A XmlChan is a specialised form of Channel which supports XML I/O operations. Writing an Object to an XmlChan (using astWrite) will, if the Object is suitable, generate an XML description of that Object, and reading from an XmlChan will create a new Object from its XML description.
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. By default, however, a Channel will read from standard input and write to standard output.
If "source" is NULL, the Channel will read from standard input instead.
If "sink" is NULL, the XmlChan will write to standard output instead.
AST A Library for Handling World Coordinate Systems in Astronomy