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NAME

       dbopen - database access methods

SYNOPSIS

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

       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
       performing  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
              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 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
              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
              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 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 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
                     structure.   (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
                     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 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
              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 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 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
       databases.

       The sync routines may  fail  and  set  errno  for  any  of  the  errors
       specified 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,
       locking, 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

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