Understanding %EOF in RPGLE File Processing

Learning Objective

By the end of this lesson, you will understand exactly what %EOF checks, and why it must be tested immediately after each READ for a read loop to work correctly.

Simple Explanation

In the Reading Physical Files in RPGLE with READ lesson, you saw %EOF used as a loop condition without a detailed explanation. %EOF (End of File) is a built-in function that reports whether the most recent input operation, such as READ, on a given file failed to find another record, meaning the end of the file has been reached.

dcl-f CUSTMAST disk;

read CUSTMAST;
dow not %eof(CUSTMAST);
  dsply CUSTNAME;

  read CUSTMAST;
enddo;

Here, %eof(CUSTMAST) becomes true the moment a READ on CUSTMAST fails to retrieve another record, which happens naturally once every record has already been read. This is exactly why READ is called both before the loop and again at the bottom: each READ attempt is immediately followed by a %EOF check, so the loop can correctly stop right when there is nothing left to read, rather than trying to process a record that was never actually retrieved.

Why It Matters

Without checking %EOF correctly, a read loop can either stop too early, skipping valid records, or try to process a record that does not actually exist once the file is exhausted. Understanding precisely what %EOF reports, and pairing it correctly with each READ, is what makes a basic file-processing loop reliable.

Practical Example

Imagine CUSTMAST contains three customer records. The program calls READ and retrieves the first record; %EOF is false, so the loop body runs, processing that record. This continues for the second and third records. After the third record is processed, the loop calls READ one more time, which fails to find a fourth record, and %EOF becomes true, correctly ending the loop without attempting to process a nonexistent fourth record.

This is a simplified, illustrative example rather than a specific real file, but it reflects exactly how %EOF governs a basic read loop's correct behavior.

Common Confusions

"Does %EOF check whether the file itself is empty?" Not directly. %EOF reports whether the most recent input operation on that file failed to find a record, which is true for an empty file from the very first READ, but also becomes true partway through a non-empty file once every record has already been retrieved.

"Do I need to specify which file %EOF is checking?" Generally yes, when a program works with more than one file, writing %eof(CUSTMAST) names the specific file being checked, since a program could otherwise be tracking end-of-file status for several different files at once.

"What happens if I check %EOF without a READ happening first?" %EOF reflects the result of the most recent input operation on that file. Checking it before any READ has actually been attempted does not give a meaningful result for that file's actual data.

Quick Recap

• %EOF reports whether the most recent input operation, such as READ, failed to find another record for the specified file.
• Calling READ before the loop and again at the bottom, checking %EOF each time, lets a loop stop exactly when there is nothing left to process.
• %EOF becoming true does not mean the file was empty; it means the most recent READ found no further record.
• Naming the file explicitly, such as %eof(CUSTMAST), matters once a program works with more than one file.

Try Asking the AI Tutor

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

• "What would happen if I forgot to check %EOF at all in a read loop?"
• "Does %EOF work the same way for CHAIN as it does for READ?"