COBOL Tutorial

COBOL Sequential File Operations

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File verbs
Verb	Description	Example
OPEN INPUT	Open an existing file for reading.	`OPEN INPUT CUSTOMER-FILE.`
OPEN OUTPUT	Create a file for writing. An existing file is replaced.	`OPEN OUTPUT REPORT-FILE.`
OPEN I-O	Open an existing file for READ and REWRITE.	`OPEN I-O CUSTOMER-FILE.`
OPEN EXTEND	Open an existing file and write records after the last one.	`OPEN EXTEND LOG-FILE.`
READ	Read the next record. AT END runs when none remain.	`READ CUSTOMER-FILE AT END MOVE "Y" TO EOF-FLAG`
READ NEXT	On a sequential file, this is the same as READ.	`READ CUSTOMER-FILE NEXT.`
WRITE	Add a new record.	`WRITE CUSTOMER-RECORD FROM WORK-RECORD.`
REWRITE	Replace the record you just read. The file must be open I-O.	`REWRITE CUSTOMER-RECORD.`
CLOSE	Finish the file and release it.	`CLOSE CUSTOMER-FILE.`

OPEN Statement (INPUT, OUTPUT, I-O, EXTEND)

The OPEN statement establishes a connection between a COBOL program and a file, making it available for processing. It is the first operation performed on a file before any other file operations can be executed. The mode used in the OPEN statement determines what operations are permitted on the file.

Basic Syntax

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OPEN [INPUT file-name-1 [file-name-2] ...]
     [OUTPUT file-name-3 [file-name-4] ...]
     [I-O file-name-5 [file-name-6] ...]
     [EXTEND file-name-7 [file-name-8] ...]

Multiple files can be opened in a single OPEN statement, and different files can be opened with different modes in the same statement.

OPEN Modes

Mode	Description	Permitted Operations	File State
INPUT	Opens an existing file for reading	READ	File must exist
OUTPUT	Creates a new file for writing	WRITE	Any existing file is deleted
I-O	Opens an existing file for both reading and writing	READ, REWRITE, DELETE	File must exist
EXTEND	Opens an existing file for adding records at the end	WRITE (appends to end)	File must exist

Examples

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* Opening a single file for reading
OPEN INPUT CUSTOMER-FILE.
 
* Opening multiple files with different modes
OPEN INPUT  CUSTOMER-MASTER
     OUTPUT REPORT-FILE
     I-O     TRANSACTION-FILE.
 
* Opening a file for appending records
OPEN EXTEND LOG-FILE.
 
* Combined with file status checking
OPEN INPUT CUSTOMER-FILE
IF CUSTOMER-FILE-STATUS = "00"
   PERFORM PROCESS-CUSTOMERS
ELSE
   PERFORM FILE-ERROR-ROUTINE
END-IF.

Common Errors and Solutions

Error	Status Code	Cause	Solution
File not found	"35"	Opening a non-existent file with INPUT or I-O	Create file first or handle error
Access denied	"37"	Insufficient permissions	Fix file permissions
File already open	"41"	Attempting to open a file already opened	Close file before reopening
File attribute conflict	"39"	File characteristics don't match FD	Ensure file and FD match

Best Practices

Always check the file status after OPEN operations
Use the appropriate mode for the intended operations
Consider using I-O instead of INPUT if you might need to update the file
Be cautious with OUTPUT mode as it erases existing files
Use EXTEND for log files and other append-only scenarios
Open files only when needed and close them as soon as possible
Include error handling for all possible file status codes

READ Statement for Sequential Files

The READ statement retrieves the next record from a sequential file. Each successful READ advances the file position to the following record. For sequential files, records are read in the order they were written, from the beginning to the end of the file.

Basic Syntax

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READ file-name [NEXT] [RECORD] [INTO identifier]
     [AT END imperative-statement-1]
     [NOT AT END imperative-statement-2]
     [END-READ]

NEXT is optional for sequential files but required for indexed or relative files
RECORD is optional and has no functional effect
INTO transfers the record to a working-storage area
AT END specifies actions when the end of file is reached
NOT AT END specifies actions when a record is successfully read
END-READ terminates the scope of the READ statement

READ Variations

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* Basic READ
READ CUSTOMER-FILE
   AT END
      SET END-OF-FILE TO TRUE
END-READ.
 
* READ with both AT END and NOT AT END
READ EMPLOYEE-FILE
   AT END
      DISPLAY "End of file reached"
      MOVE "Y" TO EOF-FLAG
   NOT AT END
      ADD 1 TO RECORD-COUNT
      PERFORM PROCESS-EMPLOYEE
END-READ.
 
* READ INTO a working-storage area
READ TRANSACTION-FILE INTO TRANSACTION-WORK-RECORD
   AT END
      MOVE "Y" TO EOF-FLAG
END-READ.

The INTO clause is particularly useful when you want to keep the file record separate from your working data, allowing you to maintain the original record while modifying a copy.

Typical Processing Loop

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IDENTIFICATION DIVISION.
PROGRAM-ID. SEQREAD.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUST-ID         PIC 9(5).
    05  CUST-NAME       PIC X(30).
    05  CUST-ADDRESS    PIC X(40).
    05  CUST-TYPE       PIC X.
       88  RETAIL         VALUE "R".
       88  WHOLESALE      VALUE "W".
       88  GOVERNMENT     VALUE "G".
    05  FILLER          PIC X(4).
 
WORKING-STORAGE SECTION.
01  FILE-VARIABLES.
    05  CUSTOMER-STATUS PIC XX.
    05  EOF-FLAG        PIC X   VALUE "N".
       88  END-OF-FILE    VALUE "Y".
01  COUNTERS.
    05  RETAIL-COUNT    PIC 9(5) VALUE ZEROS.
    05  WHOLESALE-COUNT PIC 9(5) VALUE ZEROS.
    05  GOVERNMENT-COUNT PIC 9(5) VALUE ZEROS.
    05  TOTAL-COUNT     PIC 9(5) VALUE ZEROS.
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    PERFORM PROCESS-RECORDS UNTIL END-OF-FILE
    PERFORM FINALIZATION
    STOP RUN.
 
INITIALIZATION.
    OPEN INPUT CUSTOMER-FILE
    IF CUSTOMER-STATUS NOT = "00"
       DISPLAY "Error opening file: " CUSTOMER-STATUS
       MOVE "Y" TO EOF-FLAG
    END-IF.
 
PROCESS-RECORDS.
    READ CUSTOMER-FILE
       AT END
          MOVE "Y" TO EOF-FLAG
       NOT AT END
          ADD 1 TO TOTAL-COUNT
          EVALUATE TRUE
             WHEN RETAIL
                ADD 1 TO RETAIL-COUNT
             WHEN WHOLESALE
                ADD 1 TO WHOLESALE-COUNT
             WHEN GOVERNMENT
                ADD 1 TO GOVERNMENT-COUNT
          END-EVALUATE
    END-READ.
 
FINALIZATION.
    CLOSE CUSTOMER-FILE
    DISPLAY "Processing complete"
    DISPLAY "Total records:     " TOTAL-COUNT
    DISPLAY "Retail customers:  " RETAIL-COUNT
    DISPLAY "Wholesale customers: " WHOLESALE-COUNT
    DISPLAY "Government customers: " GOVERNMENT-COUNT.

This example demonstrates a typical sequential file processing pattern. It reads customer records, counts records by customer type, and displays totals when complete. The processing loop continues until the end of file is reached.

End of File Detection

There are two common methods to detect the end of a file:

Using the AT END clause: This method executes specific code when the end of file is reached. Typically, it sets a flag that controls the main processing loop.
Checking the file status: After each READ operation, check if the file status is "10" (end of file). This is especially useful when additional error conditions need to be handled.

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* Method 1: Using AT END
READ CUSTOMER-FILE
   AT END
      MOVE "Y" TO EOF-FLAG
END-READ.
 
* Method 2: Checking file status
READ CUSTOMER-FILE
IF CUSTOMER-STATUS = "10"
   MOVE "Y" TO EOF-FLAG
END-IF.
 
* Combined approach (most robust)
READ CUSTOMER-FILE
   AT END
      MOVE "Y" TO EOF-FLAG
END-READ
IF CUSTOMER-STATUS NOT = "00" AND CUSTOMER-STATUS NOT = "10"
   DISPLAY "Error reading file: " CUSTOMER-STATUS
   PERFORM ERROR-ROUTINE
END-IF.

Best Practices for READ Operations

Always include AT END handling with each READ statement
Use a flag (typically an 88-level condition) to control processing loops
Check file status after READ operations to detect errors
Use the NOT AT END clause for clearer code structure
Consider using READ...INTO when you need to preserve the original record
Never attempt to read past the end of file
Include proper error handling for all possible file status codes

WRITE Statement Basics

The WRITE statement adds new records to a file. For sequential files, each WRITE operation adds a record at the current position, which is typically at the end of the file. The position advances automatically after each WRITE, making records appear in the file in the same order they were written.

Basic Syntax

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WRITE record-name [FROM identifier-1]
      [AFTER|BEFORE ADVANCING {identifier-2 | integer | PAGE}]
      [AT END-OF-PAGE imperative-statement-1]
      [NOT AT END-OF-PAGE imperative-statement-2]
      [END-WRITE]

record-name must be a record name defined in the FILE SECTION
FROM specifies data to be moved to the record before writing
ADVANCING controls line spacing for print files
END-OF-PAGE applies to print files (reports)
END-WRITE terminates the scope of the WRITE statement

Writing Records - Examples

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* Basic WRITE
WRITE CUSTOMER-RECORD.
 
