Printer File vs Display File vs Database File

Learning Objective

By the end of this lesson, you will be able to clearly compare printer files, display files, and database files: what each one is for, and which RPGLE operations are typically used with each.

Simple Explanation

This lesson group, together with the earlier Display Files and native RPGLE File I/O lesson groups, has now covered all three major kinds of files an RPGLE program commonly works with. Bringing them together in one place:

• A database file, a physical or logical file, uses the device type disk and stores structured business data, read and changed with READ, CHAIN, WRITE, UPDATE, and DELETE, covered throughout the RPGLE File I/O lessons.
• A display file uses the device type workstn and describes an interactive 5250 screen, worked with primarily through exfmt, covered in the RPGLE Program Calling a Display File lesson.
• A printer file uses the device type printer and describes printed report output, worked with through WRITE, covered throughout this lesson group, though WRITE here prints a line rather than adding a stored record.

All three share the same underlying idea, an externally described *FILE object defined with DDS, but each serves a genuinely different purpose, and each pairs with different RPGLE operations that make sense for that purpose.

Why It Matters

Recognizing which kind of file a program is working with, from its device type on dcl-f, immediately tells you what to expect: whether WRITE means adding a stored record or printing a line, whether the file expects interactive user input, or whether it exists purely to hold structured data. This recognition is foundational for reading any RPGLE program quickly and correctly.

Practical Example

Imagine a single RPGLE program handling the daily customer balance report end to end. It declares CUSTMAST as a database file with disk, reading through it with native READ. It declares CUSTRPTP as a printer file with printer, writing heading, detail, and total record formats to it. If the same program also needed to show a summary screen to an operator afterward, it might additionally declare a display file with workstn, using exfmt to show that screen.

This is a simplified, illustrative example rather than a specific real program, but it reflects how a single RPGLE program can genuinely combine all three kinds of files, each used for exactly what it is meant for.

Common Confusions

"Since WRITE works on both database files and printer files, does it always do the same thing?" No. WRITE on a database file adds a new stored record, as covered in the Writing Records with WRITE lesson. WRITE on a printer file prints one line using the named record format, as covered in the RPGLE Program Writing to a Printer File lesson. Same operation name, genuinely different behavior, depending on the device type.

"Can a display file also produce a spool file, the way a printer file does?" No, not in the way covered in this lesson group. Spool files, covered in the Spool Files and Output Queues lesson, come from printer file output specifically; a display file's purpose is interactive screen communication, not generating report output.

"Do all three file types require DDS?" Yes, all three are commonly defined using DDS, describing stored fields for a database file, screen layout for a display file, or printed lines for a printer file. The DDS syntax family is shared; what it describes differs by file type.

Quick Recap

• Database files (disk) store structured business data, used with READ, CHAIN, WRITE, UPDATE, and DELETE.
• Display files (workstn) describe interactive 5250 screens, used primarily with exfmt.
• Printer files (printer) describe printed report output, used with WRITE to print a specific record format.
• All three are externally described *FILE objects defined with DDS, but each serves a distinct purpose with its own typical RPGLE operations.

Try Asking the AI Tutor

Use the AI Tutor to ask follow-up questions about this lesson. For example, try asking:

• "Could a single RPGLE program realistically use all three file types at once, and if so, why?"
• "Is there a fourth device type beyond disk, workstn, and printer that RPGLE programs commonly use?"