DDNAME - Data Definition Name

Key Characteristics

• • Symbolic Link: It's a symbolic name (1-8 characters, alphanumeric, first char A-Z or #, $, @) that a program uses to request access to data.
  • JCL Statement: Defined within a DD (Data Definition) statement in JCL, which specifies the actual physical characteristics and location of the data set (e.g., DSN, DISP, UNIT, VOL).
  • Program-Defined: The DDNAME itself is often hardcoded or specified within the application program (e.g., in a SELECT statement in COBOL or a DCB macro in assembler) to identify the file.
  • Dynamic Allocation: While typically defined in JCL, DDNAMEs can also be dynamically allocated by programs using SVC 99 (DYNALLOC) or TSO ALLOCATE commands, allowing for flexible resource assignment at runtime.
  • Scope: A DDNAME is typically unique within a single job step, but it can be referenced across steps if defined in a JOBLIB or STEPLIB concatenation, or if it refers to a temporary data set passed between steps.
  • Standard DDNAMEs: Certain DDNAMEs have special meanings, such as SYSPRINT for print output, SYSIN for input data, and SYSOUT for system output.

Use Cases

• • Batch Program Input/Output: Connecting a COBOL program's SELECT ASSIGN TO DDNAME statement to a DD statement in JCL that points to an input sequential file or an output report file.
  • Utility Program Parameters: Providing input to z/OS utilities like IEBGENER or IDCAMS where SYSIN is used for control statements and SYSPRINT for messages.
  • Library Concatenation: Defining STEPLIB or JOBLIB DDNAMEs to specify private program libraries that should be searched before system libraries for program execution.
  • Temporary Data Sets: Creating work files for sorting or intermediate processing using DDNAMEs with DSN=&&TEMPDSN to manage temporary storage.
  • VSAM Data Set Access: Linking a program's DDNAME to a DD statement that defines a VSAM KSDS, ESDS, or RRDS, including its AMP parameters for performance tuning.

Related Concepts