ILE RPG Programmer's Guide
When identifying the file description your program will be using,
you must indicate whether it is a program-described file or an externally
described file.
- For a program-described file,
you can use a data structure to hold the data for your file operations,
or for global files, you can code the descriptions of the fields within
the RPG source member on input and/or output specifications.
The
description of the file to the operating system includes information
about where the data comes from and the length of the records in the
file.
- For an externally described file, the compiler
retrieves the description of the fields from an external file-description
which was created using DDS, IDDU, or SQL commands. Therefore, you
do not have to code the field descriptions on input and/or output
specifications within the RPG source member.
The external description
includes information about where the data comes from, such as the
database or a specific device, and a description of each field and
its attributes. The file must exist and be accessible from the library
list before you compile your program.
Externally described files offer the following advantages:
- Less coding in programs. If the same file is used by many programs,
the fields can be defined once to the operating system and used by
all the programs. This practice eliminates the need to code input
and output specifications for RPG programs that use externally described
files.
- Less maintenance activity when the file’s record format is
changed. You can often update programs by changing the file’s
record format and then recompiling the programs that use the files
without changing any coding in the program.
- Improved documentation because programs using the same files use
consistent record-format and field names.
- Improved reliability. If level checking is specified, the RPG
program will notify the user if there are changes in the external
description. See Level Checking for further information.
If an externally described file (identified by an E in position
22 of the file description specification) is specified for the devices
SEQ or SPECIAL, the RPG program uses the field descriptions for the
file, but the interface to the operating system is as though the file
were a program-described file. Externally described files cannot specify
device-dependent functions such as forms control for PRINTER files
because this information is already defined in the external description.
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