BTREE(3) Linux Programmer's Manual BTREE(3)
NAME
btree - btree database access method
SYNOPSIS
#include <sys/types.h>
#include <db.h>
DESCRIPTION
Note well: This page documents interfaces provided in glibc up until version 2.1. Since version 2.2, glibc no longer provides these inter-
faces. 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 descrip-
tion 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 mem-
ory 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 argu-
ment 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 com-
pared 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.
SEE ALSO
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
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latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
2017-09-15 BTREE(3)