* WRITE with FROM clause
WRITE CUSTOMER-RECORD FROM CUSTOMER-WORK-AREA.
 
* WRITE with file status checking
WRITE EMPLOYEE-RECORD
IF FILE-STATUS NOT = "00"
   PERFORM ERROR-ROUTINE
END-IF.
 
* WRITE for print files with line control
WRITE REPORT-LINE AFTER ADVANCING 2 LINES.
WRITE REPORT-LINE BEFORE ADVANCING PAGE.
WRITE REPORT-LINE AFTER ADVANCING LINE-COUNT LINES.

The FROM clause is particularly useful as it allows you to write from a working-storage area while preserving the contents of the file record area for subsequent operations.

Complete Example: Creating a File

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IDENTIFICATION DIVISION.
PROGRAM-ID. FILEWRITE.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT OUTPUT-FILE ASSIGN TO "NEWDATA.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS FILE-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  OUTPUT-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  OUTPUT-RECORD.
    05  REC-TYPE        PIC X.
    05  REC-ID          PIC 9(5).
    05  REC-NAME        PIC X(30).
    05  REC-AMOUNT      PIC 9(7)V99.
    05  REC-DATE        PIC 9(8).
    05  FILLER          PIC X(28).
 
WORKING-STORAGE SECTION.
01  FILE-STATUS         PIC XX.
01  WS-RECORD.
    05  WS-TYPE         PIC X.
    05  WS-ID           PIC 9(5).
    05  WS-NAME         PIC X(30).
    05  WS-AMOUNT       PIC 9(7)V99.
    05  WS-DATE         PIC 9(8).
    05  FILLER          PIC X(28).
01  EOF-FLAG            PIC X VALUE "N".
    88  END-OF-DATA       VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    PERFORM PROCESS-RECORDS UNTIL END-OF-DATA
    PERFORM TERMINATION
    STOP RUN.
 
INITIALIZATION.
    OPEN OUTPUT OUTPUT-FILE
    IF FILE-STATUS NOT = "00"
       DISPLAY "Error opening output file: " FILE-STATUS
       MOVE "Y" TO EOF-FLAG
    END-IF.
 
PROCESS-RECORDS.
    DISPLAY "Enter record type (or X to exit): "
    ACCEPT WS-TYPE
    
    IF WS-TYPE = "X"
       MOVE "Y" TO EOF-FLAG
    ELSE
       DISPLAY "Enter ID: "
       ACCEPT WS-ID
       DISPLAY "Enter Name: "
       ACCEPT WS-NAME
       DISPLAY "Enter Amount: "
       ACCEPT WS-AMOUNT
       DISPLAY "Enter Date (YYYYMMDD): "
       ACCEPT WS-DATE
       
       WRITE OUTPUT-RECORD FROM WS-RECORD
       IF FILE-STATUS NOT = "00"
          DISPLAY "Error writing record: " FILE-STATUS
          PERFORM ERROR-ROUTINE
       END-IF
    END-IF.
 
TERMINATION.
    CLOSE OUTPUT-FILE
    DISPLAY "File processing complete.".
 
ERROR-ROUTINE.
    DISPLAY "File operation failed with status: " FILE-STATUS
    DISPLAY "Processing terminated."
    MOVE "Y" TO EOF-FLAG.

This example demonstrates creating a sequential file by accepting user input and writing records until the user enters 'X' for the record type. It shows proper file opening, writing with the FROM clause, status checking, and closing.

WRITE Operations for Different OPEN Modes

OPEN Mode	WRITE Behavior	Common Use Case
OUTPUT	Writes records to a new file, starting at the beginning	Creating new files, replacing existing files
EXTEND	Adds records to the end of an existing file	Appending to log files, adding to existing data
I-O	WRITE is not allowed for sequential files in I-O mode	N/A (Use REWRITE instead for updates)
INPUT	WRITE is not allowed in INPUT mode	N/A (Change to OUTPUT or EXTEND mode)

Error Handling for WRITE Operations

Status Code	Error Condition	Solution
"24"	Boundary violation (record size)	Check record size matches FD
"34"	Disk full or quota exceeded	Free disk space or increase quota
"48"	WRITE not supported in current open mode	Change OPEN mode to OUTPUT or EXTEND
"90"	Other I/O error during write	Check system logs, disk hardware

Best Practices for WRITE Operations

Check file status after every WRITE operation
Use the FROM clause when working with complex record structures
Validate data before writing to ensure data integrity
Ensure the file is opened in the appropriate mode (OUTPUT or EXTEND)
For large volumes, consider buffering multiple records before writing
Include proper error handling for disk-full and other error conditions
Test both successful writes and error conditions in your programs

CLOSE Statement

The CLOSE statement terminates the connection between a COBOL program and a file, releasing any resources associated with the file. It is important to properly close all files when processing is complete to ensure data integrity and prevent resource leaks.

Basic Syntax

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CLOSE file-name-1 [file-name-2] ... [WITH LOCK]

Multiple files can be closed in a single CLOSE statement
WITH LOCK prevents the file from being reopened in the same program run
No special positioning clauses are needed for sequential files

Examples

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* Closing a single file
CLOSE CUSTOMER-FILE.
 
* Closing multiple files
CLOSE CUSTOMER-FILE
      TRANSACTION-FILE
      REPORT-FILE.
 
* Closing with lock
CLOSE SENSITIVE-DATA-FILE WITH LOCK.
 
* Closing with file status checking
CLOSE INVENTORY-FILE
IF INVENTORY-STATUS NOT = "00"
   PERFORM CLOSE-ERROR-ROUTINE
END-IF.

File Closing Process

When a file is closed, the following actions typically occur:

Any remaining buffers are flushed to the physical device
File control blocks and buffers are released
End-of-file markers or trailers are written if required
System resources associated with the file are freed
The file can no longer be accessed without reopening it

Proper closing is essential, especially for output files, to ensure all data is written to the physical device and file integrity is maintained.

Error Handling for CLOSE Operations

While CLOSE operations typically succeed, some errors can occur:

Status Code	Error Condition	Possible Cause
"42"	File not open	Attempting to close a file that isn't open
"90-99"	Physical I/O error	Hardware issue during final write operations
"34"	Disk full	Not enough space to write final buffers

Best Practices for CLOSE Operations

Always close files when processing is complete
Check file status after CLOSE operations
Close files in error handling routines
Use CLOSE WITH LOCK for sensitive files
Include CLOSE operations in program termination logic
Ensure files are properly opened before attempting to close them
Consider using a standardized file handling module to ensure consistent file closing

Example: Proper File Handling Pattern

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IDENTIFICATION DIVISION.
PROGRAM-ID. FILEOPS.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT INPUT-FILE ASSIGN TO "INPUT.DAT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS INPUT-STATUS.
    SELECT OUTPUT-FILE ASSIGN TO "OUTPUT.DAT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS OUTPUT-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  INPUT-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  INPUT-RECORD         PIC X(80).
 
FD  OUTPUT-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  OUTPUT-RECORD        PIC X(80).
 
WORKING-STORAGE SECTION.
01  FILE-STATUSES.
    05  INPUT-STATUS     PIC XX.
    05  OUTPUT-STATUS    PIC XX.
01  EOF-FLAG             PIC X VALUE "N".
    88  END-OF-FILE        VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    IF INPUT-STATUS = "00" AND OUTPUT-STATUS = "00"
       PERFORM PROCESS-FILES UNTIL END-OF-FILE
    END-IF
    PERFORM TERMINATION
    STOP RUN.
 
INITIALIZATION.
    OPEN INPUT INPUT-FILE
    IF INPUT-STATUS NOT = "00"
       DISPLAY "Error opening input file: " INPUT-STATUS
       MOVE "Y" TO EOF-FLAG
    ELSE
       OPEN OUTPUT OUTPUT-FILE
       IF OUTPUT-STATUS NOT = "00"
          DISPLAY "Error opening output file: " OUTPUT-STATUS
          MOVE "Y" TO EOF-FLAG
          * Close the file that was successfully opened
          CLOSE INPUT-FILE
       END-IF
    END-IF.
 
PROCESS-FILES.
    READ INPUT-FILE
       AT END
          MOVE "Y" TO EOF-FLAG
       NOT AT END
          MOVE INPUT-RECORD TO OUTPUT-RECORD
          WRITE OUTPUT-RECORD
          IF OUTPUT-STATUS NOT = "00"
             DISPLAY "Error writing record: " OUTPUT-STATUS
             MOVE "Y" TO EOF-FLAG
          END-IF
    END-READ.
 
TERMINATION.
    * Close files regardless of processing status
    IF INPUT-STATUS NOT = "42" * Not "file not open"
       CLOSE INPUT-FILE
    END-IF
    
    IF OUTPUT-STATUS NOT = "42" * Not "file not open"
       CLOSE OUTPUT-FILE
    END-IF
    
    DISPLAY "Processing complete."
    DISPLAY "Input status: " INPUT-STATUS
    DISPLAY "Output status: " OUTPUT-STATUS.

This example demonstrates robust file handling with proper opening, error detection, and closing. Note that the termination routine closes files even if errors occurred during processing, ensuring that resources are properly released.

File Status Checking

File status checking is an essential aspect of COBOL file handling. The file status is a two-character field that contains the result of each file operation. By checking this status after each operation, programs can detect and handle errors or exceptional conditions.

Defining File Status

File status is defined in two places:

In the SELECT statement of the ENVIRONMENT DIVISION (FILE STATUS clause)
As a two-character field in WORKING-STORAGE

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ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS     PIC XX.
    88  STATUS-OK         VALUE "00".
    88  END-OF-FILE       VALUE "10".
    88  FILE-NOT-FOUND    VALUE "35".

