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

       btree - btree database access method

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

       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.

       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

       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.

              A  suggested maximum size (in bytes) of the memory cache.  This value is only advi‐
              sory, 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 likeli‐
              hood  of  corruption or lost data if the system crashes while a tree is being modi‐
              fied.  If cachesize is 0 (no size is specified), a default cache is used.

              The maximum number of keys which will be stored on any single page.  Not  currently

              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 min‐
              imum page size is 512 bytes and the maximum page size is 64K.  If psize  is  0  (no
              page  size  is specified), a page size is chosen based on the underlying filesystem
              I/O block size.

              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 compari‐
              son 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 num‐
              ber 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.

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

       Only big and little endian byte order is supported.

       dbopen(3), hash(3), mpool(3), recno(3)

       The Ubiquitous B-tree, Douglas Comer, ACM Comput. Surv. 11, 2 (June 1979), 121-138.

       Prefix  B-trees,  Bayer  and  Unterauer,  ACM  Transactions on Database Systems, Vol. 2, 1
       (March 1977), 11-26.

       The Art of Computer Programming Vol. 3:  Sorting  and  Searching,  D.E.  Knuth,  1968,  pp

       This  page  is  part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

                                            2012-04-23                                   BTREE(3)

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