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RedHat 9 (Linux i386) - man page for dbopen (redhat section 3)

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 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 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 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.

       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 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 incon-
	      sistent 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 database.

	      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	success,  and  1  if  the
	      specified key was not in the file.

       fd     A pointer to a routine which returns a file descriptor representative of the under-
	      lying 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
	      routines 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 data-
	      base.  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 structure.  (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  structure.   (Applicable only to the DB_RECNO access
		     method.)

	      R_NOOVERWRITE
		     Enter the new key/data pair only if the key does not previously 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 meth-
		     ods.)

	      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, replac-
	      ing 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  refer-
	      enced  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.  Modi-
	      fications 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  spe-
	      cial  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 rou-
		     tine 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 success.

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 guarantees about byte string alignment.

ERRORS
       The  dbopen routine may fail and set errno for any of the errors specified 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  mis-
	      match 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 databases.

       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)

       LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer, Michael Olson, USENIX  pro-
       ceedings, 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, locking, or transac-
       tions.

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


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