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man:db

DBOPEN(3) Linux Programmer's Manual DBOPEN(3)

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

     Note  well:  This  page documents interfaces provided in glibc up until
     version 2.1.  Since version 2.2, glibc no longer provides these  inter-
     faces.   Probably,  you  are looking for the APIs provided by the libdb
     library instead.
     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  data-
            base.
            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 true only 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 data-
     bases.
     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 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

4.4 Berkeley Distribution 2017-09-15 DBOPEN(3)

/data/webs/external/dokuwiki/data/pages/man/db.txt · Last modified: 2019/05/17 09:47 by 127.0.0.1

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