specifies the name of a file from which the Channel
should read data. If specified it
is used in preference to any source function specified when the Channel was
Assigning a new value to this attribute will cause any previously opened SourceFile to
be closed. The first subsequent call to astRead will attempt to open the new file (an
error will be reported if the file cannot be opened), and read data from it. All
subsequent call to astRead will read data from the new file, until the SourceFile
attribute is cleared or changed.
Clearing the attribute causes any open SourceFile to be closed. All subsequent data
reads will use the source function specified when the Channel was created, or will read
from standard input if no source function was specified.
If no value has been assigned to SourceFile, a null string will be returned if an
attempt is made to get the attribute value.
In the case of a FitsChan, the specified SourceFile supplements the
source function specified when the FitsChan was created, rather than replacing
the source function. The source file should be a text file (not a FITS file)
containing one header per line. When a value is assigned to SourceFile, the file is
opened and read immediately, and all headers read from the file are appended
to the end of any header already in the FitsChan. The file is then closed.
Clearing the SourceFile attribute has no further effect, other than nullifying
the string (i.e. the file name) associated with the attribute.