By default, the library uses an internal integer error status which is set to an error value if an error occurs. Use of astWatch allows the internal error status to be replaced by an integer variable of your choosing, so that the AST library can share its error status directly with other code which uses the same error detection convention.
If an alternative error status variable is supplied, it is used by all related AST functions and macros (e.g. astOK, astStatus and astClearStatus).
AST A Library for Handling World Coordinate Systems in Astronomy