Example of Some Typical Relationships between Programs and Files

Figure 1. Typical Relationships between an RPG Program and Files on the IBM i

1 : The program uses the field-level description of a file that is defined to the operating system. An externally described file is identified by an E in position 22 of the file description specifications. At compilation time, the compiler copies in the external field-level description.

2 : An externally described file (that is, a file with field-level external description) is used as a program-described file in the program. A program-described file is identified by an F in position 22 of the file description specifications. This entry tells the compiler not to copy in the external field-level descriptions. This file does not have to exist at compilation time.

3 : A file is described only at the record level to the operating system. The fields in the record are described within the program; therefore, position 22 of the file description specifications must contain an F. This file does not have to exist at compilation time.

4 : A file name can be specified at compilation time (that is, coded in the RPG source member), and a different file name can be specified at run time. The E in position 22 of the file description specifications indicates that the external description of the file is to be copied in at compilation time. At run time, a file override command can be used so that a different file is accessed by the program. To override a file at run time, you must make sure that record names in both files are the same. The RPG program uses the record-format name on the input/output operations, such as a READ operation where it specifies what record type is expected. See Overriding and Redirecting File Input and Output for more information.