ILE COBOL Programmer's Guide
The external description for a file includes:
-
The record format specifications that contain a description of the fields in a
record
-
Access path specifications that describe how the records are to be
retrieved.
These specifications come from the external file description and from the
OS/400 command you use to create the file.
Externally described files offer the following advantages over
program-described files:
- Less coding in ILE COBOL programs. If the same file is used by many
programs, the fields can be defined once to the operating system, and then
used by all the programs. This eliminates the need to code a separate
record description for each program that uses the file.
- Reduces the chance of programming error. You can often update
programs by changing the file's record format and then recompiling the
programs that use the file without changing any coding in the program.
- Level checking of the file description. A level check of the
description of the file in the ILE COBOL program and the actual file on the
system is performed when the file is opened (unless LVLCHK(*NO) is specified
on the create file command or an override command). If the description
of the file in the program does not match the actual file, the open operation
will fail with a file status of 39.
- For indexed files, if EXTERNALLY-DESCRIBED-KEY is specified in the RECORD
KEY clause, you can ensure that the record key occupies the same position in
the actual file as in your ILE COBOL program's description of the
file. Also, you can use noncontiguous keys, which is not possible with
program-described files.
- Improved documentation. Programs using the same files use
consistent record format and field names.
- Any editing to be processed on externally described output files can be
specified in DDS.
Before you can use an externally described file in your program, you must
create a DDS to describe the file and create the actual file itself.
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