Hook - Intercept point

Key Characteristics

• • Predefined Entry Points: Hooks are typically well-documented entry points (e.g., EXIT=, USEREXIT=, SVC routines, PC routines) within system or application code, designed for external customization.
  • Custom Code Execution: They enable the execution of user-written routines, often referred to as user exits, at specific, critical stages of a process.
  • Control Transfer: The system temporarily transfers control to the user exit, which performs its designated function, and then returns control to the original system or application.
  • Parameter Passing: Information, such as register contents, parameter lists, or control block addresses, is often passed to the user exit, allowing it to inspect or modify system state.
  • Authorization Requirements: Many system-level hooks require the user exit code to run in an authorized state (e.g., APF-authorized, KEY 0, supervisor state) due to the sensitive nature of the intercepted operations.
  • Dynamic or Static Implementation: Hooks can be implemented statically (e.g., specified in configuration files, link-edited into modules) or dynamically (e.g., using SET EXIT commands in CICS, MODIFY commands for system exits).

Use Cases

• • System Monitoring and Auditing: SMF (System Management Facilities) exits allow custom recording of system events, beyond standard SMF records, for enhanced auditing or specialized performance analysis.
  • Security Enhancements: RACF (Resource Access Control Facility) or other security product exits can enforce additional security rules, integrate with external security systems, or perform custom logging of access attempts.
  • Subsystem Customization: CICS user exits (XPI calls, global user exits, program-specific user exits) and IMS user exits (DFS exits, DBRC exits) allow tailoring transaction processing, database access, and message handling logic.
  • Debugging and Tracing: Debugging tools often utilize hooks (e.g., SVC intercepts, PC intercepts) to gain control at specific program points, inspect memory, modify execution flow, or provide detailed tracing.
  • Performance Optimization: Intercepting I/O requests (EXCP exits) or other resource-intensive operations to implement custom caching, workload balancing, or data compression routines.

Related Concepts