Using 88-level condition names makes the code more readable when checking for specific status conditions.

Common File Status Codes

Status	Meaning	Operations
"00"	Successful completion	All operations
"10"	End of file reached	READ
"35"	File not found	OPEN INPUT, OPEN I-O
"37"	File access denied (permissions)	OPEN
"39"	File attribute mismatch	OPEN
"41"	File already open	OPEN
"42"	File not open	CLOSE, READ, WRITE
"46"	Read beyond file end	READ
"47"	Input/output error	OPEN, CLOSE, READ, WRITE
"48"	File not open in correct mode	READ, WRITE
"93"	Resource unavailable	OPEN

Comprehensive Status Checking

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IDENTIFICATION DIVISION.
PROGRAM-ID. STATUSCHK.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT DATA-FILE ASSIGN TO "DATA.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS DATA-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  DATA-FILE
    RECORD CONTAINS 100 CHARACTERS.
01  DATA-RECORD          PIC X(100).
 
WORKING-STORAGE SECTION.
01  FILE-VARIABLES.
    05  DATA-STATUS      PIC XX.
        88  STATUS-OK      VALUE "00".
        88  END-OF-FILE    VALUE "10".
        88  FILE-NOT-FOUND VALUE "35".
    05  EOF-FLAG         PIC X VALUE "N".
        88  AT-END         VALUE "Y".
    05  ERROR-FLAG       PIC X VALUE "N".
        88  ERROR-OCCURRED  VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZE-PROCESSING
    IF NOT ERROR-OCCURRED
       PERFORM PROCESS-DATA UNTIL AT-END OR ERROR-OCCURRED
    END-IF
    PERFORM TERMINATE-PROCESSING
    STOP RUN.
 
INITIALIZE-PROCESSING.
    PERFORM OPEN-DATA-FILE
    IF ERROR-OCCURRED
       DISPLAY "Cannot continue processing due to file error"
    END-IF.
 
PROCESS-DATA.
    PERFORM READ-DATA-RECORD
    IF NOT AT-END AND NOT ERROR-OCCURRED
       PERFORM PROCESS-RECORD
    END-IF.
 
TERMINATE-PROCESSING.
    PERFORM CLOSE-DATA-FILE
    DISPLAY "Processing complete".
 
OPEN-DATA-FILE.
    OPEN INPUT DATA-FILE
    PERFORM CHECK-FILE-STATUS.
 
READ-DATA-RECORD.
    READ DATA-FILE
       AT END SET AT-END TO TRUE
    END-READ
    PERFORM CHECK-FILE-STATUS.
 
CLOSE-DATA-FILE.
    CLOSE DATA-FILE
    PERFORM CHECK-FILE-STATUS.
 
PROCESS-RECORD.
    * Process the current record
    DISPLAY DATA-RECORD.
 
CHECK-FILE-STATUS.
    EVALUATE DATA-STATUS
       WHEN "00"
          * Operation successful - no action needed
          CONTINUE
       WHEN "10"
          * End of file - not an error for READ
          IF NOT AT-END
             SET AT-END TO TRUE
          END-IF
       WHEN "35"
          DISPLAY "File not found"
          SET ERROR-OCCURRED TO TRUE
       WHEN "37"
          DISPLAY "Access denied - check permissions"
          SET ERROR-OCCURRED TO TRUE
       WHEN "39"
          DISPLAY "Attribute mismatch - check file definition"
          SET ERROR-OCCURRED TO TRUE
       WHEN "41"
          DISPLAY "File already open"
          SET ERROR-OCCURRED TO TRUE
       WHEN "42"
          DISPLAY "File not open"
          SET ERROR-OCCURRED TO TRUE
       WHEN "46"
          DISPLAY "Read beyond end of file"
          SET ERROR-OCCURRED TO TRUE
       WHEN "47"
          DISPLAY "I/O error - check hardware"
          SET ERROR-OCCURRED TO TRUE
       WHEN "48"
          DISPLAY "File not open in correct mode"
          SET ERROR-OCCURRED TO TRUE
       WHEN OTHER
          DISPLAY "Unexpected file status: " DATA-STATUS
          SET ERROR-OCCURRED TO TRUE
    END-EVALUATE.

This example demonstrates a comprehensive approach to file status checking. A centralized CHECK-FILE-STATUS routine examines the status after each file operation and takes appropriate action based on the specific status code.

Extended File Status

Some COBOL implementations support an extended file status, which provides more detailed information about file operations. The extended status typically consists of:

Two-character primary status (standard file status)
Additional characters or fields with implementation-specific details

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ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE
        ASSIGN TO CUSTFILE
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
* Standard file status
01  CUSTOMER-STATUS.
    05  PRIMARY-STATUS    PIC XX.
    05  SECONDARY-STATUS  PIC XXX.
 
* Example usage
IF PRIMARY-STATUS NOT = "00"
   DISPLAY "Error: " PRIMARY-STATUS "-" SECONDARY-STATUS
END-IF.

The exact format of extended file status varies by compiler. Check your compiler documentation for specific details about extended status codes.

Best Practices for File Status Checking

Check file status after every file operation
Use 88-level condition names for commonly checked statuses
Implement a centralized status checking routine
Log detailed error information including operation type and file name
Distinguish between expected conditions (like end-of-file) and true errors
Include error recovery procedures where appropriate
Close files properly even after errors
Consider using extended file status for more detailed error information
Document the meaning of status codes in program comments

File Status Logging Example

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IDENTIFICATION DIVISION.
PROGRAM-ID. FILELOG.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT DATA-FILE ASSIGN TO "DATA.DAT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS DATA-STATUS.
    
    SELECT LOG-FILE ASSIGN TO "FILELOG.TXT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS LOG-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  DATA-FILE
    RECORD CONTAINS 100 CHARACTERS.
01  DATA-RECORD          PIC X(100).
 
FD  LOG-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  LOG-RECORD           PIC X(80).
 
WORKING-STORAGE SECTION.
01  FILE-VARIABLES.
    05  DATA-STATUS      PIC XX.
    05  LOG-STATUS       PIC XX.
    05  CURRENT-DATE-TIME.
        10  CURRENT-DATE.
            15  CURRENT-YEAR   PIC 9(4).
            15  CURRENT-MONTH  PIC 9(2).
            15  CURRENT-DAY    PIC 9(2).
        10  CURRENT-TIME.
            15  CURRENT-HOUR   PIC 9(2).
            15  CURRENT-MINUTE PIC 9(2).
            15  CURRENT-SECOND PIC 9(2).
            15  CURRENT-MS     PIC 9(2).
    05  TIMESTAMP        PIC X(19).
    05  MESSAGE          PIC X(60).
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    OPEN OUTPUT LOG-FILE
    IF LOG-STATUS NOT = "00"
       DISPLAY "Could not open log file: " LOG-STATUS
    ELSE
       * Now try to work with the data file
       PERFORM LOG-MESSAGE "Starting file processing"
       
       OPEN INPUT DATA-FILE
       IF DATA-STATUS NOT = "00"
          PERFORM LOG-ERROR "OPEN" "DATA-FILE" DATA-STATUS
       ELSE
          PERFORM PROCESS-FILE
          CLOSE DATA-FILE
          PERFORM LOG-FILE-STATUS "CLOSE" "DATA-FILE" DATA-STATUS
       END-IF
       
       PERFORM LOG-MESSAGE "Processing complete"
       CLOSE LOG-FILE
    END-IF
    STOP RUN.
 
PROCESS-FILE.
    PERFORM LOG-MESSAGE "Reading records"
    PERFORM UNTIL DATA-STATUS NOT = "00"
       READ DATA-FILE
          AT END EXIT PERFORM
       END-READ
       
       IF DATA-STATUS = "00"
          * Process the record
          CONTINUE
       ELSE IF DATA-STATUS NOT = "10"
          PERFORM LOG-ERROR "READ" "DATA-FILE" DATA-STATUS
          EXIT PERFORM
       END-IF
    END-PERFORM.
 
LOG-FILE-STATUS.
    * Parameters: operation, file-name, status
    MOVE FUNCTION CURRENT-DATE TO CURRENT-DATE-TIME
    STRING CURRENT-YEAR "-" CURRENT-MONTH "-" CURRENT-DAY " "
           CURRENT-HOUR ":" CURRENT-MINUTE ":" CURRENT-SECOND
       INTO TIMESTAMP
    
    STRING TIMESTAMP " " FUNCTION TRIM(FUNCTION-ID-1) " "
           FUNCTION TRIM(FUNCTION-ID-2) " Status: " FUNCTION-ID-3
       INTO MESSAGE
    
    PERFORM WRITE-LOG-RECORD.
 
LOG-ERROR.
    * Parameters: operation, file-name, status
    MOVE FUNCTION CURRENT-DATE TO CURRENT-DATE-TIME
    STRING CURRENT-YEAR "-" CURRENT-MONTH "-" CURRENT-DAY " "
           CURRENT-HOUR ":" CURRENT-MINUTE ":" CURRENT-SECOND
       INTO TIMESTAMP
    
    STRING TIMESTAMP " ERROR: " FUNCTION TRIM(FUNCTION-ID-1) " "
           FUNCTION TRIM(FUNCTION-ID-2) " Failed with status: " 
           FUNCTION-ID-3
       INTO MESSAGE
    
    PERFORM WRITE-LOG-RECORD.
 
LOG-MESSAGE.
    * Parameters: message-text
    MOVE FUNCTION CURRENT-DATE TO CURRENT-DATE-TIME
    STRING CURRENT-YEAR "-" CURRENT-MONTH "-" CURRENT-DAY " "
           CURRENT-HOUR ":" CURRENT-MINUTE ":" CURRENT-SECOND
       INTO TIMESTAMP
    
    STRING TIMESTAMP " " FUNCTION TRIM(FUNCTION-ID-1)
       INTO MESSAGE
    
    PERFORM WRITE-LOG-RECORD.
 
