DBOPEN(3)               NetBSD 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).

     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 struc-
     tures 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 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 data-
             base.

             The parameter flag may be set to the following value:

             R_CURSOR   Delete the record referenced by the cursor.  The cur-
                        sor 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 represen-
             tative 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 cur-
                             sor.  The cursor must have previously been ini-
                             tialized.

             R_IAFTER        Append the data immediately after the data refer-
                             enced 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 ref-
                             erenced 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 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 cur-
                        sor.  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 cur-
                        sor.  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 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 guaran-
     tees 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 func-
                        tion (for example, use of the cursor without prior
                        initialization) 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 data-
     bases.

     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", USENIX proceedings, Winter 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, lock-
     ing, or transactions.

NetBSD 5.0_STABLE               April 17, 2003               NetBSD 5.0_STABLE
