ChorusOS man pages section 3POSIX: POSIX Library Functions

btree(3POSIX)

NAME

btree- btree database access method

SYNOPSIS

#include <sys/types.h>

#include <db.h>

DESCRIPTION

dbopen() is the library interface to database files. One of the file formats supported is btree files. The general description of the database access methods is in dbopen(3POSIX). 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 accesses a method-specific data structure provided to dbopen() by the <db.h> include file, whose structure is the following:

typedef struct {
     u_long  flags;
     u_int   cachesize;
     int     maxkeypage;
     int     minkeypage;
     u_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 or'ing any of the following values:

: R_DUP permits duplicate keys in the tree, (in other words, allows insertion of the key even if it 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 memory rather than fail. As 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 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, (if a key or data item is longer than the page size 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 64K. If psize is 0 (no page size is specified) a page size is chosen based on the underlying file system I/O block size.
compare: This 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 smaller than longer keys.
prefix: This is the prefix comparison function. If specified, this routine must return the number of bytes of the second key argument, which are needed to determine that it is larger than the first key argument. If the keys are equal, the key length should be returned. Note that 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 meta data. The number should represent the order as an integer; for example, big end-Ian 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 parameters 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 smallest key to the largest.

Space freed 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 better than usual page fill factor.

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving

BUGS

Only big and little endian byte order is supported.

RESTRICTIONS

These library calls do not support multi-threaded applications.

ChorusOS 4.0 Last Revised December 1999