Using data in input and output operations

Provide the following information about the data:

• Name the input and output files that the program will use. Use the FD entry to give names to the files that the input-output statements in the PROCEDURE DIVISION can refer to.

  Data items defined in the FILE SECTION are not available to PROCEDURE DIVISION statements until the file has been successfully opened.

• In the record description that follows the FD entry, describe the fields of the records in the file:
  • You can code a level-01 description of the entire record, and then in the WORKING-STORAGE SECTION code a working copy that describes the fields of the record in more detail. Use the READ INTO statement to bring the records into WORKING-STORAGE. Processing occurs on the copy of data in WORKING-STORAGE. A WRITE FROM statement writes processed data into the record area defined in the FILE SECTION.
  • The record-name established is the object of WRITE and REWRITE statements.
  • For QSAM files only, you can set the record format in the RECORDING MODE clause. If you omit the RECORDING MODE clause, the compiler determines the record format based on the RECORD clause and on the level-01 record descriptions.
  • For QSAM files, you can set a blocking factor for the file in the BLOCK CONTAINS clause. If you omit the BLOCK CONTAINS clause, the file defaults to unblocked. However, you can override this with z/OS® data management facilities (including a DD file job-control statement).
  • For line-sequential files, you can set a blocking factor for the file in the BLOCK CONTAINS clause. When you code BLOCK CONTAINS 1 RECORDS, or BLOCK CONTAINS n CHARACTERS, where n is the length of one logical record in bytes, WRITE statements result in the record being transferred immediately to the file rather than being buffered. This technique is useful when you want each record written immediately, such as to an error log.

Programs in the same run unit can share, or have access to, common files. The method for doing this depends on whether the programs are part of a nested (contained) structure or are separately compiled (including programs compiled as part of a batch sequence).

You can use the EXTERNAL clause for separately compiled programs. A file that is defined as EXTERNAL can be referenced by any program in the run unit that describes the file.

You can use the GLOBAL clause for programs in a nested, or contained, structure. If a program contains another program (directly or indirectly), both programs can access a common file by referencing a GLOBAL file-name.