Basic File Handling: Sequential Files

COBOL Fundamentals

Chapter 8 · Basic File Handling: Sequential Files

Every real record processed so far has lived only in WORKING-STORAGE, gone the moment the program ends. Real COBOL programs exist to read and write actual files — and a sequential file is COBOL's own most basic, most common shape: records genuinely contiguous, read one after another from the front, "similarly to a linked list."

Three Places a File Touches a Program

A real COBOL file spans three separate divisions, each doing a genuinely different job:

Division	What It Declares
ENVIRONMENT DIVISION	Which real external file this program talks to (`SELECT`/`ASSIGN`)
DATA DIVISION	What one real record inside that file looks like (the `FD` and its record layout)
PROCEDURE DIVISION	The real verbs that actually move data in and out (`OPEN`, `READ`, `WRITE`, `CLOSE`)

SELECT & FD: Declaring the File

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTFILE.DAT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS WS-FILE-STATUS.

DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    LABEL RECORDS ARE STANDARD.
01  CUSTOMER-RECORD.
    05 CUST-ID       PIC 9(6).
    05 CUST-NAME     PIC X(30).
    05 CUST-BALANCE  PIC S9(7)V99.

This chapter's real CUSTOMER-RECORD deliberately mirrors Chapter 3's own WS-CUSTOMER-RECORD — a real record layout doesn't change shape just because it's now coming from a file instead of being built by hand in WORKING-STORAGE.

OPEN: Real Access Modes

Mode	What It Genuinely Allows
INPUT	Reading only, from a file that already exists
OUTPUT	Writing only; a real, existing file's contents are replaced entirely
I-O	Both reading and writing on the same open file (not valid for a real line-sequential file)
EXTEND	Writing new records appended after whatever a real file already contains

OPEN INPUT CUSTOMER-FILE.

READ ... AT END & the Real Priming-Read Pattern

A real READ pulls the next record in, and its AT END phrase fires the moment a read genuinely goes past the last record in the file:

READ CUSTOMER-FILE
    AT END MOVE "Y" TO WS-EOF-FLAG
END-READ.

Why a Single READ Inside a Loop's Condition Doesn't Work READ is a real statement, not an expression a loop condition can call directly. Chapter 5's own PERFORM UNTIL can't say "loop until the next READ hits end-of-file" in one step — it needs a genuine value to test. The standard real fix is the priming read: read once before the loop starts, then read again as the very last thing the loop body does, so WS-EOF-FLAG always reflects the most recent real read by the time the loop condition is checked again.

READ CUSTOMER-FILE
    AT END MOVE "Y" TO WS-EOF-FLAG
END-READ.

PERFORM UNTIL WS-EOF-FLAG = "Y"
    DISPLAY CUST-NAME
    READ CUSTOMER-FILE
        AT END MOVE "Y" TO WS-EOF-FLAG
    END-READ
END-PERFORM.

WRITE: Real Records, Not Files

OPEN OUTPUT CUSTOMER-FILE.
MOVE 100234 TO CUST-ID.
MOVE "SMITH, JOHN" TO CUST-NAME.
MOVE 500.00 TO CUST-BALANCE.
WRITE CUSTOMER-RECORD.

A Genuinely Easy Mistake WRITE always names the real 01-level record — CUSTOMER-RECORD — never the file itself. Writing WRITE CUSTOMER-FILE is a genuine compile error: COBOL writes whatever record is currently sitting in that record's own storage, and the file name never appears as the object being written.

CLOSE

CLOSE CUSTOMER-FILE.

A real, plain CLOSE releases the file — every file genuinely opened in a program needs a matching close before the program ends.

FILE STATUS: Real Result Codes

I/O status codes were a real COBOL-85 addition — the same standard that added Chapter 5's EVALUATE, Chapter 7's reference modification, and inline PERFORM. A real two-character FILE STATUS field, declared in WORKING-STORAGE and named in the file's own SELECT clause, is set after every single file operation:

Code	Real Meaning
00	Operation genuinely successful
10	End of file reached — the real code behind every `AT END` trigger
23	Record not found (indexed/relative files)
35	OPEN INPUT/I-O attempted against a file that genuinely doesn't exist

OPEN INPUT CUSTOMER-FILE.
IF WS-FILE-STATUS NOT = "00"
    DISPLAY "ERROR OPENING FILE: " WS-FILE-STATUS
    STOP RUN
END-IF.

Hands-On Exercises

Exercise 1

Using this chapter's own real material, explain in your own words why COBOL needs a priming read before a PERFORM UNTIL loop rather than simply writing PERFORM UNTIL the next READ reaches end-of-file directly in the loop's own condition.

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Exercise 2

Using this chapter's own real material, explain in your own words what would genuinely happen if a program opened CUSTOMER-FILE with OPEN OUTPUT when the goal was actually to read the file's existing records.

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Exercise 3

Using this chapter's own real material, explain in your own words why WS-FILE-STATUS being "00" after a READ statement genuinely can't be relied on forever within a real priming-read loop, and what real status value eventually replaces it.

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Chapter 8 Quick Reference

SELECT ... ASSIGN TO (ENVIRONMENT DIVISION) — names the real external file; FD (DATA DIVISION) — describes one real record's layout
OPEN INPUT/OUTPUT/I-O/EXTEND — real access modes; OUTPUT genuinely replaces existing content
READ ... AT END — fires once a read genuinely passes the last record; needs a real priming read before a PERFORM UNTIL loop
WRITE — always names the real 01-level record, never the file itself
CLOSE — every genuinely opened file needs a matching close
FILE STATUS (COBOL-85) — real 2-character result code after every file operation; 00 success, 10 end of file, 35 file not found on OPEN