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DBOPEN(3)							     DBOPEN(3)

NAME
       dbopen -	database access	methods

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

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

DESCRIPTION
       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
       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	 parameter  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
       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, u_int flags);
	      int (*fd)(const DB *db);
	      int (*get)(const DB *db, 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 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  inconsis-
	      tent  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 parameter 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  suc-
	      cess, 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  referenc-
	      ing  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 descrip-
	      tor 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 parameter 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
		     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 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 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	unlim-
       ited 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
       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 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	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	speci-
       fied for	the library routine fsync(2).

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

       LIBTP:  Portable, Modular Transactions for UNIX,	Margo Seltzer, Michael
       Olson, 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	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, lock-
       ing, or transactions.

4.4 Berkeley Distribution	  1994-01-02			     DBOPEN(3)

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

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