WRITE-LOG-RECORD.
    MOVE MESSAGE TO LOG-RECORD
    WRITE LOG-RECORD
    IF LOG-STATUS NOT = "00"
       DISPLAY "Error writing to log: " LOG-STATUS
    END-IF.

This example demonstrates a more advanced approach with a dedicated logging system for file operations. It creates timestamped log entries for each file operation, including the operation type, file name, and status code, providing a detailed audit trail of file processing.

SELECT and ASSIGN

OPEN, READ, and WRITE use the name from SELECT. They do not use the dataset name. SELECT CUST-FILE ASSIGN TO CUSTFILE means the program calls the file CUST-FILE, and the system looks up CUSTFILE. On z/OS that name is the DD statement in the JCL. In GnuCOBOL the same spot is usually a file name in quotes, such as ASSIGN TO "CUSTOMER.DAT".

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       FILE-CONTROL.
           SELECT CUST-FILE ASSIGN TO CUSTFILE
               ORGANIZATION IS SEQUENTIAL
               ACCESS MODE IS SEQUENTIAL
               FILE STATUS IS CUST-STAT.

The name in OPEN has to be the SELECT name. If CUSTFILE has no matching DD, or the quoted file name does not exist when you OPEN INPUT, the file status is 35.

ORGANIZATION IS SEQUENTIAL

ORGANIZATION IS SEQUENTIAL means the records stay in the order they were written. The first WRITE is the first record. The next READ returns the next record. There is no key. If you leave the clause out, Enterprise COBOL and GnuCOBOL both treat the file as sequential.

These are the organization values you will see on a SELECT:

SEQUENTIAL. Records stored in the order written. This is the z/OS form, and the organization used by the OPEN, READ, and WRITE sections above.
LINE SEQUENTIAL. A text file. Each record is a line ended by a newline, and trailing spaces are dropped on WRITE. Covered in the next section.
INDEXED. Records stored by a key. Use this when you need READ by key, and see the indexed file page for the statements.
RELATIVE. Records stored by a relative record number.

ACCESS MODE IS SEQUENTIAL

ACCESS MODE IS SEQUENTIAL means each READ and each WRITE moves one record forward from the current position. You cannot ask for a particular record. A file with ORGANIZATION IS SEQUENTIAL uses this mode even if you omit the clause.

SEQUENTIAL. The next record, every time. This is the only mode a sequential file can use.
RANDOM. One record, chosen by key. Indexed and relative files only.
DYNAMIC. A key read or a READ NEXT, in the same program. Indexed and relative files only.

LINE SEQUENTIAL

ORGANIZATION IS LINE SEQUENTIAL describes a text file. The record is one line. A newline marks the end. On WRITE, trailing spaces in the record are not stored. On READ, the newline is removed and the rest of the FD record is padded with spaces.

GnuCOBOL uses this organization, and so does IBM COBOL for Linux, AIX, and Windows. Enterprise COBOL for z/OS does not. A sequential dataset on z/OS is ORGANIZATION IS SEQUENTIAL. The record length is the length in the FD, and the file does not store a newline after each record.

WRITE AFTER ADVANCING PAGE on a line sequential file writes a form-feed character and then the line. On a z/OS print file the same phrase skips to the next page through carriage control. OPEN EXTEND still means append. If the file is missing and the SELECT does not say OPTIONAL, the status is 35.

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           SELECT REPORT-FILE ASSIGN TO "REPORT.TXT"
               ORGANIZATION IS LINE SEQUENTIAL
               ACCESS MODE IS SEQUENTIAL
               FILE STATUS IS REPORT-STAT.

OPEN INPUT

OPEN INPUT opens an existing file for READ. The current position is the first record. WRITE and REWRITE are not allowed. If you issue them, the file status is 48.

The file has to be there. Status 35 means it was not found. Put OPTIONAL in front of the file name on the SELECT when a missing file should be treated as empty. OPEN INPUT then returns status 05, and the first READ hits AT END with status 10.

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           SELECT OPTIONAL CUST-FILE ASSIGN TO CUSTFILE
               ORGANIZATION IS SEQUENTIAL
               FILE STATUS IS CUST-STAT.
      *
           OPEN INPUT CUST-FILE
           IF CUST-STAT = "35"
              DISPLAY "FILE NOT FOUND"
           END-IF

OPEN OUTPUT

OPEN OUTPUT starts a new file for WRITE. If a file with that name already exists, the records that were in it are discarded. READ is not allowed. On z/OS the ASSIGN name is still the DD name. DISP on that DD controls whether the dataset itself is new or reused. The COBOL rule is the same either way: the first WRITE goes at the start of the file.

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           OPEN OUTPUT CUST-FILE
           IF CUST-STAT NOT = "00"
              DISPLAY "OPEN OUTPUT FAILED " CUST-STAT
              STOP RUN
           END-IF
           WRITE CUST-REC FROM WORK-REC
           CLOSE CUST-FILE

OPEN EXTEND

OPEN EXTEND opens an existing file and positions after the last record. WRITE appends. READ is not allowed. This is the mode for a log, or for adding a batch of records to a file you must not empty.

The file must already exist, unless the SELECT says OPTIONAL. A missing file that is not optional returns status 35. OPTIONAL plus OPEN EXTEND creates the file, and the status on that open is 05. OUTPUT would have thrown the old records away. EXTEND keeps them.

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           OPEN EXTEND LOG-FILE
           IF LOG-STAT = "35"
              DISPLAY "LOG FILE MISSING"
              STOP RUN
           END-IF
           IF LOG-STAT = "00" OR "05"
              WRITE LOG-REC FROM LOG-LINE
           END-IF
           CLOSE LOG-FILE

REWRITE

REWRITE replaces the record you just read. The file has to be open I-O, and the last successful statement on that file has to be a READ. Change the record area, or use FROM to take the new value from working storage, then REWRITE the FD record name. You rewrite the record you just read. You do not search for a different one.

Status 43 means there is no current record, usually because the READ failed or you never issued one. Status 48 means the file is not open in a mode that allows the update. Status 44 means the rewritten record length is not allowed. Sequential organization has no DELETE. DELETE is for indexed and relative files.

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           OPEN I-O CUST-FILE
           READ CUST-FILE
              AT END
                 CONTINUE
           END-READ
           IF CUST-STAT = "00"
              MOVE NEW-BAL TO CUST-BAL
              REWRITE CUST-REC
           END-IF
           CLOSE CUST-FILE

Sequential file example

COBOL file handling for a sequential file is the same four steps every time. Name the file in SELECT, describe the record under an FD, OPEN it, then READ or WRITE until you CLOSE. This program reads a sequential file with OPEN INPUT and the READ statement.

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       IDENTIFICATION DIVISION.
       PROGRAM-ID. READSEQ.
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT CUST-FILE ASSIGN TO CUSTFILE
               ORGANIZATION IS SEQUENTIAL
               ACCESS MODE IS SEQUENTIAL
               FILE STATUS IS CUST-STAT.
       DATA DIVISION.
       FILE SECTION.
       FD  CUST-FILE
           RECORD CONTAINS 29 CHARACTERS.
       01  CUST-REC.
           05  CUST-NAME    PIC X(20).
           05  CUST-BAL     PIC 9(7)V99.
       WORKING-STORAGE SECTION.
       01  CUST-STAT        PIC XX.
       PROCEDURE DIVISION.
           OPEN INPUT CUST-FILE
           IF CUST-STAT NOT = "00"
              DISPLAY "OPEN FAILED " CUST-STAT
              STOP RUN
           END-IF
           PERFORM UNTIL CUST-STAT = "10"
              READ CUST-FILE NEXT
                 AT END
                    CONTINUE
                 NOT AT END
                    DISPLAY CUST-NAME " " CUST-BAL
              END-READ
           END-PERFORM
           CLOSE CUST-FILE
           STOP RUN.

Explain it like I'm 5

A sequential file is a stack of cards. OPEN INPUT lets you look at the next card, then the next, until the stack is empty. OPEN OUTPUT throws the stack away and you write new cards from the top. OPEN EXTEND keeps the stack and you add a card at the bottom. REWRITE means you looked at a card, changed what was written on it, and put that same card back in the same place.

Exercises

Write the SELECT

Write the FILE-CONTROL entry for a sequential customer file. The program name is CUST-FILE, the DD name is CUSTFILE, access is sequential, and the status field is CUST-STAT.

The entry is SELECT CUST-FILE ASSIGN TO CUSTFILE, then ORGANIZATION IS SEQUENTIAL, ACCESS MODE IS SEQUENTIAL, and FILE STATUS IS CUST-STAT.

Append one log line

Open a log file so the old lines stay, write one record, and decide what status 35 means.

Use OPEN EXTEND. Status 35 means the file was not found. Add OPTIONAL to the SELECT if a missing log should be created, and expect status 05 on that open.

Change a balance

You have read a customer record and you need to store a new balance in that same record. Which OPEN mode do you use, and which statement stores it?

OPEN I-O, then REWRITE after the READ. OPEN OUTPUT would wipe the file. OPEN EXTEND would add a second record and leave the old balance where it was.

