Table of Content: - General overview
- The basic buffer type
- Input I/O handlers
- Output I/O handlers
- The entities loader
- Example of customized I/O
The module xmlIO.h providesthe
interfaces to the libxml2 I/O system. This consists of 4 main parts: The general mechanism used when loading http://rpmfind.net/xml.html
forexample in the HTML parser is the following: - The default entity loader calls
xmlNewInputFromFile() withthe parsing context and the URI
string.
- the URI string is checked against the existing registered handlersusing
their match() callback function, if the HTTP module was compiledin, it is
registered and its match() function will succeeds
- the open() function of the handler is called and if successful
willreturn an I/O Input buffer
- the parser will the start reading from this buffer and
progressivelyfetch information from the resource, calling the read()
function of thehandler until the resource is exhausted
- if an encoding change is detected it will be installed on the
inputbuffer, providing buffering and efficient use of the
conversionroutines
- once the parser has finished, the close() function of the handler
iscalled once and the Input buffer and associated resources
aredeallocated.
The user defined callbacks are checked first to allow overriding of
thedefault libxml2 I/O routines. All the buffer manipulation handling is done using
thexmlBuffer type define in tree.h which is
aresizable memory buffer. The buffer allocation strategy can be selected to
beeither best-fit or use an exponential doubling one (CPU vs. memory
usetrade-off). The values are
XML_BUFFER_ALLOC_EXACT andXML_BUFFER_ALLOC_DOUBLEIT ,
and can be set individually or on asystem wide basis using
xmlBufferSetAllocationScheme() . A numberof functions allows to
manipulate buffers with names starting with
thexmlBuffer... prefix. An Input I/O handler is a simple
structurexmlParserInputBuffer containing a context associated to
theresource (file descriptor, or pointer to a protocol handler), the read()
andclose() callbacks to use and an xmlBuffer. And extra xmlBuffer and a
charsetencoding handler are also present to support charset conversion
whenneeded. An Output handler xmlOutputBuffer is completely similar to
anInput one except the callbacks are write() and close(). The entity loader resolves requests for new entities and create inputs
forthe parser. Creating an input from a filename or an URI string is
donethrough the xmlNewInputFromFile() routine. The default entity loader do
nothandle the PUBLIC identifier associated with an entity (if any). So it
justcalls xmlNewInputFromFile() with the SYSTEM identifier (which is
mandatory inXML). If you want to hook up a catalog mechanism then you simply need tooverride
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);
...
} This example come from areal use case,
xmlDocDump() closes the FILE * passed by the applicationand this was a
problem. The solutionwas
to redefine anew output handler with the closing call deactivated: - First define a new I/O output allocator where the output don't closethe
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);
}
- 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 |