COBOL by Example: Copybooks

COBOL 2002

Reusing code across programs with COPY statement, including external copybook files during compilation, maintaining consistent data structures, and managing shared record layouts centrally.

Code

       IDENTIFICATION DIVISION.
       PROGRAM-ID. COPY-DEMO.
       
       DATA DIVISION.
       WORKING-STORAGE SECTION.
           *> This line pulls in code from an external file
           COPY 'CUSTOMER-REC.CPY'.
       
       PROCEDURE DIVISION.
           MOVE 1001 TO CUST-ID.
           MOVE "Acme Corp" TO CUST-NAME.
           DISPLAY "Customer: " CUST-NAME.
           STOP RUN.

Explanation

One of COBOL's earliest forms of modularity is the Copybook. A copybook is a separate file containing a chunk of code—usually Data Division definitions—that can be included in multiple programs. This ensures that if a record layout changes, you only need to update one file and recompile all programs that use it.

The COPY statement acts like a preprocessor directive (similar to #include in C). During compilation, the compiler replaces the COPY line with the actual contents of the referenced file. This happens before the code is actually compiled into machine language.

Copybooks are not limited to variable definitions; they can also contain Procedure Division code, although this is less common. They are the standard way to enforce consistency across an organization's codebase, ensuring everyone uses the same field names and data types for shared data structures.

Code Breakdown

7

COPY 'CUSTOMER-REC.CPY'. The compiler looks for a file named 'CUSTOMER-REC.CPY' and inserts its text here.

10

MOVE 1001 TO CUST-ID. We can access CUST-ID even though it's not explicitly typed in this file, because it was defined inside the copybook.

11

MOVE "Acme Corp" TO CUST-NAME. Similarly, CUST-NAME is defined in the copybook.

Samplebadu

COBOL by Example: Copybooks

Code

Explanation

Code Breakdown