162 lines
4.4 KiB
Groff
162 lines
4.4 KiB
Groff
.\" Copyright (c) 2001 Tobias Weingartner
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $OpenBSD: hash.9,v 1.5 2003/04/17 05:08:39 jmc Exp $
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.\" $FreeBSD$
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.\"
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.Dd October 19, 2006
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.Dt HASH 9
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.Os
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.Sh NAME
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.Nm hash ,
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.Nm hash32 ,
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.Nm hash32_buf ,
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.Nm hash32_str ,
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.Nm hash32_strn ,
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.Nm hash32_stre ,
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.Nm hash32_strne
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.Nd general kernel hashing functions
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.Sh SYNOPSIS
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.In sys/hash.h
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.Ft uint32_t
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.Fn hash32_buf "const void *buf" "size_t len" "uint32_t hash"
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.Ft uint32_t
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.Fn hash32_str "const void *buf" "uint32_t hash"
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.Ft uint32_t
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.Fn hash32_strn "const void *buf" "size_t len" "uint32_t hash"
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.Ft uint32_t
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.Fn hash32_stre "const void *buf" "int end" "char **ep" "uint32_t hash"
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.Ft uint32_t
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.Fn hash32_strne "const void *buf" "size_t len" "int end" "char **ep" "uint32_t hash"
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.Sh DESCRIPTION
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The
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.Fn hash32
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functions are used to give a consistent and general interface to
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a decent hashing algorithm within the kernel.
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These functions can be used to hash
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.Tn ASCII
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.Dv NUL
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terminated strings, as well as blocks of memory.
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.Pp
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The
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.Fn hash32_buf
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function is used as a general buffer hashing function.
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The argument
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.Fa buf
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is used to pass in the location, and
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.Fa len
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is the length of the buffer.
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The argument
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.Fa hash
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is used to extend an existing hash, or is passed the initial value
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.Dv HASHINIT
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to start a new hash.
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.Pp
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The
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.Fn hash32_str
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function is used to hash a
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.Dv NUL
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terminated string passed in
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.Fa buf
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with initial hash value given in
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.Fa hash .
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.Pp
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The
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.Fn hash32_strn
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function is like the
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.Fn hash32_str
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function, except it also takes a
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.Fa len
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argument, which is the maximal length of the expected string.
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.Pp
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The
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.Fn hash32_stre
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and
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.Fn hash32_strne
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functions are helper functions used by the kernel to hash pathname
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components.
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These functions have the additional termination condition
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of terminating when they find a character given by
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.Fa end
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in the string to be hashed.
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If the argument
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.Fa ep
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is not
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.Dv NULL ,
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it is set to the point in the buffer at which the hash function
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terminated hashing.
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.Sh RETURN VALUES
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The
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.Fn hash32
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functions return a 32 bit hash value of the buffer or string.
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.Sh EXAMPLES
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.Bd -literal -offset indent
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LIST_HEAD(head, cache) *hashtbl = NULL;
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u_long mask = 0;
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void
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sample_init(void)
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{
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hashtbl = hashinit(numwanted, type, flags, &mask);
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}
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void
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sample_use(char *str, int len)
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{
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uint32_t hash;
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hash = hash32_str(str, HASHINIT);
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hash = hash32_buf(&len, sizeof(len), hash);
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hashtbl[hash & mask] = len;
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}
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.Ed
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.Sh SEE ALSO
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.Xr free 9 ,
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.Xr hashinit 9 ,
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.Xr malloc 9
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.Sh LIMITATIONS
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The
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.Fn hash32
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functions are only 32 bit functions.
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They will prove to give poor 64 bit performance, especially for the
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top 32 bits.
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At the current time, this is not seen as a great limitation, as these
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hash values are usually used to index into an array.
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Should these hash values be used for other means, this limitation should
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be revisited.
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.Sh HISTORY
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The
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.Nm
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functions were first committed to
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.Nx 1.6 .
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The
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.Ox
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versions were written and massaged for
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.Ox 2.3
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by Tobias Weingartner,
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and finally committed for
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.Ox 3.2 .
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