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A fixed-length record is a record in a file that always contains the same number of bytes as every other record in that file. Its fields occupy predictable positions, but the record’s layout—not its fixed size alone—defines what those fields mean.
What “fixed length” means
The term describes the size of each logical record within a particular file format. If a file uses fixed-length records, every record has the same byte count. It does not mean that every file must use the same record length.
For example, a layout might reserve set positions for an account number, a name, and a status code. Each record follows those positions and has the same total size. The layout explains how to interpret the fields; fixed length only guarantees the consistent size and positions. IBM defines a fixed-length record as one whose file description requires all records to contain the same number of bytes (IBM Enterprise COBOL for z/OS 6.5 glossary).
How fixed-length records are described on z/OS
IBM z/OS data set terminology uses RECFM for record format and LRECL for logical record length. In IBM’s example, RECFM=FB and LRECL=25 describe a fixed, blocked data set with 25 bytes per logical record. The F indicates fixed-length records; B indicates that records are blocked. This 25-byte value is an illustrative example, not a general requirement (IBM DFSORT documentation).
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Record length and block size are different concepts. A block can contain multiple logical records; blocking does not change the length of each record. IBM’s programming guide explains format-F records and blocking (IBM z/OS programming guide).
Fixed-length versus variable-length records
| Property | Fixed-length | Variable-length |
|---|---|---|
| Record size | Every record has the same byte count. | Record lengths can differ within a specified range. |
| Field positions | A fixed layout can place fields at predictable offsets. | Positions may depend on the record’s actual length and layout. |
| Length description | A single record length applies. | A minimum and maximum length can be specified. |
In COBOL, IBM documents RECORD IS VARYING FROM ... TO ... for records whose lengths vary within declared limits. The distinction is about records within the file: variable length does not mean the file itself has no defined structure (IBM Enterprise COBOL record clauses).
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Declaring fixed-length records in COBOL
IBM Enterprise COBOL allows a program to describe fixed record size with RECORD CONTAINS integer. Alternatively, the clause can be omitted if all level-01 record descriptions have the same fixed size. The stated record size should match the data set’s length attribute. See IBM’s documentation for the RECORD CONTAINS clause.
When reading a COBOL file definition, distinguish the declared record size from the fields within it: the declaration establishes the length, while the record descriptions establish the data layout.
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