Table of Contents
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)
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.
Only big and little endian byte order is supported.
Table of Contents