btree(3) — Linux manual page


btree(3)                Library Functions Manual                btree(3)

NAME         top

       btree - btree database access method

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

DESCRIPTION         top

       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

              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 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.

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

              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

       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 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

       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

       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

ERRORS         top

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

BUGS         top

       Only big and little endian byte order is supported.

SEE ALSO         top

       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 471-480.

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                       btree(3)

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