Test Your Knowledge

1. Which OPEN mode allows both reading and writing to an existing file?

INPUT
OUTPUT
I-O
EXTEND

2. What happens when you open a file with OUTPUT mode that already exists?

The file is opened for reading
The existing file is deleted and a new empty file is created
The existing contents are preserved and new records are appended
An error occurs and the file remains unchanged

3. What is the purpose of the AT END clause in a READ statement?

To specify what happens when no more records exist in the file
To limit the number of records read
To skip to the end of the file
To read only the last record

4. What is the correct sequence of operations for processing a sequential file?

OPEN, WRITE, CLOSE
OPEN, READ, PROCESS, CLOSE
READ, PROCESS, WRITE, CLOSE
WRITE, OPEN, PROCESS, CLOSE

5. What is the purpose of checking the file status after file operations?

To improve performance
To detect and handle errors or exceptional conditions
To count the number of records processed
To determine the file size

6. On a sequential file, what does READ NEXT do?

It reads the previous record
It reads the next record, the same as READ
It reads a record by key
It appends a record

7. Which OPEN mode adds records after the last record already in the file?

INPUT
OUTPUT
EXTEND
I-O

Frequently Asked Questions

Related Concepts

File Organization Types

Understanding different file organizations in COBOL.

File Description (FD)

Learn about describing file structures in the DATA DIVISION.

Indexed File Operations

Working with indexed files for random access.

Exception Handling

Handling errors and exceptions in COBOL.

File Status and Error Handling

Comprehensive approach to file errors.

File Description (FD)Indexed File Operations

COBOL Tutorial

COBOL Sequential File Operations

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File verbs
Verb	Description	Example
OPEN INPUT	Open an existing file for reading.	`OPEN INPUT CUSTOMER-FILE.`
OPEN OUTPUT	Create a file for writing. An existing file is replaced.	`OPEN OUTPUT REPORT-FILE.`
OPEN I-O	Open an existing file for READ and REWRITE.	`OPEN I-O CUSTOMER-FILE.`
OPEN EXTEND	Open an existing file and write records after the last one.	`OPEN EXTEND LOG-FILE.`
READ	Read the next record. AT END runs when none remain.	`READ CUSTOMER-FILE AT END MOVE "Y" TO EOF-FLAG`
READ NEXT	On a sequential file, this is the same as READ.	`READ CUSTOMER-FILE NEXT.`
WRITE	Add a new record.	`WRITE CUSTOMER-RECORD FROM WORK-RECORD.`
REWRITE	Replace the record you just read. The file must be open I-O.	`REWRITE CUSTOMER-RECORD.`
CLOSE	Finish the file and release it.	`CLOSE CUSTOMER-FILE.`

OPEN Statement (INPUT, OUTPUT, I-O, EXTEND)

Basic Syntax

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OPEN [INPUT file-name-1 [file-name-2] ...]
     [OUTPUT file-name-3 [file-name-4] ...]
     [I-O file-name-5 [file-name-6] ...]
     [EXTEND file-name-7 [file-name-8] ...]

Multiple files can be opened in a single OPEN statement, and different files can be opened with different modes in the same statement.

OPEN Modes

Mode	Description	Permitted Operations	File State
INPUT	Opens an existing file for reading	READ	File must exist
OUTPUT	Creates a new file for writing	WRITE	Any existing file is deleted
I-O	Opens an existing file for both reading and writing	READ, REWRITE, DELETE	File must exist
EXTEND	Opens an existing file for adding records at the end	WRITE (appends to end)	File must exist

Examples

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* Opening a single file for reading
OPEN INPUT CUSTOMER-FILE.
 
* Opening multiple files with different modes
OPEN INPUT  CUSTOMER-MASTER
     OUTPUT REPORT-FILE
     I-O     TRANSACTION-FILE.
 
* Opening a file for appending records
OPEN EXTEND LOG-FILE.
 
* Combined with file status checking
OPEN INPUT CUSTOMER-FILE
IF CUSTOMER-FILE-STATUS = "00"
   PERFORM PROCESS-CUSTOMERS
ELSE
   PERFORM FILE-ERROR-ROUTINE
END-IF.

Common Errors and Solutions

Error	Status Code	Cause	Solution
File not found	"35"	Opening a non-existent file with INPUT or I-O	Create file first or handle error
Access denied	"37"	Insufficient permissions	Fix file permissions
File already open	"41"	Attempting to open a file already opened	Close file before reopening
File attribute conflict	"39"	File characteristics don't match FD	Ensure file and FD match

Best Practices

Always check the file status after OPEN operations
Use the appropriate mode for the intended operations
Consider using I-O instead of INPUT if you might need to update the file
Be cautious with OUTPUT mode as it erases existing files
Use EXTEND for log files and other append-only scenarios
Open files only when needed and close them as soon as possible
Include error handling for all possible file status codes

READ Statement for Sequential Files

Basic Syntax

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READ file-name [NEXT] [RECORD] [INTO identifier]
     [AT END imperative-statement-1]
     [NOT AT END imperative-statement-2]
     [END-READ]

NEXT is optional for sequential files but required for indexed or relative files
RECORD is optional and has no functional effect
INTO transfers the record to a working-storage area
AT END specifies actions when the end of file is reached
NOT AT END specifies actions when a record is successfully read
END-READ terminates the scope of the READ statement

READ Variations

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* Basic READ
READ CUSTOMER-FILE
   AT END
      SET END-OF-FILE TO TRUE
END-READ.
 
* READ with both AT END and NOT AT END
READ EMPLOYEE-FILE
   AT END
      DISPLAY "End of file reached"
      MOVE "Y" TO EOF-FLAG
   NOT AT END
      ADD 1 TO RECORD-COUNT
      PERFORM PROCESS-EMPLOYEE
END-READ.
 
* READ INTO a working-storage area
READ TRANSACTION-FILE INTO TRANSACTION-WORK-RECORD
   AT END
      MOVE "Y" TO EOF-FLAG
END-READ.

The INTO clause is particularly useful when you want to keep the file record separate from your working data, allowing you to maintain the original record while modifying a copy.

Typical Processing Loop

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IDENTIFICATION DIVISION.
PROGRAM-ID. SEQREAD.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  CUSTOMER-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  CUSTOMER-RECORD.
    05  CUST-ID         PIC 9(5).
    05  CUST-NAME       PIC X(30).
    05  CUST-ADDRESS    PIC X(40).
    05  CUST-TYPE       PIC X.
       88  RETAIL         VALUE "R".
       88  WHOLESALE      VALUE "W".
       88  GOVERNMENT     VALUE "G".
    05  FILLER          PIC X(4).
 
WORKING-STORAGE SECTION.
01  FILE-VARIABLES.
    05  CUSTOMER-STATUS PIC XX.
    05  EOF-FLAG        PIC X   VALUE "N".
       88  END-OF-FILE    VALUE "Y".
01  COUNTERS.
    05  RETAIL-COUNT    PIC 9(5) VALUE ZEROS.
    05  WHOLESALE-COUNT PIC 9(5) VALUE ZEROS.
    05  GOVERNMENT-COUNT PIC 9(5) VALUE ZEROS.
    05  TOTAL-COUNT     PIC 9(5) VALUE ZEROS.
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    PERFORM PROCESS-RECORDS UNTIL END-OF-FILE
    PERFORM FINALIZATION
    STOP RUN.
 
INITIALIZATION.
    OPEN INPUT CUSTOMER-FILE
    IF CUSTOMER-STATUS NOT = "00"
       DISPLAY "Error opening file: " CUSTOMER-STATUS
       MOVE "Y" TO EOF-FLAG
    END-IF.
 
PROCESS-RECORDS.
    READ CUSTOMER-FILE
       AT END
          MOVE "Y" TO EOF-FLAG
       NOT AT END
          ADD 1 TO TOTAL-COUNT
          EVALUATE TRUE
             WHEN RETAIL
                ADD 1 TO RETAIL-COUNT
             WHEN WHOLESALE
                ADD 1 TO WHOLESALE-COUNT
             WHEN GOVERNMENT
                ADD 1 TO GOVERNMENT-COUNT
          END-EVALUATE
    END-READ.
 
FINALIZATION.
    CLOSE CUSTOMER-FILE
    DISPLAY "Processing complete"
    DISPLAY "Total records:     " TOTAL-COUNT
    DISPLAY "Retail customers:  " RETAIL-COUNT
    DISPLAY "Wholesale customers: " WHOLESALE-COUNT
    DISPLAY "Government customers: " GOVERNMENT-COUNT.

End of File Detection

There are two common methods to detect the end of a file:

Using the AT END clause: This method executes specific code when the end of file is reached. Typically, it sets a flag that controls the main processing loop.
Checking the file status: After each READ operation, check if the file status is "10" (end of file). This is especially useful when additional error conditions need to be handled.

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* Method 1: Using AT END
READ CUSTOMER-FILE
   AT END
      MOVE "Y" TO EOF-FLAG
END-READ.
 
* Method 2: Checking file status
READ CUSTOMER-FILE
IF CUSTOMER-STATUS = "10"
   MOVE "Y" TO EOF-FLAG
END-IF.
 
* Combined approach (most robust)
READ CUSTOMER-FILE
   AT END
      MOVE "Y" TO EOF-FLAG
END-READ
IF CUSTOMER-STATUS NOT = "00" AND CUSTOMER-STATUS NOT = "10"
   DISPLAY "Error reading file: " CUSTOMER-STATUS
   PERFORM ERROR-ROUTINE
END-IF.

