Overview
In COBOL, FILE CONTROL entries are crucial as they serve as the interface between the program and the external files it needs to interact with. These entries, defined in the Input-Output Section of the Environment Division, specify the files' characteristics and how they should be accessed, enabling data processing tasks on files such as reading, writing, updating, and deleting records.
Key Concepts
- File Organization: Describes how records are organized in the file (e.g., sequential, indexed, or relative).
- Access Mode: Defines how the records are accessed, whether sequentially, randomly, or dynamically.
- File Status: A mechanism to check the outcome of the last file operation, aiding in error handling.
Common Interview Questions
Basic Level
- What is the purpose of FILE CONTROL entries in a COBOL program?
- How do you define a sequential file in the FILE CONTROL section?
Intermediate Level
- Explain the significance of specifying ACCESS MODE in FILE CONTROL.
Advanced Level
- How does the FILE STATUS clause enhance error handling in file operations?
Detailed Answers
1. What is the purpose of FILE CONTROL entries in a COBOL program?
Answer: FILE CONTROL entries link the logical file descriptions in a COBOL program to the physical files on the storage medium. They provide essential details about how files should be processed, including their organization, access mode, and the device they reside on. This is crucial for performing any file operations within the program.
Key Points:
- Specifies the file's organization and access method.
- Connects the file described in the DATA DIVISION to the actual file.
- Enables the program to perform operations like READ, WRITE, REWRITE, and DELETE on the file.
Example:
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
SELECT myFile ASSIGN TO 'example.dat'
ORGANIZATION IS SEQUENTIAL
ACCESS MODE IS SEQUENTIAL.
This example shows how to define a sequential file named myFile and assign it to a physical file named example.dat with sequential access.
2. How do you define a sequential file in the FILE CONTROL section?
Answer: To define a sequential file in the FILE CONTROL section, you must specify the file's logical name using the SELECT statement, assign it to a physical file using ASSIGN TO, and indicate that its organization is SEQUENTIAL. Sequential files are accessed in a fixed order, from the beginning to the end.
Key Points:
- Used for files where records are processed in a sequential order.
- Requires the ORGANIZATION IS SEQUENTIAL clause.
- Often used in batch processing where the entire file is read or written in one go.
Example:
FILE-CONTROL.
SELECT customerFile ASSIGN TO 'customers.dat'
ORGANIZATION IS SEQUENTIAL.
This code snippet defines a sequential file customerFile that is linked to a physical file named customers.dat.
3. Explain the significance of specifying ACCESS MODE in FILE CONTROL.
Answer: The ACCESS MODE clause in the FILE CONTROL section determines how records within a file are accessed by the program. It can be set to SEQUENTIAL for reading records from beginning to end, RANDOM for accessing records directly based on a key, or DYNAMIC which allows both sequential and random access. Specifying the correct access mode is crucial for optimizing file operations and ensuring the program runs efficiently.
Key Points:
- Determines the efficiency of file operations.
- Must align with the file's organization and the program's processing needs.
- Influences the choice of READ statements in the program.
Example:
FILE-CONTROL.
SELECT orderFile ASSIGN TO 'orders.dat'
ORGANIZATION IS INDEXED
ACCESS MODE IS DYNAMIC.
This configuration allows orderFile, an indexed file, to be accessed both sequentially and randomly, providing flexibility in file operations.
4. How does the FILE STATUS clause enhance error handling in file operations?
Answer: The FILE STATUS clause allows a program to check the outcome of the last file operation by assigning a two-character code to a specified data item. This facilitates detailed error checking and handling, enabling the program to take corrective action or notify the user when an operation fails or encounters issues.
Key Points:
- Provides detailed information on the success or failure of file operations.
- Essential for robust error handling in file processing applications.
- The status code can indicate errors such as file not found, read past end of file, or write error.
Example:
FILE-CONTROL.
SELECT employeeFile ASSIGN TO 'employees.dat'
ORGANIZATION IS SEQUENTIAL
FILE STATUS IS WS-FILE-STATUS.
DATA DIVISION.
FILE SECTION.
FD employeeFile.
01 employeeRecord.
05 employeeID PIC X(5).
05 employeeName PIC X(25).
WORKING-STORAGE SECTION.
01 WS-FILE-STATUS PIC XX.
After each file operation on employeeFile, the program can check WS-FILE-STATUS to determine if the operation was successful or if an error occurred, allowing for appropriate error handling or logging.
