MirOS Manual: db(3), dbopen(3)

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

NAME

     dbopen - database access methods

SYNOPSIS

     #include <sys/types.h>
     #include <fcntl.h>
     #include <limits.h>
     #include <db.h>

     DB *
     dbopen(const char *file, int flags, int mode, DBTYPE type,
             const void *openinfo);

DESCRIPTION

     The dbopen() function 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 parameter 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_NOFOLLOW, O_NONBLOCK,
     O_RDONLY, O_RDWR, O_SHLOCK, O_SYNC, 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 er-
     ror. 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, u_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 re-
     turned by dbopen(), and sometimes one or more pointers to key/data struc-
     tures 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, fail-
                  ing 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 da-
                  tabase.

                  The parameter flags may be set to the following value:

                  R_CURSOR       Delete the record referenced by the cursor.
                                 The cursor must have previously been initial-
                                 ized.

                  del 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
                  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 re-
                  trieval 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 er-
                  ror (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 data-
                  base.

                  The parameter flags 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 previously exist.

                  R_SETCURSOR    Store the key/data pair, setting or initial-
                                 izing 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 in-
                  herent 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 suc-
                  cess, 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, i.e., records inserted behind the cursor will not be
                  returned while records inserted in front of the cursor will
                  be returned.

                  The flags 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 smal-
                                 lest key greater than or equal to the speci-
                                 fied 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 initial-
                                 ized to reference it.

                  R_LAST         The last key/data pair of the database is re-
                                 turned, 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. (Appli-
                                 cable only to the DB_BTREE and DB_RECNO ac-
                                 cess 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 suc-
                  cess, 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 flags 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 guaran-
     tees about byte string alignment.

ERRORS

     The dbopen() routine may fail and set errno for any of the errors speci-
     fied 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 the 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 data-
     bases.

     The sync routines may fail and set errno for any of the errors specified
     for the library routine fsync(2).

SEE ALSO

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

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

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.

MirOS BSD #10-current          January 2, 1994                               4

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