методы доступа к базе данных (database access methods)
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.
dbopen() is the library interface to database files. The
supported file formats are btree, hashed, and UNIX file oriented.
The btree format is a representation of a sorted, balanced tree
structure. The hashed format is an extensible, dynamic hashing
scheme. The flat-file format is a byte stream file with fixed or
variable length records. The formats and file-format-specific
information are described in detail in their respective manual
pages btree(3), hash(3), and recno(3).
dbopen() opens file for reading and/or writing. Files never
intended to be preserved on disk may be created by setting the
file argument to NULL.
The flags and mode arguments are as specified to the open(2)
routine, however, only the O_CREAT, O_EXCL, O_EXLOCK, O_NONBLOCK,
O_RDONLY, O_RDWR, O_SHLOCK, and O_TRUNC flags are meaningful.
(Note, opening a database file O_WRONLY is not possible.)
The type argument is of type DBTYPE (as defined in the <db.h>
include file) and may be set to DB_BTREE, DB_HASH, or DB_RECNO.
The openinfo argument is a pointer to an access-method-specific
structure described in the access method's manual page. If
openinfo is NULL, each access method will use defaults
appropriate for the system and the access method.
dbopen() returns a pointer to a DB structure on success and NULL
on error. The DB structure is defined in the <db.h> include
file, and contains at least the following fields:
typedef struct {
DBTYPE type;
int (*close)(const DB *db);
int (*del)(const DB *db, const DBT *key, unsigned int flags);
int (*fd)(const DB *db);
int (*get)(const DB *db, DBT *key, DBT *data,
unsigned int flags);
int (*put)(const DB *db, DBT *key, const DBT *data,
unsigned int flags);
int (*sync)(const DB *db, unsigned int flags);
int (*seq)(const DB *db, DBT *key, DBT *data,
unsigned int flags);
} DB;
These elements describe a database type and a set of functions
performing various actions. These functions take a pointer to a
structure as returned by dbopen(), and sometimes one or more
pointers to key/data structures and a flag value.
type The type of the underlying access method (and file
format).
close A pointer to a routine to flush any cached information to
disk, free any allocated resources, and close the
underlying file(s). Since key/data pairs may be cached in
memory, failing to sync the file with a close or sync
function may result in inconsistent or lost information.
close routines return -1 on error (setting errno) and 0 on
success.
del A pointer to a routine to remove key/data pairs from the
database.
The argument flag may be set to the following value:
R_CURSOR
Delete the record referenced by the cursor. The
cursor must have previously been initialized.
delete routines return -1 on error (setting errno), 0 on
success, and 1 if the specified key was not in the file.
fd A pointer to a routine which returns a file descriptor
representative of the underlying database. A file
descriptor referencing the same file will be returned to
all processes which call dbopen() with the same file name.
This file descriptor may be safely used as an argument to
the fcntl(2) and flock(2) locking functions. The file
descriptor is not necessarily associated with any of the
underlying files used by the access method. No file
descriptor is available for in memory databases. fd
routines return -1 on error (setting errno), and the file
descriptor on success.
get A pointer to a routine which is the interface for keyed
retrieval from the database. The address and length of
the data associated with the specified key are returned in
the structure referenced by data. get routines return -1
on error (setting errno), 0 on success, and 1 if the key
was not in the file.
put A pointer to a routine to store key/data pairs in the
database.
The argument flag may be set to one of the following
values:
R_CURSOR
Replace the key/data pair referenced by the cursor.
The cursor must have previously been initialized.
R_IAFTER
Append the data immediately after the data
referenced by key, creating a new key/data pair.
The record number of the appended key/data pair is
returned in the key structure. (Applicable only to
the DB_RECNO access method.)
R_IBEFORE
Insert the data immediately before the data
referenced by key, creating a new key/data pair.
The record number of the inserted key/data pair is
returned in the key structure. (Applicable only to
the DB_RECNO access method.)
R_NOOVERWRITE
Enter the new key/data pair only if the key does
not previously exist.
R_SETCURSOR
Store the key/data pair, setting or initializing
the position of the cursor to reference it.
(Applicable only to the DB_BTREE and DB_RECNO
access methods.)
R_SETCURSOR is available only for the DB_BTREE and
DB_RECNO access methods because it implies that the keys
have an inherent order which does not change.
R_IAFTER and R_IBEFORE are available only for the DB_RECNO
access method because they each imply that the access
method is able to create new keys. This is true only if
the keys are ordered and independent, record numbers for
example.
The default behavior of the put routines is to enter the
new key/data pair, replacing any previously existing key.
put routines return -1 on error (setting errno), 0 on
success, and 1 if the R_NOOVERWRITE flag was set and the
key already exists in the file.
seq A pointer to a routine which is the interface for
sequential retrieval from the database. The address and
length of the key are returned in the structure referenced
by key, and the address and length of the data are
returned in the structure referenced by data.
Sequential key/data pair retrieval may begin at any time,
and the position of the "cursor" is not affected by calls
to the del, get, put, or sync routines. Modifications to
the database during a sequential scan will be reflected in
the scan, that is, records inserted behind the cursor will
not be returned while records inserted in front of the
cursor will be returned.
The flag value must be set to one of the following values:
R_CURSOR
The data associated with the specified key is
returned. This differs from the get routines in
that it sets or initializes the cursor to the
location of the key as well. (Note, for the
DB_BTREE access method, the returned key is not
necessarily an exact match for the specified key.
The returned key is the smallest key greater than
or equal to the specified key, permitting partial
key matches and range searches.)
R_FIRST
The first key/data pair of the database is
returned, and the cursor is set or initialized to
reference it.
R_LAST The last key/data pair of the database is returned,
and the cursor is set or initialized to reference
it. (Applicable only to the DB_BTREE and DB_RECNO
access methods.)
R_NEXT Retrieve the key/data pair immediately after the
cursor. If the cursor is not yet set, this is the
same as the R_FIRST flag.
R_PREV Retrieve the key/data pair immediately before the
cursor. If the cursor is not yet set, this is the
same as the R_LAST flag. (Applicable only to the
DB_BTREE and DB_RECNO access methods.)
R_LAST and R_PREV are available only for the DB_BTREE and
DB_RECNO access methods because they each imply that the
keys have an inherent order which does not change.
seq routines return -1 on error (setting errno), 0 on
success and 1 if there are no key/data pairs less than or
greater than the specified or current key. If the
DB_RECNO access method is being used, and if the database
file is a character special file and no complete key/data
pairs are currently available, the seq routines return 2.
sync A pointer to a routine to flush any cached information to
disk. If the database is in memory only, the sync routine
has no effect and will always succeed.
The flag value may be set to the following value:
R_RECNOSYNC
If the DB_RECNO access method is being used, this
flag causes the sync routine to apply to the btree
file which underlies the recno file, not the recno
file itself. (See the bfname field of the recno(3)
manual page for more information.)
sync routines return -1 on error (setting errno) and 0 on
success.
Key/data pairs
Access to all file types is based on key/data pairs. Both keys
and data are represented by the following data structure:
typedef struct {
void *data;
size_t size;
} DBT;
The elements of the DBT structure are defined as follows:
data A pointer to a byte string.
size The length of the byte string.
Key and data byte strings may reference strings of essentially
unlimited length although any two of them must fit into available
memory at the same time. It should be noted that the access
methods provide no guarantees about byte string alignment.
