метод доступа к базе данных номеров записей (record number database access method)
Имя (Name)
recno - record number database access method
Синопсис (Synopsis)
#include <sys/types.h>
#include <db.h>
Описание (Description)
Note well: This page documents interfaces provided in glibc up
until version 2.1. Since version 2.2, glibc no longer provides
these interfaces. Probably, you are looking for the APIs
provided by the libdb library instead.
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 {
unsigned 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 ORing 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 filesystem 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 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 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.
Ошибки (Error)
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.
Ошибки (баги) (Bugs)
Only big and little endian byte order is supported.
Смотри также (See also)
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.
