dbopen(3) — Linux manual page

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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 5.11 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)

Pages that refer to this page: btree(3)hash(3)mpool(3)recno(3)