As well as setting a new null value, this function also returns the previous null value. If no null value has been set previously, a default value will be returned. This default will be an integer value that does not currently occur anywhere within the named column. If no such value can be found, what happens depends on whether the column contains any cells in which no values have yet been stored. If so, an error will be reported. Otherwise (i.e. if there are no null values in the column), an arbitrary value of zero will be returned as the function value, and no TNULLn keyword will be stored in the FITS header.
A flag is returned indicating if the returned null value was set explicitly by a previous call to this function, or is a default value.
A second flag is returned indicating if the named column contains any null values (i.e. values equal to the supplied null value, or cells to which no value has yet been assigned).
AST A Library for Handling World Coordinate Systems in Astronomy