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COBOL Files description Entries. |
Introduction
Sequential files are an integral part of data processing in COBOL. To effectively work with these files, COBOL programmers utilize File Description (FD) entries along with other supporting entries. This article provides a comprehensive overview of COBOL File Description entries for a sequential file, discussing their structure, purpose, and usage.
COBOL File Description Entries
COBOL File Description entries are used to describe the structure and characteristics of files in COBOL programs. These entries play a crucial role in defining and manipulating sequential files. Each entry within the File Description section provides specific details about the file, such as its organization, access mode, record length, and more.
Sequential File
A sequential file is a type of file organization where records are stored and accessed in sequential order, one after the other. In COBOL, sequential files are widely used for various purposes, such as input/output operations and batch processing. They are especially suitable for applications where data needs to be processed in a specific order.
File Control Entry
The File Control Entry is an essential part of working with sequential files in COBOL. It acts as a bridge between the program and the actual file, providing necessary information about the file's physical attributes, such as its name, device type, organization, and access mode. The FCB is typically declared in the File Control section of the program.
There are two types of entries that are required in a program for any file. They are file description entries and record description entries.
The file description entries specify the physical aspects of the data such as the size relationship between physical and logical records, the size and name(s) of the logical record(s), labeling information, etc., These entries are made in the ENVIRONMENT DIVISION.
The record description entries describe the logical records in the file, including the category and format of data within each field of the logical record, different values the data might be assigned, etc,. These entries are made in the DATA DIVISION.
The file description entries are made in the FILE-CONTROL paragraph of the ENVIRONMENT DIVISION using the SELECT . . . ASSIGN . . . clause. Its format is given below.
Format :
Corresponding to every file there must be a SELECT . . . ASSIGN . . . clause. The purpose of this is to establish a relationship between the logical file name (internal to COBOL) used in the program and the physical file name (external file name) used to store the file on DASD.
After the relationship between physical and logical records has been established, only logical records are made available to the programmer. For this reason, when we say record, we mean the logical record and not the physical record.
Example: File Description for Sequential File.
Best Practices
When working with COBOL File Description entries for sequential files, consider the following best practices:
- Use descriptive names for files and entries to enhance readability and maintainability.
- Ensure consistency between the declared structure in the FD and RD entries.
- Verify that the record length specified in the FD entry matches the actual record length.
- Always close the file after its use to free system resources.
- Test and validate file operations to ensure proper functioning.
Conclusion
COBOL File Description entries play a vital role in defining and manipulating sequential files in COBOL programs. By utilizing these entries effectively, programmers can ensure accurate file operations and efficient data processing. Understanding the structure, purpose, and usage of COBOL File Description entries is essential for developing robust and reliable COBOL applications.
FAQs
Can a sequential file have multiple FD entries?
No, a sequential file can have only one FD entry associated with it. Multiple FD entries are typically used for different files within a program.
Can I change the structure of a sequential file?
Changing the structure of a sequential file requires modifying the associated File Description entry and potentially the Record Description entry. It is important to ensure data consistency and handle existing data when altering the file structure.
What is the purpose of the SD entry?
The SD (Sort Description) entry is used to specify the sorting criteria and order for records within a sequential file when sorting operations are required.
How do I declare a sequential file in COBOL?
Sequential files are declared using the FD (File Description) entry within the Data Division of a COBOL program. The FD entry provides details about the file's organization, access mode, record length, and more.
Can I use COBOL File Description Entries with other file types?
COBOL File Description entries are primarily used for sequential files. Other file types, such as indexed files or relative files, require different file description entries and handling methods.
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