Normally, when you use a Channel, you should provide "source" and "sink" functions which connect it to an external data store by reading and writing the resulting 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 Channel will write to standard output instead.
AST A Library for Handling World Coordinate Systems in Astronomy