NAME | SYNOPSIS | DESCRIPTION | ERRORS | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       dbopen - database access methods

SYNOPSIS         top

       #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         top

       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         top

       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         top

       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         top

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

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

COLOPHON         top

       This page is part of release 4.08 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        2012-05-04                        DBOPEN(3)