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DFHEIBLK

EIB block
Enhanced Definition

The `DFHEIBLK`, or Executive Interface Block, is a control block automatically provided by CICS to every application program. It contains critical information about the current CICS task, the CICS command just executed, and the environment in which the program is running. It serves as the primary communication area between the CICS system and the executing application program.

Key Characteristics

    • Task-Specific: Each active CICS task has its own unique DFHEIBLK, which is initialized at the start of the task and updated by CICS throughout its execution.
    • Read-Only for Applications: Application programs can read the fields within the DFHEIBLK to obtain task information, but they cannot directly modify its contents.
    • Automatic Population: CICS automatically populates and updates the DFHEIBLK fields before and after each CICS command execution, reflecting the status and outcome of the command.
    • Standard Structure: It has a predefined structure with fields like EIBTRNID (transaction ID), EIBTRMID (terminal ID), EIBDATE (current date), EIBTIME (current time), EIBRESP (response code), and EIBRESP2 (secondary response code).
    • Language Agnostic: The DFHEIBLK structure is defined and accessible from CICS application programs written in various languages, including COBOL, PL/I, C/C++, and Assembler.
    • Implicitly Passed: In COBOL, the DFHEIBLK is often implicitly available to the program without explicit definition in the LINKAGE SECTION when using the EXEC CICS preprocessor.

Use Cases

    • Error Handling and Response Checking: Programs frequently check EIBRESP and EIBRESP2 after executing CICS commands to determine if the command was successful or if an error occurred, enabling appropriate error handling logic.
    • Transaction and Terminal Identification: Retrieving EIBTRNID allows a program to identify the transaction it is processing, while EIBTRMID provides information about the originating terminal, useful for logging, security, or terminal-specific processing.
    • Date and Time Stamping: EIBDATE and EIBTIME can be used to obtain the current date and time of the CICS task, useful for audit trails, record creation timestamps, or time-sensitive business logic.
    • Program Flow Control: Examining EIBFN (function code) and EIBRCODE (return code) can provide details about the last CICS command executed, which can be useful in complex error recovery or debugging scenarios.
    • Resource Availability Checks: Before attempting to access a resource (e.g., a file or temporary storage queue), a program might check EIBRESP from a previous command to ensure the resource is available or the operation is valid.

Related Concepts

The DFHEIBLK is fundamental to CICS application programming, acting as the primary interface for an application program to query its execution environment and the results of CICS API calls. It works in conjunction with CICS commands (EXEC CICS ...) by providing the immediate feedback on their success or failure. It is implicitly linked to the CICS Task Control Area (TCA) and other internal CICS control blocks, which CICS uses to manage the overall task. While the DFHEIBLK provides task-specific information, other CICS facilities like DFHCOMMAREA (or CHANNEL/CONTAINER) are used for passing data between programs.

Best Practices:
  • Always Check EIBRESP: After every CICS command, explicitly check EIBRESP for DFHRESP(NORMAL) to ensure the command executed successfully. Implement robust error handling for non-normal responses.
  • Use Symbolic Conditions: Leverage CICS-provided symbolic conditions (e.g., DFHRESP(NOTFND), DFHRESP(DUPREC)) rather than hardcoding numeric EIBRESP values for improved readability and maintainability.
  • Avoid Direct Modification: Never attempt to modify the contents of the DFHEIBLK directly, as this can lead to unpredictable CICS behavior, system abends, or data corruption.
  • Understand Common Responses: Familiarize yourself with the common EIBRESP and EIBRESP2 values for the CICS commands your application uses most frequently.
  • Utilize EIBDATE and EIBTIME Judiciously: While convenient, be aware that EIBDATE and EIBTIME reflect the time at the start of the task or the last CICS command, not necessarily the exact current system time. For highly precise timing, consider using ASKTIME or system functions.

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