btree(3) Library Functions Manual btree(3)
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 de-
scribes 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-
ues:
R_DUP Permit duplicate keys in the tree, that is, permit inser-
tion 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 in-
sert duplicate keys into the tree will fail.
If the database contains duplicate keys, the order of re-
trieval 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 substan-
tially 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 corrup-
tion 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 inte-
ger 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 argu-
ment 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 signifi-
cantly 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 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 great-
est.
Space freed up by deleting key/data pairs from the tree is never re-
claimed, 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 or-
dered data into btrees results in a low fill factor. This implementa-
tion has been modified to make ordered insertion the best case, result-
ing 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 Sys-
tems, Vol. 2, 1 (March 1977), 11-26.
The Art of Computer Programming Vol. 3: Sorting and Searching, D.E.
Knuth, 1968, pp 471-480.
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