Debug Tool

Enhanced Definition

IBM Debug Tool is an interactive source-level debugger for applications running on z/OS. It allows developers to step through program execution, inspect variables, set breakpoints, and modify program flow to diagnose and resolve issues in COBOL, PL/I, C/C++, and Assembler programs. Its primary purpose is to facilitate efficient problem determination and code validation within the mainframe environment. IBM Debug Tool is the primary interactive source-level debugger for applications running on z/OS. It enables developers to control program execution, inspect and modify variables, and set breakpoints to diagnose and resolve issues in COBOL, PL/I, C/C++, and Assembler programs. It is an essential utility for identifying logic errors, data corruption, and performance bottlenecks in mainframe applications.

Key Characteristics

- Source-level debugging: Enables debugging using the original source code, providing a high-level view rather than just machine instructions.
- Multi-language support: Supports a wide range of z/OS programming languages, including Enterprise COBOL, Enterprise PL/I, C/C++, and Assembler.
- Multiple debugging modes: Offers full-screen mode (ISPF-based), remote debug mode (using a client GUI like IBM Developer for z/OS), and batch mode (non-interactive).
- Integration with z/OS subsystems: Seamlessly integrates with critical z/OS subsystems such as CICS, IMS, DB2 stored procedures, and traditional batch jobs.
- Conditional breakpoints and watchpoints: Allows setting breakpoints based on specific conditions or monitoring changes to particular variables, enhancing diagnostic precision.
- Program flow control: Provides capabilities to step over, step into, step out, run to cursor, and modify variable values or program flow during execution.

Use Cases

- Debugging batch COBOL programs: Stepping through a COBOL batch job to identify logic errors, data manipulation issues, or unexpected program termination.
- Troubleshooting CICS transactions: Interactively debugging a CICS transaction program (e.g., COBOL or PL/I) to resolve runtime errors, investigate abnormal transaction behavior, or validate new functionality.
- Debugging DB2 stored procedures: Examining the execution of a COBOL or PL/I stored procedure called from a DB2 application to ensure correct data access and business logic.
- Analyzing IMS programs: Debugging IMS BMP (Batch Message Processing) or MPP (Message Processing Program) applications to diagnose issues related to database interaction or message processing.
- Performance tuning and code optimization: Identifying bottlenecks or inefficient code paths by observing variable values and execution flow, leading to more performant applications.

Related Concepts

Debug Tool is intrinsically linked to the compilers (e.g., Enterprise COBOL, Enterprise PL/I) that generate the executable code, as programs must be compiled with specific options (TEST or DEBUG) to embed debugging information. It operates within the z/OS operating system and interacts with various subsystems like CICS, IMS, and DB2 to provide a comprehensive debugging experience. It often works in conjunction with JCL for setting up batch debugging sessions or transaction definitions for CICS debugging. Modern usage frequently involves IBM Developer for z/OS (IDz), which provides a rich graphical user interface for remote debugging sessions, enhancing developer productivity.

Best Practices:

Compile with TEST option: Always compile programs intended for debugging with the TEST compiler option (or DEBUG for older versions) to generate the necessary debug information.
Use appropriate debugging mode: Choose between full -screen (ISPF) for quick interactive sessions, remote GUI for complex debugging and modern development workflows, or batch for non-interactive analysis.
Manage debug profiles: For CICS and IMS, utilize the Debug Profile Service or DTCN (Debug Tool CICS Control) to define and activate debug profiles, ensuring only specific transactions or users are debugged.
Optimize debug data: After debugging, recompile programs without the TEST option to remove debug hooks and associated overhead, improving performance in production environments.
Understand EQAOPTS: Leverage the EQAOPTS file to configure Debug Tool behavior, such as automatic start, message logging, and file allocations, for tailored debugging sessions.