freebsd-skq/share/man/man9/hash.9

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.\"     $OpenBSD: hash.9,v 1.5 2003/04/17 05:08:39 jmc Exp $
.\" $FreeBSD$
.\"
.Dd April 3, 2007
.Dt HASH 9
.Os
.Sh NAME
.Nm hash ,
.Nm hash32 ,
.Nm hash32_buf ,
.Nm hash32_str ,
.Nm hash32_strn ,
.Nm hash32_stre ,
.Nm hash32_strne
.Nd general kernel hashing functions
.Sh SYNOPSIS
.In sys/hash.h
.Ft uint32_t
.Fn hash32_buf "const void *buf" "size_t len" "uint32_t hash"
.Ft uint32_t
.Fn hash32_str "const void *buf" "uint32_t hash"
.Ft uint32_t
.Fn hash32_strn "const void *buf" "size_t len" "uint32_t hash"
.Ft uint32_t
.Fn hash32_stre "const void *buf" "int end" "const char **ep" "uint32_t hash"
.Ft uint32_t
.Fn hash32_strne "const void *buf" "size_t len" "int end" "const char **ep" "uint32_t hash"
.Sh DESCRIPTION
The
.Fn hash32
functions are used to give a consistent and general interface to
a decent hashing algorithm within the kernel.
These functions can be used to hash
.Tn ASCII
.Dv NUL
terminated strings, as well as blocks of memory.
.Pp
The
.Fn hash32_buf
function is used as a general buffer hashing function.
The argument
.Fa buf
is used to pass in the location, and
.Fa len
is the length of the buffer.
The argument
.Fa hash
is used to extend an existing hash, or is passed the initial value
.Dv HASHINIT
to start a new hash.
.Pp
The
.Fn hash32_str
function is used to hash a
.Dv NUL
terminated string passed in
.Fa buf
with initial hash value given in
.Fa hash .
.Pp
The
.Fn hash32_strn
function is like the
.Fn hash32_str
function, except it also takes a
.Fa len
argument, which is the maximal length of the expected string.
.Pp
The
.Fn hash32_stre
and
.Fn hash32_strne
functions are helper functions used by the kernel to hash pathname
components.
These functions have the additional termination condition
of terminating when they find a character given by
.Fa end
in the string to be hashed.
If the argument
.Fa ep
is not
.Dv NULL ,
it is set to the point in the buffer at which the hash function
terminated hashing.
.Sh RETURN VALUES
The
.Fn hash32
functions return a 32 bit hash value of the buffer or string.
.Sh EXAMPLES
.Bd -literal -offset indent
LIST_HEAD(head, cache) *hashtbl = NULL;
u_long mask = 0;

void
sample_init(void)
{

        hashtbl = hashinit(numwanted, type, flags, &mask);
}

void
sample_use(char *str, int len)
{
        uint32_t hash;

        hash = hash32_str(str, HASHINIT);
        hash = hash32_buf(&len, sizeof(len), hash);
        hashtbl[hash & mask] = len;
}
.Ed
.Sh SEE ALSO
.Xr free 9 ,
.Xr hashinit 9 ,
.Xr malloc 9
.Sh LIMITATIONS
The
.Fn hash32
functions are only 32 bit functions.
They will prove to give poor 64 bit performance, especially for the
top 32 bits.
At the current time, this is not seen as a great limitation, as these
hash values are usually used to index into an array.
Should these hash values be used for other means, this limitation should
be revisited.
.Sh HISTORY
The
.Nm
functions were first committed to
.Nx 1.6 .
The
.Ox
versions were written and massaged for
.Ox 2.3
by Tobias Weingartner,
and finally committed for
.Ox 3.2 .