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 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 func-
       tions performing various actions.  These functions take  a
       pointer  to  a  structure as returned by dbopen, and some-
       times 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 informa-
              tion  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  incon-
              sistent 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 ini-
                     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
              database.  A file descriptor referencing  the  same
              file  will  be returned to all processes which call
              dbopen with the same file name.  This file descrip-
              tor  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 follow-
              ing 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 previously exist.

              R_SETCURSOR
                     Store the key/data pair, setting or initial-
                     izing  the  position of the cursor to refer-
                     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 indepen-
              dent, record numbers for example.

              The default behavior of  the  put  routines  is  to
              enter  the  new key/data pair, replacing any previ-
              ously 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 rou-
                     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 par-
                     tial 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 to reference it.

              R_LAST The last key/data pair of  the  database  is
                     returned,  and the cursor is set or initial-
                     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 informa-
              tion  to  disk.  If the database 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 struc-
       ture:

       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 essen-
       tially 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 mal-
       loc(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  with-
              out  prior  initialization)  or there is a mismatch
              between the version number of file  and  the  soft-
              ware.

       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)

       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  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.

                         January 2, 1994                        1