FSR - Function Status Return

Enhanced Definition

In the context of IBM mainframe systems and z/OS, FSR typically refers to a **Function Status Return** code or register used by various system components, utilities, or application programs to indicate the outcome of an operation or function call. It serves as a crucial mechanism for communicating success, failure, or specific conditions back to the calling entity.

Key Characteristics

- Purpose: To convey the result of a called function, routine, or system service, enabling the caller to react appropriately.
- Implementation: Often implemented as a specific general-purpose register (e.g., Register 15 in z/OS assembler programs following standard linkage conventions) or as a field within a parameter list or control block.
- Values: FSRs are typically numeric, with 0 often indicating successful completion and non-zero values representing various warning, error, or exceptional conditions.
- Standardization: While the concept is general, specific z/OS components, SVC calls, macro expansions, and utility programs define their own unique FSR conventions and detailed meanings.
- Debugging Aid: FSRs are indispensable for debugging and error handling, providing immediate, granular feedback on the status of an operation.
- Control Flow: Programs frequently check the FSR value to determine subsequent processing steps, such as branching to error recovery routines, retrying an operation, or continuing normal execution.

Use Cases

- System Service Calls: After an application program issues a z/OS SVC (Supervisor Call) or calls a system service (e.g., OPEN, CLOSE, GETMAIN, FREEMAIN), the FSR (often in Register 15) indicates the success or failure of that service.
- Utility Program Execution: Many z/OS utility programs (e.g., IDCAMS, IEBGENER, DFSRRC00 for IMS) set a return code (a specific type of FSR) in the JCL STEP or JOB RC to indicate their completion status.
- COBOL/PL/I Subprogram Calls: When a COBOL or PL/I program calls another subprogram, the called program can pass a return code back to the caller, typically via the RETURN-CODE special register in COBOL or a parameter in PL/I.
- Assembler Subroutines: Assembler language subroutines consistently use Register 15 to return a status code to the calling routine, adhering to the z/OS standard linkage conventions.
- DB2 Stored Procedures: Stored procedures in DB2 for z/OS often return a status code to the calling application, indicating the outcome of the procedure's execution.

Related Concepts

FSRs are fundamental to inter-program communication and error handling in the z/OS environment. They are closely related to return codes (RC), condition codes (CC), and abend codes (ABEND). An FSR provides immediate, granular feedback on a function's outcome, while a return code is often a specific type of FSR used at a higher level (e.g., JCL step completion status). Condition codes are typically set by CPU instructions, reflecting the result of arithmetic or logical operations, whereas abend codes signify abnormal program termination, often triggered by a severe error indicated by an FSR that was not handled.

Best Practices:

Always Check FSRs: Programs should consistently check the FSR after calling system services, macros, or subroutines to ensure proper execution and implement robust error handling.
Document FSR Meanings: For custom routines and applications, thoroughly document the meaning of all possible FSR values to facilitate debugging, maintenance, and future development.
Standardize FSR Usage: Adhere to established z/OS conventions (e.g., Register 15 for assembler, RETURN-CODE for COBOL) for consistency and predictability across the enterprise.
Provide Meaningful Error Messages: Translate raw FSR values into user-friendly error messages or detailed log entries to aid in problem determination and operational support.
Implement Error Recovery: Based on the FSR, implement appropriate error recovery logic, such as retries, alternative processing paths, or controlled program termination with diagnostic information.