dbopen(3) - Linux manual page (original) (raw)
dbopen(3) Library Functions Manual dbopen(3)
NAME top
dbopen - database access methodsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 top
_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 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)](../man3/btree.3.html), [hash(3)](../man3/hash.3.html), and [recno(3)](../man3/recno.3.html).
**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)](../man2/open.2.html)
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](../man3/errno.3.html)_) 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](../man3/errno.3.html)_), 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)](../man2/fcntl.2.html) and [flock(2)](../man2/flock.2.html) 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](../man3/errno.3.html)_), 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](../man3/errno.3.html)_), 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](../man3/errno.3.html)_), 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](../man3/errno.3.html)_), 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)](../man3/recno.3.html)
manual page for more information.)
_sync_ routines return -1 on error (setting _[errno](../man3/errno.3.html)_) 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.ERRORS top
The **dbopen**() routine may fail and set _[errno](../man3/errno.3.html)_ for any of the errors
specified for the library routines [open(2)](../man2/open.2.html) and [malloc(3)](../man3/malloc.3.html) 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 cursor 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](../man3/errno.3.html)_ for any of the errors
specified for the library routines [close(2)](../man2/close.2.html), [read(2)](../man2/read.2.html), [write(2)](../man2/write.2.html),
[free(3)](../man3/free.3.html), or [fsync(2)](../man2/fsync.2.html).
The _del_, _get_, _put_, and _seq_ routines may fail and set _[errno](../man3/errno.3.html)_ for any
of the errors specified for the library routines [read(2)](../man2/read.2.html),
[write(2)](../man2/write.2.html), [free(3)](../man3/free.3.html), or [malloc(3)](../man3/malloc.3.html).
The _fd_ routines will fail and set _[errno](../man3/errno.3.html)_ to **ENOENT** for in memory
databases.
The _sync_ routines may fail and set _[errno](../man3/errno.3.html)_ for any of the errors
specified for the library routine [fsync(2)](../man2/fsync.2.html).BUGS top
The typedef _DBT_ is a mnemonic for "data base thang", and was used
because 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
future version of the interface.
None of the access methods provide any form of concurrent access,
locking, or transactions.SEE ALSO top
[btree(3)](../man3/btree.3.html), [hash(3)](../man3/hash.3.html), [mpool(3)](../man3/mpool.3.html), [recno(3)](../man3/recno.3.html)
_LIBTP: Portable, Modular Transactions for UNIX_, Margo Seltzer,
Michael Olson, USENIX proceedings, Winter 1992.COLOPHON top
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Pages that refer to this page:btree(3), hash(3), mpool(3), recno(3)