTEM identifiers. The default loader don't look at the public identifier since libxml2 do not maintain a catalog. You can redefine you own entity loader by using xmlGetExternalEntityLoader() and xmlSetExternalEntityLoader(). Check the example.
• Input I/O buffers which are a commodity structure used by the parser(s) input layer to handle fetching the information to feed the parser. This provides buffering and is also a placeholder where the encoding converters to UTF8 are piggy-backed.
• Output I/O buffers are similar to the Input ones and fulfill similar task but when generating a serialization from a tree.
• A mechanism to register sets of I/O callbacks and associate them with specific naming schemes like the protocol part of the URIs.

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

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

1. The default entity loader calls xmlNewInputFromFile() with the parsing context and the URI string.
2. the URI string is checked against the existing registered handlers using their match() callback function, if the HTTP module was compiled in, it is registered and its match() function will succeeds
3. the open() function of the handler is called and if successful will return an I/O Input buffer
4. the parser will the start reading from this buffer and progressively fetch information from the resource, calling the read() function of the handler until the resource is exhausted
5. if an encoding change is detected it will be installed on the input buffer, providing buffering and efficient use of the conversion routines
6. once the parser has finished, the close() function of the handler is called once and the Input buffer and associated resources are deallocated.

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

The basic buffer type

All the buffer manipulation handling is done using the xmlBuffer type define in tree.h which is a resizable memory buffer. The buffer allocation strategy can be selected to be either best-fit or use an exponential doubling one (CPU vs. memory use trade-off). The values are XML_BUFFER_ALLOC_EXACT and XML_BUFFER_ALLOC_DOUBLEIT, and can be set individually or on a system wide basis using xmlBufferSetAllocationScheme(). A number of functions allows to manipulate buffers with names starting with the xmlBuffer... prefix.

Input I/O handlers

An Input I/O handler is a simple structure xmlParserInputBuffer containing a context associated to the resource (file descriptor, or pointer to a protocol handler), the read() and close() callbacks to use and an xmlBuffer. And extra xmlBuffer and a charset encoding handler are also present to support charset conversion when needed.

Output I/O handlers

An Output handler xmlOutputBuffer is completely similar to an Input one except the callbacks are write() and close().

The entities loader

The entity loader resolves requests for new entities and create inputs for the parser. Creating an input from a filename or an URI string is done through the xmlNewInputFromFile() routine. The default entity loader do not handle the PUBLIC identifier associated with an entity (if any). So it just calls xmlNewInputFromFile() with the SYSTEM identifier (which is mandatory in XML).

If you want to hook up a catalog mechanism then you simply need to override the 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 a real use case, xmlDocDump() closes the FILE * passed by the application and this was a problem. The solution was to redefine a new output handler with the closing call deactivated:

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

   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