# dbopen

## NAME



           dbopen - database access methods
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/types.h>**
           **#include <limits.h>**
           **#include <db.h>**
           **#include <fcntl.h>**
    
           **DB *dbopen(const char ***_path_**, int** _flags_**, int** _mode_**, DBTYPE** _type_**,**
                      **const void ***_openinfo_**);**
    


## DESCRIPTION



           _Note well_ : This page documents interfaces provided up until glibc
           2.1.  Since glibc 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 _path_ for reading and/or writing.  Files never
           intended to be preserved on disk may be created by setting the
           _path_ 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 _path_ 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.
    


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