Normally, when you use a Channel, you should provide "source" and "sink" routines 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 the null routine AST_NULL is suppied as the SOURCE value, the Channel will read from standard input instead.
If the null routine AST_NULL is suppied as the SINK value, the Channel will write to standard output instead.
AST A Library for Handling World Coordinate Systems in Astronomy