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DBOPEN(3)		   Library Functions Manual		     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  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, 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  re-
	      trieval  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  po-
		     sition  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  ac-
	      cess  method  because  they each imply that the access method is
	      able to create new keys.	This is	only true if the keys are  or-
	      dered 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, 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  re-
		     turned  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 ef-
	      fect 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 be-
       cause 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  fu-
       ture 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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