A file can contain records written in only one of three formats:
fixed-length, variable-length, and undefined-length. You can specify
the format that you want for your records, either with the
HPFOPEN/FOPEN intrinsic or the MPE/iX BUILD or FILE
commands.
Files residing on disk or magnetic tape may contain records in any of
the three formats. For files on other devices, the file system
overrides any specifications that you supply, and treats the records as
undefined-length records.
Fixed-length records |
 |
When you create a file and request fixed-length records, all the
records in the file will be the same size. The file system knows
how much space has been allocated for each record, and that all of the
space is to be available for data.
Figure 2-2 “Fixed-Length Records” depicts a file with fixed-length records. A record size
of n bytes has been specified. Note that each record is the same
size and contains the same amount of information.
Variable-length records |
|
There may be a time when you want a disk file in which
the logical records need not be the same size. In this case, you can request
that the format of the records be variable-length. The file system knows
the size of each logical record because each record is preceded by a two-byte
(16-bit) counter giving the length of the record in bytes; thus, the data for
each record is accompanied by an indication of its length. When you build a
file containing variable-length records, specify a record size at least large
enough to accommodate your longest record.
Figure 2-3 “Variable-length Records” depicts a file with variable-length records. The byte count
preceding the first byte of each record gives its record's length.
Undefined-length records |
|
When your file contains undefined-length records, the
file system does not know the amount of good data in any given logical record.
The data length is "undefined." Undefined-length records are especially useful
when you are reading tapes of unknown record length produced on other systems.
The file system knows the maximum room available in each record because the
same amount of space is allocated for each record; however, the data in the
records may vary in length, so MPE/iX pads the unused space with "filler"
instead of good data. The file system supplies this filler during writes to
the file when the length of the data being written is less than the maximum
record length. The file system cannot distinguish between valid data and
filler. When you read data from a file you must be able to distinguish between
the valid data and the filler.
Figure 2-4 “Undefined-Length Records” depicts a file with undefined-length records. When data does not
fill the space allocated, filler occupies the unused space.
The three record formats, fixed-length, variable-length, and undefined-length
are summarized in Table 2-5 “Comparison of Logical Record Formats”.
Table 2-5 Comparison of Logical Record Formats
| Fixed-Length | Variable-Length | Undefined-Length |
|---|
| Data length known to file system. | Data length known to file system. | Data length not known to file system. |
| Same length for all records. | Record length varies. | Same length for all records. |
| Record space contains data only. | Record space contains data plus byte count. | Record space contains data plus filler. |
| Request actual size for records. | Request maximum size for records. | Request maximum size for records. |
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