Best Practices for READ Operations

Always include AT END handling with each READ statement
Use a flag (typically an 88-level condition) to control processing loops
Check file status after READ operations to detect errors
Use the NOT AT END clause for clearer code structure
Consider using READ...INTO when you need to preserve the original record
Never attempt to read past the end of file
Include proper error handling for all possible file status codes

WRITE Statement Basics

Basic Syntax

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WRITE record-name [FROM identifier-1]
      [AFTER|BEFORE ADVANCING {identifier-2 | integer | PAGE}]
      [AT END-OF-PAGE imperative-statement-1]
      [NOT AT END-OF-PAGE imperative-statement-2]
      [END-WRITE]

record-name must be a record name defined in the FILE SECTION
FROM specifies data to be moved to the record before writing
ADVANCING controls line spacing for print files
END-OF-PAGE applies to print files (reports)
END-WRITE terminates the scope of the WRITE statement

Writing Records - Examples

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* Basic WRITE
WRITE CUSTOMER-RECORD.
 
* WRITE with FROM clause
WRITE CUSTOMER-RECORD FROM CUSTOMER-WORK-AREA.
 
* WRITE with file status checking
WRITE EMPLOYEE-RECORD
IF FILE-STATUS NOT = "00"
   PERFORM ERROR-ROUTINE
END-IF.
 
* WRITE for print files with line control
WRITE REPORT-LINE AFTER ADVANCING 2 LINES.
WRITE REPORT-LINE BEFORE ADVANCING PAGE.
WRITE REPORT-LINE AFTER ADVANCING LINE-COUNT LINES.

The FROM clause is particularly useful as it allows you to write from a working-storage area while preserving the contents of the file record area for subsequent operations.

Complete Example: Creating a File

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IDENTIFICATION DIVISION.
PROGRAM-ID. FILEWRITE.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT OUTPUT-FILE ASSIGN TO "NEWDATA.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS FILE-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  OUTPUT-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  OUTPUT-RECORD.
    05  REC-TYPE        PIC X.
    05  REC-ID          PIC 9(5).
    05  REC-NAME        PIC X(30).
    05  REC-AMOUNT      PIC 9(7)V99.
    05  REC-DATE        PIC 9(8).
    05  FILLER          PIC X(28).
 
WORKING-STORAGE SECTION.
01  FILE-STATUS         PIC XX.
01  WS-RECORD.
    05  WS-TYPE         PIC X.
    05  WS-ID           PIC 9(5).
    05  WS-NAME         PIC X(30).
    05  WS-AMOUNT       PIC 9(7)V99.
    05  WS-DATE         PIC 9(8).
    05  FILLER          PIC X(28).
01  EOF-FLAG            PIC X VALUE "N".
    88  END-OF-DATA       VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    PERFORM PROCESS-RECORDS UNTIL END-OF-DATA
    PERFORM TERMINATION
    STOP RUN.
 
INITIALIZATION.
    OPEN OUTPUT OUTPUT-FILE
    IF FILE-STATUS NOT = "00"
       DISPLAY "Error opening output file: " FILE-STATUS
       MOVE "Y" TO EOF-FLAG
    END-IF.
 
PROCESS-RECORDS.
    DISPLAY "Enter record type (or X to exit): "
    ACCEPT WS-TYPE
    
    IF WS-TYPE = "X"
       MOVE "Y" TO EOF-FLAG
    ELSE
       DISPLAY "Enter ID: "
       ACCEPT WS-ID
       DISPLAY "Enter Name: "
       ACCEPT WS-NAME
       DISPLAY "Enter Amount: "
       ACCEPT WS-AMOUNT
       DISPLAY "Enter Date (YYYYMMDD): "
       ACCEPT WS-DATE
       
       WRITE OUTPUT-RECORD FROM WS-RECORD
       IF FILE-STATUS NOT = "00"
          DISPLAY "Error writing record: " FILE-STATUS
          PERFORM ERROR-ROUTINE
       END-IF
    END-IF.
 
TERMINATION.
    CLOSE OUTPUT-FILE
    DISPLAY "File processing complete.".
 
ERROR-ROUTINE.
    DISPLAY "File operation failed with status: " FILE-STATUS
    DISPLAY "Processing terminated."
    MOVE "Y" TO EOF-FLAG.

WRITE Operations for Different OPEN Modes

OPEN Mode	WRITE Behavior	Common Use Case
OUTPUT	Writes records to a new file, starting at the beginning	Creating new files, replacing existing files
EXTEND	Adds records to the end of an existing file	Appending to log files, adding to existing data
I-O	WRITE is not allowed for sequential files in I-O mode	N/A (Use REWRITE instead for updates)
INPUT	WRITE is not allowed in INPUT mode	N/A (Change to OUTPUT or EXTEND mode)

Error Handling for WRITE Operations

Status Code	Error Condition	Solution
"24"	Boundary violation (record size)	Check record size matches FD
"34"	Disk full or quota exceeded	Free disk space or increase quota
"48"	WRITE not supported in current open mode	Change OPEN mode to OUTPUT or EXTEND
"90"	Other I/O error during write	Check system logs, disk hardware

Best Practices for WRITE Operations

Check file status after every WRITE operation
Use the FROM clause when working with complex record structures
Validate data before writing to ensure data integrity
Ensure the file is opened in the appropriate mode (OUTPUT or EXTEND)
For large volumes, consider buffering multiple records before writing
Include proper error handling for disk-full and other error conditions
Test both successful writes and error conditions in your programs

CLOSE Statement

Basic Syntax

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CLOSE file-name-1 [file-name-2] ... [WITH LOCK]

Multiple files can be closed in a single CLOSE statement
WITH LOCK prevents the file from being reopened in the same program run
No special positioning clauses are needed for sequential files

Examples

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* Closing a single file
CLOSE CUSTOMER-FILE.
 
* Closing multiple files
CLOSE CUSTOMER-FILE
      TRANSACTION-FILE
      REPORT-FILE.
 
* Closing with lock
CLOSE SENSITIVE-DATA-FILE WITH LOCK.
 
* Closing with file status checking
CLOSE INVENTORY-FILE
IF INVENTORY-STATUS NOT = "00"
   PERFORM CLOSE-ERROR-ROUTINE
END-IF.

File Closing Process

When a file is closed, the following actions typically occur:

Any remaining buffers are flushed to the physical device
File control blocks and buffers are released
End-of-file markers or trailers are written if required
System resources associated with the file are freed
The file can no longer be accessed without reopening it

Proper closing is essential, especially for output files, to ensure all data is written to the physical device and file integrity is maintained.

Error Handling for CLOSE Operations

While CLOSE operations typically succeed, some errors can occur:

Status Code	Error Condition	Possible Cause
"42"	File not open	Attempting to close a file that isn't open
"90-99"	Physical I/O error	Hardware issue during final write operations
"34"	Disk full	Not enough space to write final buffers

Best Practices for CLOSE Operations

Always close files when processing is complete
Check file status after CLOSE operations
Close files in error handling routines
Use CLOSE WITH LOCK for sensitive files
Include CLOSE operations in program termination logic
Ensure files are properly opened before attempting to close them
Consider using a standardized file handling module to ensure consistent file closing

Example: Proper File Handling Pattern

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IDENTIFICATION DIVISION.
PROGRAM-ID. FILEOPS.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT INPUT-FILE ASSIGN TO "INPUT.DAT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS INPUT-STATUS.
    SELECT OUTPUT-FILE ASSIGN TO "OUTPUT.DAT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS OUTPUT-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  INPUT-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  INPUT-RECORD         PIC X(80).
 
FD  OUTPUT-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  OUTPUT-RECORD        PIC X(80).
 
WORKING-STORAGE SECTION.
01  FILE-STATUSES.
    05  INPUT-STATUS     PIC XX.
    05  OUTPUT-STATUS    PIC XX.
01  EOF-FLAG             PIC X VALUE "N".
    88  END-OF-FILE        VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZATION
    IF INPUT-STATUS = "00" AND OUTPUT-STATUS = "00"
       PERFORM PROCESS-FILES UNTIL END-OF-FILE
    END-IF
    PERFORM TERMINATION
    STOP RUN.
 
INITIALIZATION.
    OPEN INPUT INPUT-FILE
    IF INPUT-STATUS NOT = "00"
       DISPLAY "Error opening input file: " INPUT-STATUS
       MOVE "Y" TO EOF-FLAG
    ELSE
       OPEN OUTPUT OUTPUT-FILE
       IF OUTPUT-STATUS NOT = "00"
          DISPLAY "Error opening output file: " OUTPUT-STATUS
          MOVE "Y" TO EOF-FLAG
          * Close the file that was successfully opened
          CLOSE INPUT-FILE
       END-IF
    END-IF.
 
PROCESS-FILES.
    READ INPUT-FILE
       AT END
          MOVE "Y" TO EOF-FLAG
       NOT AT END
          MOVE INPUT-RECORD TO OUTPUT-RECORD
          WRITE OUTPUT-RECORD
          IF OUTPUT-STATUS NOT = "00"
             DISPLAY "Error writing record: " OUTPUT-STATUS
             MOVE "Y" TO EOF-FLAG
          END-IF
    END-READ.
 
TERMINATION.
    * Close files regardless of processing status
    IF INPUT-STATUS NOT = "42" * Not "file not open"
       CLOSE INPUT-FILE
    END-IF
    
    IF OUTPUT-STATUS NOT = "42" * Not "file not open"
       CLOSE OUTPUT-FILE
    END-IF
    
    DISPLAY "Processing complete."
    DISPLAY "Input status: " INPUT-STATUS
    DISPLAY "Output status: " OUTPUT-STATUS.

File Status Checking

Defining File Status

File status is defined in two places:

In the SELECT statement of the ENVIRONMENT DIVISION (FILE STATUS clause)
As a two-character field in WORKING-STORAGE

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ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE ASSIGN TO "CUSTOMER.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
01  CUSTOMER-STATUS     PIC XX.
    88  STATUS-OK         VALUE "00".
    88  END-OF-FILE       VALUE "10".
    88  FILE-NOT-FOUND    VALUE "35".

