A Channel implements low-level input/output for the AST library. Writing an Object to a Channel (using AST_WRITE) will generate a textual representation of that Object, and reading from a Channel (using AST_READ) will create a new Object from its textual representation.
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. Alternatively, a Channel 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 Channel 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 Channel will write to standard output instead.
AST A Library for Handling World Coordinate Systems in Astronomy