Originální popis anglicky:
recno - record number database access method
Návod, kniha: Library Functions Manual
#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.
