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

NAME
     dbopen - database access methods



SYNOPSIS
     #include <sys/types.h>
     #include <fcntl.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 re-
     turned 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
                  database.

                  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 rep-
                  resentative 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 de-
                  scriptor 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 (set-
                  ting errno), and the file descriptor on success.

           get    A pointer to a routine which is the interface for keyed re-
                  trieval 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 er-
                  ror (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 val-
                  ues:

                  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 refer-
                         enced by key, creating a new key/data pair.  The
                         record number of the inserted key/data pair is re-
                         turned 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 in-
                  herent 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 suc-
                  cess, 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 re-
                         turned.  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 search-
                         es.)

                  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
                         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 suc-
                  cess 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
                  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 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 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 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 databas-
     es.

     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 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,
     locking, or transactions.

OpenBSD 2.6                     January 2, 1994                              5