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COBOLintermediate

REDEFINES Clause

Understand how REDEFINES lets multiple data descriptions share the same storage location and why this is both powerful and risky.

REDEFINES gives a second name and a second layout to the same bytes. No data is copied and nothing is converted.

REDEFINES is like a single sheet of graph paper viewed through two different stencils — one stencil reads it as a bar chart, the other as a table of numbers, but it's the same ink on the same paper the whole time.

Key Concepts

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    05  WS-DATE-NUM   PIC 9(8).
    05  WS-DATE-PARTS REDEFINES WS-DATE-NUM.
        10  WS-YEAR   PIC 9(4).
        10  WS-MONTH  PIC 9(2).
        10  WS-DAY    PIC 9(2).
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One eight-byte area. WS-DATE-NUM sees 20260315; WS-YEAR sees 2026. Write through one name and the other sees the change immediately, because there is only one piece of storage.
WS-DATE-NUMWS-YEAR
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It is a C union without the discriminator, and without any checking.
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The main use: one record, several layouts. A transaction file where the record type decides the structure:
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    01  TRANS-RECORD.
        05  TRANS-TYPE      PIC X(2).
        05  TRANS-BODY      PIC X(98).
        05  PAYMENT-DETAIL  REDEFINES TRANS-BODY.
            10  PAY-AMOUNT  PIC S9(9)V99 COMP-3.
            10  PAY-METHOD  PIC X(10).
            ...
        05  ADDRESS-DETAIL  REDEFINES TRANS-BODY.
            10  ADDR-LINE-1 PIC X(40).
            ...
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Read the type, then use the matching view. No copying, no second record area.
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Nothing enforces that you picked the right one. Read a payment record and look at ADDR-LINE-1 and you get characters — the packed-decimal amount interpreted as text. No error, because bytes are bytes.
ADDR-LINE-1
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The type code is the discriminator and the programmer is the only thing enforcing it.
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The standard question: a COMP-3 field viewed as PIC X.
    stored as COMP-3:   12 34 5C        (the value 12345)
    read as PIC X:      unprintable characters
PIC X
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People expect to see "12345". REDEFINES does not convert; it re-describes. To get a display value you must MOVE it to a display field, which is a conversion.
REDEFINESMOVE
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The rules. The redefining item must immediately follow the one it redefines, at the same level number. It may be shorter and, in most implementations, may not extend beyond the original's storage. It cannot carry a VALUE clause.
VALUE
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And the caution worth stating. REDEFINES is powerful and defeats the type checking COBOL otherwise gives you. Used for genuine multi-layout records it is the right tool; used to save a few bytes by overlaying unrelated fields, it produces code where changing one thing breaks another for reasons nobody can see.
REDEFINES
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What the interviewer is probing.1. "What does REDEFINES actually do?" Probing: the mechanism. Stalls: "Copies data." *Moves up:* it gives the same storage a second layout — no data is moved, so both names read the same bytes.
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2. "What is the legitimate use case?" Probing: judgement. Stalls: "Saving memory." *Moves up:* a record whose layout varies by a type code, so each variant is a redefinition read after checking the code.
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3. "What is the danger?" Probing: the hazard. Stalls: "None." Moves up: reading through the wrong redefinition gives garbage that looks like data, and packed fields read as display will abend.
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4. "What restriction applies to a REDEFINES entry?" Probing: the detail. Stalls: "None." Moves up: it must be at the same level as the item it redefines and must immediately follow it, and it cannot carry a VALUE clause.
Use cases:redefinesstorage-overlaymulti-format-recordsmemory-layout