Using 88-level condition names makes the code more readable when checking for specific status conditions.

Common File Status Codes

Status	Meaning	Operations
"00"	Successful completion	All operations
"10"	End of file reached	READ
"35"	File not found	OPEN INPUT, OPEN I-O
"37"	File access denied (permissions)	OPEN
"39"	File attribute mismatch	OPEN
"41"	File already open	OPEN
"42"	File not open	CLOSE, READ, WRITE
"46"	Read beyond file end	READ
"47"	Input/output error	OPEN, CLOSE, READ, WRITE
"48"	File not open in correct mode	READ, WRITE
"93"	Resource unavailable	OPEN

Comprehensive Status Checking

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IDENTIFICATION DIVISION.
PROGRAM-ID. STATUSCHK.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT DATA-FILE ASSIGN TO "DATA.DAT"
        ORGANIZATION IS SEQUENTIAL
        ACCESS MODE IS SEQUENTIAL
        FILE STATUS IS DATA-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  DATA-FILE
    RECORD CONTAINS 100 CHARACTERS.
01  DATA-RECORD          PIC X(100).
 
WORKING-STORAGE SECTION.
01  FILE-VARIABLES.
    05  DATA-STATUS      PIC XX.
        88  STATUS-OK      VALUE "00".
        88  END-OF-FILE    VALUE "10".
        88  FILE-NOT-FOUND VALUE "35".
    05  EOF-FLAG         PIC X VALUE "N".
        88  AT-END         VALUE "Y".
    05  ERROR-FLAG       PIC X VALUE "N".
        88  ERROR-OCCURRED  VALUE "Y".
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    PERFORM INITIALIZE-PROCESSING
    IF NOT ERROR-OCCURRED
       PERFORM PROCESS-DATA UNTIL AT-END OR ERROR-OCCURRED
    END-IF
    PERFORM TERMINATE-PROCESSING
    STOP RUN.
 
INITIALIZE-PROCESSING.
    PERFORM OPEN-DATA-FILE
    IF ERROR-OCCURRED
       DISPLAY "Cannot continue processing due to file error"
    END-IF.
 
PROCESS-DATA.
    PERFORM READ-DATA-RECORD
    IF NOT AT-END AND NOT ERROR-OCCURRED
       PERFORM PROCESS-RECORD
    END-IF.
 
TERMINATE-PROCESSING.
    PERFORM CLOSE-DATA-FILE
    DISPLAY "Processing complete".
 
OPEN-DATA-FILE.
    OPEN INPUT DATA-FILE
    PERFORM CHECK-FILE-STATUS.
 
READ-DATA-RECORD.
    READ DATA-FILE
       AT END SET AT-END TO TRUE
    END-READ
    PERFORM CHECK-FILE-STATUS.
 
CLOSE-DATA-FILE.
    CLOSE DATA-FILE
    PERFORM CHECK-FILE-STATUS.
 
PROCESS-RECORD.
    * Process the current record
    DISPLAY DATA-RECORD.
 
CHECK-FILE-STATUS.
    EVALUATE DATA-STATUS
       WHEN "00"
          * Operation successful - no action needed
          CONTINUE
       WHEN "10"
          * End of file - not an error for READ
          IF NOT AT-END
             SET AT-END TO TRUE
          END-IF
       WHEN "35"
          DISPLAY "File not found"
          SET ERROR-OCCURRED TO TRUE
       WHEN "37"
          DISPLAY "Access denied - check permissions"
          SET ERROR-OCCURRED TO TRUE
       WHEN "39"
          DISPLAY "Attribute mismatch - check file definition"
          SET ERROR-OCCURRED TO TRUE
       WHEN "41"
          DISPLAY "File already open"
          SET ERROR-OCCURRED TO TRUE
       WHEN "42"
          DISPLAY "File not open"
          SET ERROR-OCCURRED TO TRUE
       WHEN "46"
          DISPLAY "Read beyond end of file"
          SET ERROR-OCCURRED TO TRUE
       WHEN "47"
          DISPLAY "I/O error - check hardware"
          SET ERROR-OCCURRED TO TRUE
       WHEN "48"
          DISPLAY "File not open in correct mode"
          SET ERROR-OCCURRED TO TRUE
       WHEN OTHER
          DISPLAY "Unexpected file status: " DATA-STATUS
          SET ERROR-OCCURRED TO TRUE
    END-EVALUATE.

Extended File Status

Some COBOL implementations support an extended file status, which provides more detailed information about file operations. The extended status typically consists of:

Two-character primary status (standard file status)
Additional characters or fields with implementation-specific details

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ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT CUSTOMER-FILE
        ASSIGN TO CUSTFILE
        FILE STATUS IS CUSTOMER-STATUS.
 
DATA DIVISION.
WORKING-STORAGE SECTION.
* Standard file status
01  CUSTOMER-STATUS.
    05  PRIMARY-STATUS    PIC XX.
    05  SECONDARY-STATUS  PIC XXX.
 
* Example usage
IF PRIMARY-STATUS NOT = "00"
   DISPLAY "Error: " PRIMARY-STATUS "-" SECONDARY-STATUS
END-IF.

The exact format of extended file status varies by compiler. Check your compiler documentation for specific details about extended status codes.

Best Practices for File Status Checking

Check file status after every file operation
Use 88-level condition names for commonly checked statuses
Implement a centralized status checking routine
Log detailed error information including operation type and file name
Distinguish between expected conditions (like end-of-file) and true errors
Include error recovery procedures where appropriate
Close files properly even after errors
Consider using extended file status for more detailed error information
Document the meaning of status codes in program comments

File Status Logging Example

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IDENTIFICATION DIVISION.
PROGRAM-ID. FILELOG.
 
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
    SELECT DATA-FILE ASSIGN TO "DATA.DAT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS DATA-STATUS.
    
    SELECT LOG-FILE ASSIGN TO "FILELOG.TXT"
        ORGANIZATION IS SEQUENTIAL
        FILE STATUS IS LOG-STATUS.
 
DATA DIVISION.
FILE SECTION.
FD  DATA-FILE
    RECORD CONTAINS 100 CHARACTERS.
01  DATA-RECORD          PIC X(100).
 
FD  LOG-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  LOG-RECORD           PIC X(80).
 
WORKING-STORAGE SECTION.
01  FILE-VARIABLES.
    05  DATA-STATUS      PIC XX.
    05  LOG-STATUS       PIC XX.
    05  CURRENT-DATE-TIME.
        10  CURRENT-DATE.
            15  CURRENT-YEAR   PIC 9(4).
            15  CURRENT-MONTH  PIC 9(2).
            15  CURRENT-DAY    PIC 9(2).
        10  CURRENT-TIME.
            15  CURRENT-HOUR   PIC 9(2).
            15  CURRENT-MINUTE PIC 9(2).
            15  CURRENT-SECOND PIC 9(2).
            15  CURRENT-MS     PIC 9(2).
    05  TIMESTAMP        PIC X(19).
    05  MESSAGE          PIC X(60).
 
PROCEDURE DIVISION.
MAIN-PROCESS.
    OPEN OUTPUT LOG-FILE
    IF LOG-STATUS NOT = "00"
       DISPLAY "Could not open log file: " LOG-STATUS
    ELSE
       * Now try to work with the data file
       PERFORM LOG-MESSAGE "Starting file processing"
       
       OPEN INPUT DATA-FILE
       IF DATA-STATUS NOT = "00"
          PERFORM LOG-ERROR "OPEN" "DATA-FILE" DATA-STATUS
       ELSE
          PERFORM PROCESS-FILE
          CLOSE DATA-FILE
          PERFORM LOG-FILE-STATUS "CLOSE" "DATA-FILE" DATA-STATUS
       END-IF
       
       PERFORM LOG-MESSAGE "Processing complete"
       CLOSE LOG-FILE
    END-IF
    STOP RUN.
 
PROCESS-FILE.
    PERFORM LOG-MESSAGE "Reading records"
    PERFORM UNTIL DATA-STATUS NOT = "00"
       READ DATA-FILE
          AT END EXIT PERFORM
       END-READ
       
       IF DATA-STATUS = "00"
          * Process the record
          CONTINUE
       ELSE IF DATA-STATUS NOT = "10"
          PERFORM LOG-ERROR "READ" "DATA-FILE" DATA-STATUS
          EXIT PERFORM
       END-IF
    END-PERFORM.
 
LOG-FILE-STATUS.
    * Parameters: operation, file-name, status
    MOVE FUNCTION CURRENT-DATE TO CURRENT-DATE-TIME
    STRING CURRENT-YEAR "-" CURRENT-MONTH "-" CURRENT-DAY " "
           CURRENT-HOUR ":" CURRENT-MINUTE ":" CURRENT-SECOND
       INTO TIMESTAMP
    
    STRING TIMESTAMP " " FUNCTION TRIM(FUNCTION-ID-1) " "
           FUNCTION TRIM(FUNCTION-ID-2) " Status: " FUNCTION-ID-3
       INTO MESSAGE
    
    PERFORM WRITE-LOG-RECORD.
 
LOG-ERROR.
    * Parameters: operation, file-name, status
    MOVE FUNCTION CURRENT-DATE TO CURRENT-DATE-TIME
    STRING CURRENT-YEAR "-" CURRENT-MONTH "-" CURRENT-DAY " "
           CURRENT-HOUR ":" CURRENT-MINUTE ":" CURRENT-SECOND
       INTO TIMESTAMP
    
    STRING TIMESTAMP " ERROR: " FUNCTION TRIM(FUNCTION-ID-1) " "
           FUNCTION TRIM(FUNCTION-ID-2) " Failed with status: " 
           FUNCTION-ID-3
       INTO MESSAGE
    
    PERFORM WRITE-LOG-RECORD.
 
