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Specifying a Record Format [ Accessing Files Programmer's Guide ] MPE/iX 5.0 Documentation

Accessing Files Programmer's Guide

Specifying a Record Format

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 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.

Figure 2-2. Fixed-Length Records

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 depicts a file with variable-length records. The byte
count preceding the first byte of each record gives its record's length.

Figure 2-3. Variable-length Records

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 depicts a file with undefined-length records. When data
does not fill the space allocated, filler occupies the unused space.

Figure 2-4. Undefined-Length Records

The three record formats, fixed-length, variable-length, and
undefined-length are summarized in Table 2-5 .

MPE/iX 5.0 Documentation