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

NAME         top

       btree - btree database access method

SYNOPSIS         top

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

DESCRIPTION         top

       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 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 val‐

              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 insert‐
                     ing 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.  Obvi‐
              ously, using a cache increases (but only increases) the like‐
              lihood of corruption or lost data if the system crashes while
              a tree is being modified.  If cachesize is 0 (no size is spec‐
              ified), 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 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 maxi‐
              mum page size is 64 KiB.  If psize is 0 (no page size is spec‐
              ified), a page size is chosen based on the underlying filesys‐
              tem 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 compar‐
              ison function must be used on a given tree every time it is
              opened.  If compare is NULL (no comparison function is speci‐
              fied), the keys are compared lexically, with shorter keys con‐
              sidered 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 rou‐
              tine is very data-dependent, but, in some data sets can pro‐
              duce significantly reduced tree sizes and search times.  If
              prefix is NULL (no prefix function is specified), and no com‐
              parison 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 exam‐
              ple, 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 solu‐
       tions 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 imple‐
       mentation has been modified to make ordered insertion the best case,
       resulting in a much better than normal page fill factor.

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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       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

                                 2017-09-15                         BTREE(3)

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