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DBOPEN(3)	       FreeBSD Library Functions Manual		     DBOPEN(3)

NAME
     dbopen -- database	access methods

SYNOPSIS
     #include <sys/types.h>
     #include <db.h>
     #include <fcntl.h>
     #include <limits.h>

     DB	*
     dbopen(const char *file, int flags, int mode, DBTYPE type,
	 const void *openinfo);

DESCRIPTION
     The dbopen() function 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).

     The dbopen() function opens file for reading and/or writing.  Files never
     intended to be preserved on disk may be created by	setting	the file argu-
     ment 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_NOFOLLOW, O_NONBLOCK,
     O_RDONLY, O_RDWR, O_SHLOCK, O_SYNC	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.

     The dbopen() function 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)(DB *db);
	     int (*del)(const DB *db, const DBT	*key, u_int flags);
	     int (*fd)(const DB	*db);
	     int (*get)(const DB *db, const DBT	*key, DBT *data, u_int flags);
	     int (*put)(const DB *db, DBT *key,	const DBT *data,
		  u_int	flags);
	     int (*sync)(const DB *db, u_int flags);
	     int (*seq)(const DB *db, DBT *key,	DBT *data, u_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 data-
	     base.

	     The flags argument	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 flags argument	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
		     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 only true 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, i.e., records
	     inserted behind the cursor	will not be returned while records
	     inserted in front of the cursor will be returned.

	     The flags argument	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 flags argument	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 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 guaran-
     tees 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]		An argument has	been specified (hash function, pad
			byte etc.) that	is incompatible	with the current file
			specification or which is not meaningful for the func-
			tion (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 for any of the errors specified
     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).

SEE ALSO
     btree(3), hash(3),	mpool(3), recno(3)

     Margo Seltzer and Michael Olson, LIBTP: Portable, Modular Transactions
     for UNIX, USENIX proceedings, Winter 1992.

BUGS
     The typedef DBT is	a mnemonic for ``data base thang'', and	was used
     because noone could think of a reasonable name that was not already used.

     The file descriptor interface is a	kluge and will be deleted in a future
     version of	the interface.

     None of the access	methods	provide	any form of concurrent access, lock-
     ing, or transactions.

FreeBSD	9.3		      September	10, 2010		   FreeBSD 9.3

NAME | SYNOPSIS | DESCRIPTION | KEY/DATA PAIRS | ERRORS | SEE ALSO | BUGS

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