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
   interfaces.   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
   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 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
          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

   This page is part of release 4.09 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/.





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