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db

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 for-
       mat 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 struc-
       ture 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 perform-
       ing  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  inconsis-
              tent  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  suc-
              cess, 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  referenc-
              ing  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  rou-
              tines  return -1 on error (setting errno), and the file descrip-
              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 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  struc-
                     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.  (Applicable only to the DB_RECNO access method.)

              R_NOOVERWRITE
                     Enter the new key/data pair only if the key does not pre-
                     viously 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 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 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 unlim-
       ited 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  speci-
       fied 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 speci-
       fied 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  speci-
       fied 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 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 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.



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