db/man/hash.3

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.\"	@(#)hash.3	8.6 (Berkeley) 8/18/94
.\" $FreeBSD: src/lib/libc/db/man/hash.3,v 1.4.2.3 2003/02/23 19:45:52 trhodes Exp $
.\" $DragonFly: src/lib/libc/db/man/hash.3,v 1.6 2007/08/18 20:48:47 swildner Exp $
.\"
.Dd August 18, 1994
.Dt HASH 3
.Os
.Sh NAME
.Nm hash
.Nd "hash database access method"
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In db.h
.Sh DESCRIPTION
The routine
.Fn dbopen
is the library interface to database files.
One of the supported file formats is
.Nm
files.
The general description of the database access methods is in
.Xr dbopen 3 ,
this manual page describes only the
.Nm
specific information.
.Pp
The
.Nm
data structure is an extensible, dynamic hashing scheme.
.Pp
The access method specific data structure provided to
.Fn dbopen
is defined in the
.In db.h
include file as follows:
.Bd -literal
typedef struct {
	u_int bsize;
	u_int ffactor;
	u_int nelem;
	u_int cachesize;
	u_int32_t (*hash)(const void *, size_t);
	int lorder;
} HASHINFO;
.Ed
.Pp
The elements of this structure are as follows:
.Bl -tag -width indent
.It Va bsize
The
.Va bsize
element
defines the
.Nm
table bucket size, and is, by default, 256 bytes.
It may be preferable to increase the page size for disk-resident tables
and tables with large data items.
.It Va ffactor
The
.Va ffactor
element
indicates a desired density within the
.Nm
table.
It is an approximation of the number of keys allowed to accumulate in any
one bucket, determining when the
.Nm
table grows or shrinks.
The default value is 8.
.It Va nelem
The
.Va nelem
element
is an estimate of the final size of the
.Nm
table.
If not set or set too low,
.Nm
tables will expand gracefully as keys
are entered, although a slight performance degradation may be noticed.
The default value is 1.
.It Va cachesize
A suggested maximum size, in bytes, of the memory cache.
This value is
.Em only
advisory, and the access method will allocate more memory rather
than fail.
.It Va hash
The
.Va hash
element
is a user defined
.Nm
function.
Since no
.Nm
function performs equally well on all possible data, the
user may find that the built-in
.Nm
function does poorly on a particular
data set.
User specified
.Nm
functions must take two arguments (a pointer to a byte
string and a length) and return a 32-bit quantity to be used as the
.Nm
value.
.It Va 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
.Va lorder
is 0 (no order is specified) the current host order is used.
If the file already exists, the specified value is ignored and the
value specified when the tree was created is used.
.El
.Pp
If the file already exists (and the
.Dv O_TRUNC
flag is not specified), the
values specified for the
.Va bsize , ffactor , lorder
and
.Va nelem
arguments
are
ignored and the values specified when the tree was created are used.
.Pp
If a
.Nm
function is specified,
.Fn __hash_open
will attempt to determine if the
.Nm
function specified is the same as
the one with which the database was created, and will fail if it is not.
.Pp
Backward compatible interfaces to the older
.Em dbm
and
.Em ndbm
routines are provided, however these interfaces are not compatible with
previous file formats.
.Sh ERRORS
The
.Nm
access method routines may fail and set
.Va errno
for any of the errors specified for the library routine
.Xr dbopen 3 .
.Sh SEE ALSO
.Xr btree 3 ,
.Xr dbopen 3 ,
.Xr mpool 3 ,
.Xr recno 3
.Rs
.%T "Dynamic Hash Tables"
.%A Per-Ake Larson
.%R "Communications of the ACM"
.%D April 1988
.Re
.Rs
.%T "A New Hash Package for UNIX"
.%A Margo Seltzer
.%R "USENIX Proceedings"
.%D Winter 1991
.Re
.Sh BUGS
Only big and little endian byte order is supported.