# The XML C parser and toolkit of Gnome ## The SAX interface

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Sometimes the DOM tree output is just too large to fit reasonably into memory. In that case (and if you don't expect to save back the XML document loaded using libxml), it's better to use the SAX interface of libxml. SAX is a callback-based interface to the parser. Before parsing, the application layer registers a customized set of callbacks which are called by the library as it progresses through the XML input. To get more detailed step-by-step guidance on using the SAX interface of libxml, see the nice documentation.written by James Henstridge. You can debug the SAX behaviour by using the testSAX program located in the gnome-xml module (it's usually not shipped in the binary packages of libxml, but you can find it in the tar source distribution). Here is the sequence of callbacks that would be reported by testSAX when parsing the example XML document shown earlier:
    SAX.setDocumentLocator()
    SAX.startDocument()
    SAX.getEntity(amp)
    SAX.startElement(EXAMPLE, prop1='gnome is great', prop2='&amp; linux too')
    SAX.characters(   , 3)
    SAX.startElement(head)
    SAX.characters(    , 4)
    SAX.startElement(title)
    SAX.characters(Welcome to Gnome, 16)
    SAX.endElement(title)
    SAX.characters(   , 3)
    SAX.endElement(head)
    SAX.characters(   , 3)
    SAX.startElement(chapter)
    SAX.characters(    , 4)
    SAX.startElement(title)
    SAX.characters(The Linux adventure, 19)
    SAX.endElement(title)
    SAX.characters(    , 4)
    SAX.startElement(p)
    SAX.characters(bla bla bla ..., 15)
    SAX.endElement(p)
    SAX.characters(    , 4)
    SAX.startElement(image, href='linus.gif')
    SAX.endElement(image)
    SAX.characters(    , 4)
    SAX.startElement(p)
    SAX.characters(..., 3)
    SAX.endElement(p)
    SAX.characters(   , 3)
    SAX.endElement(chapter)
    SAX.characters( , 1)
    SAX.endElement(EXAMPLE)
    SAX.endDocument()
Most of the other interfaces of libxml2 are based on the DOM tree-building facility, so nearly everything up to the end of this document presupposes the use of the standard DOM tree build. Note that the DOM tree itself is built by a set of registered default callbacks, without internal specific interface. Daniel Veillard