The routine
dbopen(3)
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(3)
is defined in the
<db.h>
include file as follows:
typedef struct {
unisgned long flags;
unsigned int cachesize;
unsigned int psize;
int lorder;
size_t reclen;
unsigned 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(3),
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(3)
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 filename for a
dbopen(3)
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 th
<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 metadata 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(3),
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.
ERRORS
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.
BUGS
Only big and little endian byte order is supported.
Document Processing in a Relational Database System,
Michael Stonebraker, Heidi Stettner, Joseph Kalash, Antonin Guttman,
Nadene Lynn, Memorandum No. UCB/ERL M82/32, May 1982.
COLOPHON
This page is part of release 3.14 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
