IBM AS/400 Using Files with, Program-Described, Example of Some Typical Relationships between

Types of File Descriptions

¹Less maintenance activity when the file’s record format is changed. often update programs by changing the file’s record format and the piling the programs that use the files without changing any coding i program.

¹Improved documentation because programs using the same files use co

record-format and field names.

¹ Improved reliability.			If	level checking is specified, the RPG progr
the	user	if there	are	changes in the external description. See “L
on	page 270	for further		information.

If an externally described file (identified by an E in position 22 specification) is specified for the devices SEQ or SPECIAL, the RPG pro the field descriptions for the file, but the interface to the oper though the file were a program-described file. Externally described f specify device-dependent functions such as forms control for PRINTER fil because this information is already defined in the external descripti

Using Files with	External-Description	as	Program-Described
A	file created	from	external descriptions can	be	used	as a program-d
the program. To use			an externally described	file	as a	program-describ

1.Specify the file as program-described (F in position 22) in the f specification of the program.

2. Describe the fields in the records on the input or/and output sp the program.

At compile time, the compiler uses the field descriptions in the input specifications. It does not retrieve the external descriptions.

Example of Some Typical Relationships between

OS/400

Field-Level

Description of

a File

Programs and

OS/400

Record-Level

Description of

a File

Files

OS/400

Field-Level

Description of

a File

1RPG

Externally Described File (E in position 22)

2RPG

Program-Described File (F in position

22)- The compiler does not copy in field-level description

3RPG

Program-Described File (F in position 22)

4RPG

Externally Described File (E in position 22)

Figure 129. Typical Relationships between an RPG Program and Files on the AS/400 System
.1/	The program uses	the field-level description	of a file that is
	the operating system. An externally described file is identifi
	position 22 of the file description specifications. At compilat
	compiler copies in	the external field-level	description.

264 ILE RPG for AS/400 Programmer's Guide