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A fixed-length record is a logical record in a file that always contains the same number of bytes as every other record in that file. Its length stays constant; the record layout—not the fixed length itself—defines what the bytes and fields mean.
What does “fixed-length record” mean?
IBM defines a fixed-length record as a record associated with a file whose description requires all records to contain the same number of bytes. In practical terms, each record occupies a predictable amount of space, so the next record begins at a predictable position when records are stored consecutively.
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Fixed length applies to records within the relevant file format. It does not mean that different files must use the same record length.
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What does fixed length tell you—and what does it not?
It tells you the size of each record, not how to interpret its contents. A separate record layout or schema defines the fields, their types, and their positions. For example, a 25-byte record might have fields assigned to particular byte ranges, but the length alone does not reveal what those fields represent.
With a consistent layout, software can look for a field at the same offset in every record. The format therefore offers predictable boundaries, while the schema supplies meaning.
Fixed-length and variable-length records compared
| Characteristic | Fixed-length record | Variable-length record |
|---|---|---|
| Record size | The same number of bytes for every record in the file. | May differ from record to record within a specified range. |
| Field positions | Can be read at consistent offsets when records share the same layout. | Positions may depend on the record’s contents or length and the layout rules. |
| Length description | A single fixed record length. | A minimum and maximum length can be declared; IBM COBOL uses a varying-record clause for this case. |
Variable length does not mean that record boundaries are unknowable: the file format must provide a way to determine each record’s actual length. The exact representation depends on the format.
How IBM z/OS identifies fixed records
In IBM z/OS data set terminology, RECFM identifies the record format and LRECL gives the logical record length. IBM’s DFSORT documentation uses RECFM=FB and LRECL=25 as an example: each logical record is 25 bytes, and B indicates that the records are blocked. Blocking concerns how logical records are grouped in a physical block; it does not change the meaning of a record’s logical length.
IBM’s programming guide describes format-F logical records as fixed length and notes that a block can contain multiple logical records. A block’s size and a record’s size are therefore related storage concepts, but they are not interchangeable.
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How fixed record length is declared in COBOL
In IBM Enterprise COBOL, a program can specify a fixed record size with a RECORD CONTAINS integer clause. Alternatively, the clause may be omitted when all level-01 record descriptions for the file have the same fixed size. IBM says the stated record size should match the data set’s length attribute.
For records whose sizes can vary, IBM COBOL documents a RECORD IS VARYING FROM ... TO ... clause to declare the permitted minimum and maximum lengths. That is a different arrangement from a fixed-length file, where every record has one shared byte count.
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Sources
- IBM Enterprise COBOL for z/OS 6.5 glossary: “fixed-length record”
- IBM Enterprise COBOL documentation on record length
- IBM z/OS DFSORT documentation on record formats
- IBM z/OS programming guide on record formats
- Government-hosted COBOL programming guide
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