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DBOPEN(3)			   BSD Library Functions Manual 			DBOPEN(3)

NAME
     dbopen, db -- database access methods

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

     DB *
     dbopen(const char *file, int flags, mode_t 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).

     The dbopen() function 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.

   The DB Structure
     The dbopen() function 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 allo-
		   cated 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 inconsistent 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
		   underlying 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
		   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 cur-
				   sor 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 previ-
				   ously 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 rou-
		   tines.  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 dif-
			      fers from the get routines in that it sets or initializes the cur-
			      sor 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 data-
		   base is in memory only, the sync routine has no effect and will always suc-
		   ceed.

		   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 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 meaning-
			ful for the function (for example, use of the cursor without prior ini-
			tialization) or there is a mismatch between the version number of file
			and the software.

     EFBIG		The key could not be inserted due to limitations in the DB file format
			(e.g., a hash database was out of overflow pages).

     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)

     Margo Seltzer and Michael Olson, "LIBTP: Portable, Modular Transactions for UNIX",
     Proceedings of the 1992 Winter USENIX Technical Conference, USENIX Association, 9-25, 1992.

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, locking, or transactions.

BSD					December 16, 2010				      BSD
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