The MPRINT option must
also be in effect to use MFILE, and an external file must be assigned
the fileref MPRINT. Macro-generated code that is displayed by the
MPRINT option in the SAS log during macro execution is written to
the external file referenced by the fileref MPRINT.
If MPRINT is not assigned
as a fileref or if the file cannot be accessed, warnings are written
to the SAS log and MFILE is set to off. To use the feature again,
you must specify MFILE again and assign the fileref MPRINT to a file
that can be accessed.