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A fixed-length field is a data element assigned a predetermined width. In a fixed-format record, it starts and ends at the same positions in every record, so a reader can locate it from the layout rather than by searching for a delimiter. The width might count bytes or characters, and padding and overflow rules depend on the specific format or software.
How a fixed-length field works
A record layout defines where a field begins and how wide it is. For example, a file specification might assign an account code 8 characters, a status 1 character, and an amount 7 characters. A parser can read each value from those positions in every record. The example assumes the specification defines the width units, encoding, and any padding rules.
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IBM Informix documentation describes fixed-format records this way: “each field starts and ends at the same place in every record.” IBM Informix 12.10: Fixed-length records.
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How a field differs from a record
A fixed-length field has a predetermined width within a data structure. A fixed-length record has a predetermined total size. A record can contain several fields whose widths, along with any filler or reserved positions, make up that total.
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IBM’s Enterprise COBOL for z/OS documentation describes a particular VSAM record definition as fixed in size by its declared length. That is a record-level rule, not a general definition of every fixed-length field. IBM Enterprise COBOL for z/OS 6.4: Defining fixed-length records.
Fixed-width layout versus delimited data
In fixed-width data, a layout tells the reader each field’s position and width. In delimited data, a separator such as a comma marks a boundary, so the length of a field can vary. Fixed-width parsing depends on applying the correct layout; delimited parsing depends on recognizing separators and handling them consistently.
Oracle’s SQL*Loader documentation describes fixed-format input records as having the same byte length, with specified fields assigned positions and lengths. This describes that product documentation and should not be taken as a universal rule for every file format. Oracle SQL*Loader Concepts.
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A declared width such as 10 is incomplete unless you know whether it means 10 bytes or 10 characters. The distinction matters for text that includes non-ASCII characters, because a character may occupy more than one byte in an encoding.
Oracle Database 26 documents that a CHAR column’s size can be expressed in characters or bytes, depending on the character set. IBM’s Enterprise COBOL for z/OS 6.5 guide also distinguishes fixed character-length UTF-8 items from fixed byte-length items: fixed character-length items reserve storage based on a maximum of four bytes per character, while byte-length behavior must account for character boundaries. Oracle Database 26: Datatypes and Host Variables; IBM Enterprise COBOL for z/OS 6.5 Programming Guide.
Padding, trimming, and database columns
Fixed width does not guarantee that every system pads or trims values in the same way. Oracle defines its CHAR datatype as fixed-length character data and says the value is fixed for every row, with trailing blanks. Its VARCHAR2 datatype is variable in length per row. These are Oracle datatype behaviors, not rules for all databases or external record formats.
Oracle’s Pro*COBOL documentation describes another layer of behavior: PICX settings affect whether trailing blanks are stripped on input, and fixed-length host variables are blank-padded on output. Identify whether a width refers to a database column, a language variable, or an external record before assuming how spaces will be handled. Oracle Database 26: Datatypes and Host Variables; Oracle Database 19: Datatypes and Host Variables.
What happens when a value exceeds the width
There is no universal overflow rule. In Oracle’s documented Pro*COBOL CHARF cases, input longer than the target database column’s defined width causes an error. When output is assigned to a shorter host variable, Oracle truncates the value before assignment. Other languages, databases, and file formats may reject, truncate, or handle excess data differently.
Check the target system’s documentation and validate values against the field width before writing or transferring data. The meaning of “too long” should also match the format’s unit: bytes and characters can produce different results.
When fixed-length fields are useful
Fixed-length fields fit formats that assign stable positions to values, such as specified external records. They can also suit standardized-width codes; Firebird’s language reference gives codes with a definite standard width as an example for CHAR. Firebird 2.5 Language Reference.
When choosing or interpreting a layout, check the details that determine whether it will work for your data:
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- Width unit and encoding: Does the width count bytes or characters, and which character encoding applies?
- Padding and trimming: Are spaces added, retained, or removed at a particular layer?
- Overflow: Does an over-width value cause an error, get truncated, or follow another documented rule?
- Parsing: Can the reader reliably apply the layout, or does the data use delimiters instead?
Those details—not the phrase “fixed length” alone—determine how a particular field is represented and handled.
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