dbopen(3) Library Functions Manual dbopen(3)
NAME
dbopen - database access methods
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <limits.h>
#include <db.h>
#include <fcntl.h>
DB *dbopen(const char *file, int flags, int mode, DBTYPE type,
const void *openinfo);
DESCRIPTION
Note well: This page documents interfaces provided up until glibc 2.1.
Since glibc 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 for-
mat 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 re-
spective 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 struc-
ture 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 er-
ror. The DB structure is defined in the <db.h> include file, and con-
tains 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 perform-
ing 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 er-
rno) and 0 on success.
del A pointer to a routine to remove key/data pairs from the data-
base.
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 represen-
tative 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 rou-
tines 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 struc-
ture. (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 struc-
ture. (Applicable only to the DB_RECNO access method.)
R_NOOVERWRITE
Enter the new key/data pair only if the key does not pre-
viously exist.
R_SETCURSOR
Store the key/data pair, setting or initializing the posi-
tion 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 ac-
cess 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 ex-
ists in the file.
seq A pointer to a routine which is the interface for sequential re-
trieval 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 in-
serted 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 ini-
tializes 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. (Ap-
plicable only to the DB_BTREE and DB_RECNO access meth-
ods.)
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 in-
formation.)
sync routines return -1 on error (setting errno) and 0 on suc-
cess.
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 guar-
antees about byte string alignment.
ERRORS
The dbopen() routine may fail and set errno for any of the errors speci-
fied for the library routines open(2) and malloc(3) or the following:
EFTYPE A file is incorrectly formatted.
EINVAL A parameter has been specified (hash function, pad byte, etc.)
that is incompatible with the current file specification or which
is not meaningful for the function (for example, use of the cur-
sor without prior initialization) or there is a mismatch between
the version number of file and the software.
The close routines may fail and set errno for any of the errors speci-
fied for the library routines close(2), read(2), write(2), free(3), or
fsync(2).
The del, get, put, and seq routines may fail and set errno for any of
the errors specified for the library routines read(2), write(2),
free(3), or malloc(3).
The fd routines will fail and set errno to ENOENT for in memory data-
bases.
The sync routines may fail and set errno for any of the errors specified
for the library routine fsync(2).
BUGS
The typedef DBT is a mnemonic for "data base thang", and was used be-
cause no one could think of a reasonable name that wasn't already used.
The file descriptor interface is a kludge and will be deleted in a fu-
ture version of the interface.
None of the access methods provide any form of concurrent access, lock-
ing, or transactions.
SEE ALSO
btree(3), hash(3), mpool(3), recno(3)
LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer, Michael
Olson, USENIX proceedings, Winter 1992.
4.4 Berkeley Distribution 2024-05-02 dbopen(3)
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