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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	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.

4.4 Berkeley Distribution	January 2, 1994			     DBOPEN(3)
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