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DBOPEN(3)                  Linux Programmer's Manual                 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  for-
       mat  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 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  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, unsigned int flags);
               int (*fd)(const DB *db);
               int (*get)(const DB *db, DBT *key, DBT *data,
                          unsigned int flags);
               int (*put)(const DB *db, DBT *key, const DBT *data,
                          unsigned int flags);
               int (*sync)(const DB *db, unsigned int flags);
               int (*seq)(const DB *db, DBT *key, DBT *data,
                          unsigned 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 argument 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 argument 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, that is,
              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 spec-
       ified 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).

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.

SEE ALSO
       btree(3), hash(3), mpool(3), recno(3)

       LIBTP: Portable, Modular Transactions for UNIX, Margo Seltzer,  Michael
       Olson, USENIX proceedings, Winter 1992.

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

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

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