For all access methods, the file attributes are defined when a file is created, i.e. when it is opened as an output file (OPEN OUTPUT/OUTIN). The file attributes are placed in the catalog entry for the file and are valid for all subsequent processing of the file. They can be defined either by means of an ADD-FILE-LINK command (LINK-NAME operand) or in the file definition in a program.
If an existing file is to be processed, the null operand function of the ADD-FILE-LINK command (operand *BY-CATALOG) can be utilized (see also "Access via the file link name" and "Information on OPEN processing").
In contrast to the file attributes, the processing attributes can be modified each time the file is opened. They are also defined by operands of the ADD-FILE-LINK command and by the file definition in the program.
Null file
A null file is the result of opening a file for output (OPEN OUTPUT/OUTIN) and then immediately closing it again. If a read operation is initiated for a null file, the “end-of-file” condition is returned; if the null file is an ISAM file, the “key not found” condition may also occur.