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DBOPEN(3)		   Linux Programmer's Manual		     DBOPEN(3)

NAME
       dbopen - database access methods

SYNOPSIS
       #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
       Note  well:  This  page documents interfaces provided in glibc up until
       version 2.1.  Since version 2.2, glibc no longer provides these	inter‐
       faces.	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 for‐
       mat 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).

       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)  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, 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 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 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), 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 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	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 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), 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),	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 spec‐
       ified 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).

BUGS
       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, lock‐
       ing, or transactions.

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

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

COLOPHON
       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

4.4 Berkeley Distribution	  2012-05-04			     DBOPEN(3)
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