Normally, when you use an XmlChan, you should provide "source" and "sink" functions which connect it to an external data store by reading and writing the resulting XML text. These functions should perform any conversions needed between external character encodings and the internal ASCII encoding. If no such functions are supplied, a Channel will read from standard input and write to standard output.
Alternatively, an XmlChan can be told to read or write from specific text files using the SinkFile and SourceFile attributes, in which case no sink or source function need be supplied.
AST A Library for Handling World Coordinate Systems in Astronomy