DETOUR

Program Flow Alteration

Enhanced Definition

In the context of z/OS system programming, a "detour" refers to a mechanism that alters the normal sequential execution flow of a program, typically to handle asynchronous events like program abends, synchronous program interrupts, or to provide user-defined processing at specific system points. It establishes an alternative execution path to intercept and manage these events.

Key Characteristics

- Event-Driven Activation: Activated by specific events, such as a program check, an abend, or a system-defined exit point, rather than being part of the linear program logic.
- Context Preservation: When a detour occurs (e.g., via ESTAE), the z/OS system preserves the execution context (registers, PSW, storage) of the interrupted program, allowing the detour routine to analyze the state.
- Recovery/Diagnostic Focus: Primarily used for error recovery, diagnostic data collection, or graceful termination, aiming to prevent uncontrolled system abends or provide custom handling.
- System Macros: Implemented using specific z/OS system macros like ESTAE (Extended Specified Task Abnormal Exit), ESTAEX (Extended Specified Task Abnormal Exit Extended), or SPIE (Specify Program Interrupt Exit).
- Scope of Control: Can be established at the task level (ESTAE/ESTAEX) to intercept abends for the entire task, or for specific program interrupts within a task (SPIE).
- Re-establishment: Detour routines often need to explicitly re-establish themselves if they wish to remain active after successfully handling an event and returning control.

Use Cases

- Abend Recovery: A common use is to intercept program abends (ESTAE/ESTAEX) to perform cleanup (e.g., closing datasets, releasing enqueues), log diagnostic information, or attempt to terminate the task gracefully instead of letting the system abend handler take over.
- Program Interrupt Handling: Using SPIE to catch specific program checks (e.g., decimal data errors, addressing exceptions) to allow the program to correct the error, log it, and potentially resume execution at the point of interruption.
- System Exit Routines: User-written routines that "detour" the normal system processing flow at predefined exit points (e.g., SMF exits, JES exits, CICS user exits) to add custom functionality, monitoring, or security checks.
- Debugging and Tracing: Temporarily installing a detour routine to intercept specific events or instructions for debugging purposes, logging program state changes, or altering execution paths for testing.

Related Concepts

Detour mechanisms are fundamental to error handling and system customization in z/OS. They are closely related to Program Event Recording (PER) for debugging, System Completion Codes (for abends), and User Exits. They provide a lower-level, more granular control over program flow compared to high -level language exception handling (e.g., ON ERROR in PL/I) and are often used to build robust recovery routines that prevent system outages or data corruption by intercepting and managing critical events.

Best Practices:

Minimal Processing: Detour routines should perform minimal processing to avoid introducing new errors or complexities. Focus on essential diagnostics, cleanup, and