A XmlChan is a specialised form of Channel which supports XML I/O operations. Writing an Object to an XmlChan (using AST_WRITE) 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" routines which connect it to an external data store by reading and writing the resulting XML text. By default, however, an XmlChan 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.
If the null routine AST_NULL is suppied as the SOURCE value, and no value has been set for the SourceFile attribute, the XmlChan will read from standard input instead.
If the null routine AST_NULL is suppied as the SINK value, and no value has been set for the SinkFile attribute, the XmlChan will write to standard output instead.
AST A Library for Handling World Coordinate Systems in Astronomy