DTF - Define The File
DTF (Define The File) is a control statement used in **DOS/VSE** (Disk Operating System/Virtual Storage Extended) to define the characteristics of a file to be processed by an application program. It provides the operating system with crucial information about the file's organization, access method, and physical attributes, enabling the program to perform I/O operations. DTF, standing for "Define The File," refers to the process and, historically, specific macros or statements used in older IBM mainframe operating systems (like DOS/VSE) to explicitly describe the characteristics of a data file to the operating system and access method. While not a direct component in modern z/OS in the same way, the *functionality* of defining file attributes is fundamental and is primarily handled by JCL `DD` statements and the Data Control Block (`DCB`) within application programs.
Key Characteristics
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- VSE-Specific: DTF statements are exclusively used within the DOS/VSE operating system environment, not z/OS.
- File Attributes: Specifies essential file attributes such as file organization (e.g., sequential, indexed sequential), record format (fixed, variable), record length, block size, and the access method to be used.
- Symbolic Unit: Links a logical file name (often referred to as a symbolic unit or
SYSnnn) used within a program to a physical device or dataset defined in the JCL. - Access Method Implication: The specific DTF statement used (e.g.,
DTFSDfor sequential disk,DTFISfor indexed sequential) implies the underlying access method the system will employ. - JCL Integration: DTF statements are typically embedded within the Job Control Language (JCL) stream of a VSE job, preceding the program execution step.
Use Cases
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- Batch Program I/O: Defining input files (e.g., transaction data, master files) and output files (e.g., reports, updated master files) for COBOL, PL/I, or Assembler batch applications.
- Sequential File Processing: Specifying
DTFSD(Define The File Sequential Disk) for reading or writing sequential files on disk, orDTFPR(Define The File Printer) for print output. - Indexed Sequential File Management: Using
DTFIS(Define The File Indexed Sequential) to define files that require both sequential and direct access capabilities via a primary key. - Program-to-File Linkage: Establishing the critical connection between the
SELECTclause in a COBOL program (which refers to a logical file name) and the actual physical file definition provided by the DTF statement in the JCL.
Related Concepts
DTF is the DOS/VSE equivalent of the DD (Data Definition) statement found in z/OS JCL. Both serve the fundamental purpose of describing files to the operating system for program access. In VSE, DTF statements often work in conjunction with DLBL (Define Label) and EXTENT statements, which provide more detailed information about disk file labels and physical storage allocation, respectively. It directly maps to the file names declared in a COBOL program's SELECT statements.
- Accurate Attribute Matching: Ensure that all file attributes specified in the DTF statement (e.g., record length, block size, file organization) precisely match the actual characteristics of the file and the expectations of the application program to prevent I/O errors or data corruption.
- Clear Naming Conventions: Use consistent and descriptive symbolic unit names that align with the program's logical file names for improved readability and maintainability of JCL.
- Resource Allocation: For disk files, ensure that associated
DLBLandEXTENTstatements provide sufficient and correct space allocation, especially for new or growing files. - Error Handling Awareness: Understand that discrepancies between DTF definitions and actual file properties or program logic are common causes of job abends (abnormal terminations) in VSE environments.
- Documentation: Thoroughly document the purpose and specific attributes of each DTF statement within the JCL or external documentation to aid in troubleshooting and future modifications.