# btree

## NAME



           btree - btree database access method
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/types.h>**
           **#include <db.h>**
    


## 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.
    
           The routine dbopen(3) is the library interface to database files.
           One of the supported file formats is btree files.  The general
           description of the database access methods is in dbopen(3), this
           manual page describes only the btree-specific information.
    
           The btree data structure is a sorted, balanced tree structure
           storing associated key/data pairs.
    
           The btree access-method-specific data structure provided to
           dbopen(3) is defined in the _< db.h>_ include file as follows:
    
               typedef struct {
                   unsigned long flags;
                   unsigned int  cachesize;
                   int           maxkeypage;
                   int           minkeypage;
                   unsigned int  psize;
                   int         (*compare)(const DBT *key1, const DBT *key2);
                   size_t      (*prefix)(const DBT *key1, const DBT *key2);
                   int           lorder;
               } BTREEINFO;
    
           The elements of this structure are as follows:
    
           _flags_  The flag value is specified by ORing any of the following
                  values:
    
                  **R_DUP** Permit duplicate keys in the tree, that is, permit
                         insertion if the key to be inserted already exists
                         in the tree.  The default behavior, as described in
                         dbopen(3), is to overwrite a matching key when
                         inserting a new key or to fail if the **R_NOOVERWRITE**
                         flag is specified.  The **R_DUP** flag is overridden by
                         the **R_NOOVERWRITE** flag, and if the **R_NOOVERWRITE**
                         flag is specified, attempts to insert duplicate keys
                         into the tree will fail.
    
                         If the database contains duplicate keys, the order
                         of retrieval of key/data pairs is undefined if the
                         _get_ routine is used, however, _seq_ routine calls with
                         the **R_CURSOR** flag set will always return the logical
                         "first" of any group of duplicate keys.
    
           _cachesize_
                  A suggested maximum size (in bytes) of the memory cache.
                  This value is _only_ advisory, and the access method will
                  allocate more memory rather than fail.  Since every search
                  examines the root page of the tree, caching the most
                  recently used pages substantially improves access time.  In
                  addition, physical writes are delayed as long as possible,
                  so a moderate cache can reduce the number of I/O operations
                  significantly.  Obviously, using a cache increases (but
                  only increases) the likelihood of corruption or lost data
                  if the system crashes while a tree is being modified.  If
                  _cachesize_ is 0 (no size is specified), a default cache is
                  used.
    
           _maxkeypage_
                  The maximum number of keys which will be stored on any
                  single page.  Not currently implemented.
    
           _minkeypage_
                  The minimum number of keys which will be stored on any
                  single page.  This value is used to determine which keys
                  will be stored on overflow pages, that is, if a key or data
                  item is longer than the pagesize divided by the minkeypage
                  value, it will be stored on overflow pages instead of in
                  the page itself.  If _minkeypage_ is 0 (no minimum number of
                  keys is specified), a value of 2 is used.
    
           _psize_  Page size is the size (in bytes) of the pages used for
                  nodes in the tree.  The minimum page size is 512 bytes and
                  the maximum page size is 64 KiB.  If _psize_ is 0 (no page
                  size is specified), a page size is chosen based on the
                  underlying filesystem I/O block size.
    
           _compare_
                  Compare is the key comparison function.  It must return an
                  integer less than, equal to, or greater than zero if the
                  first key argument is considered to be respectively less
                  than, equal to, or greater than the second key argument.
                  The same comparison function must be used on a given tree
                  every time it is opened.  If _compare_ is NULL (no comparison
                  function is specified), the keys are compared lexically,
                  with shorter keys considered less than longer keys.
    
           _prefix_ Prefix is the prefix comparison function.  If specified,
                  this routine must return the number of bytes of the second
                  key argument which are necessary to determine that it is
                  greater than the first key argument.  If the keys are
                  equal, the key length should be returned.  Note, the
                  usefulness of this routine is very data-dependent, but, in
                  some data sets can produce significantly reduced tree sizes
                  and search times.  If _prefix_ is NULL (no prefix function is
                  specified), _and_ no comparison function is specified, a
                  default lexical comparison routine is used.  If _prefix_ is
                  NULL and a comparison routine is specified, no prefix
                  comparison is done.
    
           _lorder_ The byte order for integers in the stored database
                  metadata.  The number should represent the order as an
                  integer; for example, big endian order would be the number
                  4,321.  If _lorder_ is 0 (no order is specified), the current
                  host order is used.
    
           If the file already exists (and the **O_TRUNC** flag is not
           specified), the values specified for the arguments _flags_ , _lorder_ ,
           and _psize_ are ignored in favor of the values used when the tree
           was created.
    
           Forward sequential scans of a tree are from the least key to the
           greatest.
    
           Space freed up by deleting key/data pairs from the tree is never
           reclaimed, although it is normally made available for reuse.  This
           means that the btree storage structure is grow-only.  The only
           solutions are to avoid excessive deletions, or to create a fresh
           tree periodically from a scan of an existing one.
    
           Searches, insertions, and deletions in a btree will all complete
           in O lg base N where base is the average fill factor.  Often,
           inserting ordered data into btrees results in a low fill factor.
           This implementation has been modified to make ordered insertion
           the best case, resulting in a much better than normal page fill
           factor.
    


## ERRORS



           The _btree_ access method routines may fail and set _errno_ for any of
           the errors specified for the library routine dbopen(3).
    


## BUGS



           Only big and little endian byte order is supported.
    


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