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Character String

Enhanced Definition

A character string, in the mainframe context, is a contiguous sequence of bytes representing textual data, where each byte (or sequence of bytes) corresponds to a specific character. It is a fundamental data type used for storing, manipulating, and displaying human-readable information within z/OS applications and data structures. Its primary encoding on z/OS is EBCDIC.

Key Characteristics

- EBCDIC Encoding: On IBM z/OS systems, character strings are predominantly encoded using EBCDIC (Extended Binary Coded Decimal Interchange Code), which differs from ASCII used on most other platforms.
- Fixed or Variable Length: Character strings can be defined with a fixed length (e.g., PIC X(20) in COBOL) or a variable length (e.g., VARCHAR in DB2, or using OCCURS DEPENDING ON in COBOL).
- Storage: Stored as a sequence of bytes in memory, data sets, or database fields. Fixed-length strings are often padded with spaces to their defined length.
- Manipulation: Common operations include concatenation, substring extraction, comparison, searching, and conversion between different character sets (e.g., EBCDIC to ASCII).
- Data Type Representation: Often represented as PIC X in COBOL, CHAR or VARCHAR in DB2, DS CLn in Assembler, or character arrays in C programs.

Use Cases

- Storing Alphanumeric Data: Used extensively for names, addresses, product descriptions, comments, and other textual fields in business applications.
- JCL Parameters: Passing textual information to batch programs via PARM fields on EXEC statements or as values in DD statements (e.g., DSN, UNIT).
- Database Fields: Defining columns in DB2 tables (e.g., CHAR, VARCHAR, GRAPHIC) or IMS segments to hold textual data.
- Report Generation: Formatting and presenting human-readable output, including headers, labels, and data values in batch reports or online screens.
- Message Processing: Constructing and parsing messages in transaction processing systems like CICS or message queuing systems like IBM MQ.

Related Concepts

Character strings are intrinsically linked to EBCDIC, which dictates their byte -level representation on z/OS. In COBOL, they are defined using PIC X data types and are manipulated through MOVE, STRING, UNSTRING, and intrinsic functions. They form the content of many data sets (e.g., sequential files, PDS members, VSAM KSDS data components) and are critical for defining parameters and data set names in JCL. Within DB2, they correspond to CHAR and VARCHAR column types, and in CICS and IMS, they are used for data fields in screens, transactions, and database segments.

Best Practices:

EBCDIC-ASCII Conversion: Be acutely aware of character encoding when exchanging data with non-mainframe systems. Implement explicit EBCDIC-ASCII conversions using utilities like ICONV or programmatically when necessary.
Data Validation: Always validate the content of character strings (e.g., length, allowed characters, format) upon input to prevent data corruption, buffer overflows, or application errors.
Fixed vs. Variable Length Selection: Choose fixed-length strings (PIC X(n)) for data with consistent lengths (e.g., codes) and variable-length strings (VARCHAR, OCCURS DEPENDING ON) for data with varying lengths (e.g., descriptions) to optimize storage and I/O.
Padding and Truncation: Understand how COBOL handles padding with spaces (for shorter source to longer target) and truncation (for longer source to shorter target) during MOVE operations to avoid unexpected data loss or extra spaces.
Performance Considerations: For extensive string manipulations, leverage optimized built-in functions or system services where available, rather than manual byte -by-byte processing, to improve performance.