LOG-MESSAGE.
    * Parameters: message-text
    MOVE FUNCTION CURRENT-DATE TO CURRENT-DATE-TIME
    STRING CURRENT-YEAR "-" CURRENT-MONTH "-" CURRENT-DAY " "
           CURRENT-HOUR ":" CURRENT-MINUTE ":" CURRENT-SECOND
       INTO TIMESTAMP
    
    STRING TIMESTAMP " " FUNCTION TRIM(FUNCTION-ID-1)
       INTO MESSAGE
    
    PERFORM WRITE-LOG-RECORD.
 
WRITE-LOG-RECORD.
    MOVE MESSAGE TO LOG-RECORD
    WRITE LOG-RECORD
    IF LOG-STATUS NOT = "00"
       DISPLAY "Error writing to log: " LOG-STATUS
    END-IF.

SELECT and ASSIGN

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       FILE-CONTROL.
           SELECT CUST-FILE ASSIGN TO CUSTFILE
               ORGANIZATION IS SEQUENTIAL
               ACCESS MODE IS SEQUENTIAL
               FILE STATUS IS CUST-STAT.

The name in OPEN has to be the SELECT name. If CUSTFILE has no matching DD, or the quoted file name does not exist when you OPEN INPUT, the file status is 35.

ORGANIZATION IS SEQUENTIAL

These are the organization values you will see on a SELECT:

SEQUENTIAL. Records stored in the order written. This is the z/OS form, and the organization used by the OPEN, READ, and WRITE sections above.
LINE SEQUENTIAL. A text file. Each record is a line ended by a newline, and trailing spaces are dropped on WRITE. Covered in the next section.
INDEXED. Records stored by a key. Use this when you need READ by key, and see the indexed file page for the statements.
RELATIVE. Records stored by a relative record number.

ACCESS MODE IS SEQUENTIAL

SEQUENTIAL. The next record, every time. This is the only mode a sequential file can use.
RANDOM. One record, chosen by key. Indexed and relative files only.
DYNAMIC. A key read or a READ NEXT, in the same program. Indexed and relative files only.

LINE SEQUENTIAL

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           SELECT REPORT-FILE ASSIGN TO "REPORT.TXT"
               ORGANIZATION IS LINE SEQUENTIAL
               ACCESS MODE IS SEQUENTIAL
               FILE STATUS IS REPORT-STAT.

OPEN INPUT

OPEN INPUT opens an existing file for READ. The current position is the first record. WRITE and REWRITE are not allowed. If you issue them, the file status is 48.

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           SELECT OPTIONAL CUST-FILE ASSIGN TO CUSTFILE
               ORGANIZATION IS SEQUENTIAL
               FILE STATUS IS CUST-STAT.
      *
           OPEN INPUT CUST-FILE
           IF CUST-STAT = "35"
              DISPLAY "FILE NOT FOUND"
           END-IF

OPEN OUTPUT

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           OPEN OUTPUT CUST-FILE
           IF CUST-STAT NOT = "00"
              DISPLAY "OPEN OUTPUT FAILED " CUST-STAT
              STOP RUN
           END-IF
           WRITE CUST-REC FROM WORK-REC
           CLOSE CUST-FILE

OPEN EXTEND

OPEN EXTEND opens an existing file and positions after the last record. WRITE appends. READ is not allowed. This is the mode for a log, or for adding a batch of records to a file you must not empty.

cobol

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           OPEN EXTEND LOG-FILE
           IF LOG-STAT = "35"
              DISPLAY "LOG FILE MISSING"
              STOP RUN
           END-IF
           IF LOG-STAT = "00" OR "05"
              WRITE LOG-REC FROM LOG-LINE
           END-IF
           CLOSE LOG-FILE

REWRITE

cobol

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           OPEN I-O CUST-FILE
           READ CUST-FILE
              AT END
                 CONTINUE
           END-READ
           IF CUST-STAT = "00"
              MOVE NEW-BAL TO CUST-BAL
              REWRITE CUST-REC
           END-IF
           CLOSE CUST-FILE

Sequential file example

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       IDENTIFICATION DIVISION.
       PROGRAM-ID. READSEQ.
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT CUST-FILE ASSIGN TO CUSTFILE
               ORGANIZATION IS SEQUENTIAL
               ACCESS MODE IS SEQUENTIAL
               FILE STATUS IS CUST-STAT.
       DATA DIVISION.
       FILE SECTION.
       FD  CUST-FILE
           RECORD CONTAINS 29 CHARACTERS.
       01  CUST-REC.
           05  CUST-NAME    PIC X(20).
           05  CUST-BAL     PIC 9(7)V99.
       WORKING-STORAGE SECTION.
       01  CUST-STAT        PIC XX.
       PROCEDURE DIVISION.
           OPEN INPUT CUST-FILE
           IF CUST-STAT NOT = "00"
              DISPLAY "OPEN FAILED " CUST-STAT
              STOP RUN
           END-IF
           PERFORM UNTIL CUST-STAT = "10"
              READ CUST-FILE NEXT
                 AT END
                    CONTINUE
                 NOT AT END
                    DISPLAY CUST-NAME " " CUST-BAL
              END-READ
           END-PERFORM
           CLOSE CUST-FILE
           STOP RUN.

Explain it like I'm 5

Exercises

Write the SELECT

Write the FILE-CONTROL entry for a sequential customer file. The program name is CUST-FILE, the DD name is CUSTFILE, access is sequential, and the status field is CUST-STAT.

The entry is SELECT CUST-FILE ASSIGN TO CUSTFILE, then ORGANIZATION IS SEQUENTIAL, ACCESS MODE IS SEQUENTIAL, and FILE STATUS IS CUST-STAT.

Append one log line

Open a log file so the old lines stay, write one record, and decide what status 35 means.

Use OPEN EXTEND. Status 35 means the file was not found. Add OPTIONAL to the SELECT if a missing log should be created, and expect status 05 on that open.

Change a balance

You have read a customer record and you need to store a new balance in that same record. Which OPEN mode do you use, and which statement stores it?

OPEN I-O, then REWRITE after the READ. OPEN OUTPUT would wipe the file. OPEN EXTEND would add a second record and leave the old balance where it was.

Test Your Knowledge

1. Which OPEN mode allows both reading and writing to an existing file?

INPUT
OUTPUT
I-O
EXTEND

2. What happens when you open a file with OUTPUT mode that already exists?

The file is opened for reading
The existing file is deleted and a new empty file is created
The existing contents are preserved and new records are appended
An error occurs and the file remains unchanged

3. What is the purpose of the AT END clause in a READ statement?

To specify what happens when no more records exist in the file
To limit the number of records read
To skip to the end of the file
To read only the last record

4. What is the correct sequence of operations for processing a sequential file?

OPEN, WRITE, CLOSE
OPEN, READ, PROCESS, CLOSE
READ, PROCESS, WRITE, CLOSE
WRITE, OPEN, PROCESS, CLOSE

5. What is the purpose of checking the file status after file operations?

To improve performance
To detect and handle errors or exceptional conditions
To count the number of records processed
To determine the file size

6. On a sequential file, what does READ NEXT do?

It reads the previous record
It reads the next record, the same as READ
It reads a record by key
It appends a record

7. Which OPEN mode adds records after the last record already in the file?

INPUT
OUTPUT
EXTEND
I-O

COBOL Tutorial

COBOL Sequential File Operations

OPEN Statement (INPUT, OUTPUT, I-O, EXTEND)

Basic Syntax

OPEN Modes

Examples

Common Errors and Solutions

Best Practices

READ Statement for Sequential Files

Basic Syntax

READ Variations

Typical Processing Loop

End of File Detection

Best Practices for READ Operations

WRITE Statement Basics

Basic Syntax

Writing Records - Examples

Complete Example: Creating a File

WRITE Operations for Different OPEN Modes

Error Handling for WRITE Operations

Best Practices for WRITE Operations

CLOSE Statement

Basic Syntax

Examples

File Closing Process

Error Handling for CLOSE Operations

Best Practices for CLOSE Operations

Example: Proper File Handling Pattern

File Status Checking

Defining File Status

Common File Status Codes

Comprehensive Status Checking

Extended File Status

Best Practices for File Status Checking

File Status Logging Example

SELECT and ASSIGN

ORGANIZATION IS SEQUENTIAL

ACCESS MODE IS SEQUENTIAL

LINE SEQUENTIAL

OPEN INPUT

OPEN OUTPUT

OPEN EXTEND

READ NEXT

REWRITE

Sequential file example

Explain it like I'm 5

Exercises

Write the SELECT

Append one log line

Change a balance

Test Your Knowledge

Frequently Asked Questions

What is the difference between INPUT, OUTPUT, I-O, and EXTEND modes?

How do I handle end-of-file conditions properly?

What are the most common file status codes I should check for?

What is the best way to handle errors during file operations?

How can I improve performance when processing large sequential files?

When do you use OPEN EXTEND?

What is the difference between READ and READ NEXT on a sequential file?

What does ORGANIZATION IS LINE SEQUENTIAL mean?

Related Concepts

File Organization Types

File Description (FD)

Indexed File Operations

Exception Handling

File Status and Error Handling

Related Pages

COBOL Tutorial

COBOL Sequential File Operations

OPEN Statement (INPUT, OUTPUT, I-O, EXTEND)

Basic Syntax

OPEN Modes

Examples

Common Errors and Solutions

Best Practices

READ Statement for Sequential Files

Basic Syntax

READ Variations

Typical Processing Loop

End of File Detection