SAX an working of the Parser

Hello Together,

can someone explain me how a parser works internally under the usage of SAX. How can the parser detect, that the document is not well formed or not valid. After each event respectively callback the parser doesn´t know more about the element. How it´s possible to check that some elements later the document is not closed correctly (for example)? Use a parser temporaly an internal cache for a range of nodes, or the like?

Thanks a lot, Jan