ADC Home > Reference Library > Reference > Mac OS X > Mac OS X Man Pages

 

This document is a Mac OS X manual page. Manual pages are a command-line technology for providing documentation. You can view these manual pages locally using the man(1) command. These manual pages come from many different sources, and thus, have a variety of writing styles.

For more information about the manual page format, see the manual page for manpages(5).



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

NAME
     dbopen -- database access methods

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

     DB *
     dbopen(const char *file, int flags, int mode, DBTYPE type, const void *openinfo);

DESCRIPTION
     The dbopen() function 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 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 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 dbopen() function returns a pointer to a DB structure on success and NULL on error.  The DB struc-ture structure
     ture 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 func-tions functions
     tions 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 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 flags argument may be set to the following value:

             R_CURSOR
                     Delete the record referenced by the cursor.  The cursor must have previously been ini-tialized. initialized.
                     tialized.

             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 data-base. database.
             base.  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 descrip-tor descriptor
             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 flags argument 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. structure.
                     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. structure.
                     ture.  (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 refer-ence reference
                     ence 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 pre-viously previously
             viously 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 cur-sor cursor
             sor will not be returned while records inserted in front of the cursor will be returned.

             The flags argument 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 rou-tines routines
                     tines 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 initial-ized initialized
                     ized to reference it.

             R_LAST  The last key/data pair of the database is returned, and the cursor is set or initial-ized initialized
                     ized 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 flags argument 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 follow-ing following
     ing 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 pro-vide provide
     vide 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]           An argument 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 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, USENIX proceedings,
     Winter 1992.

BUGS
     The typedef DBT is a mnemonic for ``data base thang'', and was used because noone could think of a rea-sonable reasonable
     sonable 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.

BSD                             January 2, 1994                            BSD

Did this document help you?
Yes: Tell us what works for you.
It’s good, but: Report typos, inaccuracies, and so forth.
It wasn’t helpful: Tell us what would have helped.