recno
—
record number database access method
#include
<sys/types.h>
#include
<db.h>
The routine
dbopen
() is the library interface
to database files. One of the supported file formats is record number files.
The general description of the database access methods is in
dbopen(3),
this manual page describes only the
recno
specific information.
The record number data structure is either variable or fixed-length records
stored in a flat-file format, accessed by the logical record number. The
existence of record number five implies the existence of records one through
four, and the deletion of record number one causes record number five to be
renumbered to record number four, as well as the cursor, if positioned after
record number one, to shift down one record.
The
recno
access method specific data
structure provided to
dbopen
() is defined
in the
<db.h>
include file as follows:
typedef struct {
u_long flags;
u_int cachesize;
u_int psize;
int lorder;
size_t reclen;
u_char bval;
char *bfname;
} RECNOINFO;
The elements of this structure are defined as follows:
- flags
- The flag value is specified by or'ing any of
the following values:
R_FIXEDLEN
- The records are fixed-length, not byte delimited. The structure
element reclen specifies the length
of the record, and the structure element
bval is used as the pad character.
Any records, inserted into the database, that are less than
reclen bytes long are automatically
padded.
R_NOKEY
- In the interface specified by
dbopen
(), the sequential record
retrieval fills in both the caller's key and data structures. If the
R_NOKEY
flag is specified, the
cursor routines are not required to fill
in the key structure. This permits applications to retrieve records at
the end of files without reading all of the intervening records.
R_SNAPSHOT
- This flag requires that a snapshot of the file be taken when
dbopen
() is called, instead of
permitting any unmodified records to be read from the original
file.
- cachesize
- A suggested maximum size, in bytes, of the memory cache. This value is
only advisory, and the access method will
allocate more memory rather than fail. If
cachesize is 0 (no size is specified) a
default cache is used.
- psize
- The
recno
access method stores the
in-memory copies of its records in a btree. This value is the size (in
bytes) of the pages used for nodes in that tree. If
psize is 0 (no page size is specified) a
page size is chosen based on the underlying file system I/O block size.
See
btree(3)
for more information.
- lorder
- The byte order for integers in the stored database metadata. The number
should represent the order as an integer; for example, big endian order
would be the number 4,321. If lorder is 0
(no order is specified) the current host order is used.
- reclen
- The length of a fixed-length record.
- bval
- The delimiting byte to be used to mark the end of a record for
variable-length records, and the pad character for fixed-length records.
If no value is specified, newlines (“\n”) are used to mark
the end of variable-length records and fixed-length records are padded
with spaces.
- bfname
- The
recno
access method stores the
in-memory copies of its records in a btree. If
bfname is
non-NULL
, it
specifies the name of the btree file, as if specified as the file name for
a dbopen
() of a btree file.
The data part of the key/data pair used by the
recno
access method is the same as other
access methods. The key is different. The
data field of the key should be a pointer to
a memory location of type
recno_t, as defined
in the
<db.h>
include file. This type is normally the largest unsigned integral type
available to the implementation. The
size
field of the key should be the size of that type.
Because there can be no meta-data associated with the underlying
recno
access method files, any changes made
to the default values (e.g. fixed record length or byte separator value) must
be explicitly specified each time the file is opened.
In the interface specified by
dbopen
(), using
the
put interface to create a new record will
cause the creation of multiple, empty records if the record number is more
than one greater than the largest record currently in the database.
The
recno
access method routines may fail and
set
errno for any of the errors specified for
the library routine
dbopen(3)
or the following:
- [
EINVAL
]
- An attempt was made to add a record to a fixed-length database that was
too large to fit.
btree(3),
dbopen(3),
hash(3),
mpool(3)
Michael Stonebraker,
Heidi Stettner, Joseph
Kalash, Antonin Guttman, and
Nadene Lynn, Document Processing in
a Relational Database System, Memorandum No. UCB/ERL
M82/32, May 1982.
Only big and little endian byte order is supported.