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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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       pages/.