179 lines
4.8 KiB
Groff
179 lines
4.8 KiB
Groff
.\"
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.\" Copyright (c) 2004 Joseph Koshy
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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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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS''
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.\" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
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.\" LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd January 23, 2016
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.Dt HASHINIT 9
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.Os
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.Sh NAME
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.Nm hashinit , hashinit_flags , hashdestroy , phashinit
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.Nd manage kernel hash tables
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.Sh SYNOPSIS
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.In sys/malloc.h
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.In sys/systm.h
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.In sys/queue.h
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.Ft "void *"
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.Fn hashinit "int nelements" "struct malloc_type *type" "u_long *hashmask"
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.Ft void
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.Fo hashinit_flags
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.Fa "int nelements" "struct malloc_type *type" "u_long *hashmask" "int flags"
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.Fc
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.Ft void
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.Fn hashdestroy "void *hashtbl" "struct malloc_type *type" "u_long hashmask"
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.Ft "void *"
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.Fn phashinit "int nelements" "struct malloc_type *type" "u_long *nentries"
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.Sh DESCRIPTION
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The
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.Fn hashinit ,
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.Fn hashinit_flags
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and
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.Fn phashinit
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functions allocate space for hash tables of size given by the argument
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.Fa nelements .
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.Pp
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The
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.Fn hashinit
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function allocates hash tables that are sized to largest power of two
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less than or equal to argument
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.Fa nelements .
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The
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.Fn phashinit
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function allocates hash tables that are sized to the largest prime
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number less than or equal to argument
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.Fa nelements .
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The
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.Fn hashinit_flags
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function operates like
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.Fn hashinit
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but also accepts an additional argument
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.Fa flags
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which control various options during allocation.
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Allocated hash tables are contiguous arrays of
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.Xr LIST_HEAD 3
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entries, allocated using
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.Xr malloc 9 ,
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and initialized using
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.Xr LIST_INIT 3 .
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The malloc arena to be used for allocation is pointed to by argument
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.Fa type .
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.Pp
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The
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.Fn hashdestroy
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function frees the space occupied by the hash table pointed to by argument
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.Fa hashtbl .
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Argument
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.Fa type
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determines the malloc arena to use when freeing space.
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The argument
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.Fa hashmask
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should be the bit mask returned by the call to
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.Fn hashinit
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that allocated the hash table.
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The argument
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.Fa flags
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must be used with one of the following values.
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.Pp
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.Bl -tag -width ".Dv HASH_NOWAIT" -offset indent -compact
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.It Dv HASH_NOWAIT
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Any malloc performed by the
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.Fn hashinit_flags
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function will not be allowed to wait, and therefore may fail.
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.It Dv HASH_WAITOK
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Any malloc performed by
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.Fn hashinit_flags
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function is allowed to wait for memory.
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This is also the behavior of
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.Fn hashinit .
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.El
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.Sh IMPLEMENTATION NOTES
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The largest prime hash value chosen by
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.Fn phashinit
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is 32749.
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.Sh RETURN VALUES
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The
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.Fn hashinit
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function returns a pointer to an allocated hash table and sets the
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location pointed to by
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.Fa hashmask
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to the bit mask to be used for computing the correct slot in the
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hash table.
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.Pp
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The
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.Fn phashinit
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function returns a pointer to an allocated hash table and sets the
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location pointed to by
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.Fa nentries
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to the number of rows in the hash table.
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.Sh EXAMPLES
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A typical example is shown below:
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.Bd -literal -offset indent
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\&...
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static LIST_HEAD(foo, foo) *footable;
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static u_long foomask;
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\&...
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footable = hashinit(32, M_FOO, &foomask);
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.Ed
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.Pp
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Here we allocate a hash table with 32 entries from the malloc arena
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pointed to by
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.Dv M_FOO .
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The mask for the allocated hash table is returned in
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.Va foomask .
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A subsequent call to
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.Fn hashdestroy
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uses the value in
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.Va foomask :
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.Bd -literal -offset indent
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\&...
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hashdestroy(footable, M_FOO, foomask);
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.Ed
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.Sh DIAGNOSTICS
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The
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.Fn hashinit
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and
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.Fn phashinit
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functions will panic if argument
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.Fa nelements
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is less than or equal to zero.
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.Pp
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The
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.Fn hashdestroy
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function will panic if the hash table
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pointed to by
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.Fa hashtbl
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is not empty.
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.Sh SEE ALSO
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.Xr LIST_HEAD 3 ,
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.Xr malloc 9
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.Sh BUGS
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There is no
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.Fn phashdestroy
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function, and using
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.Fn hashdestroy
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to free a hash table allocated by
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.Fn phashinit
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usually has grave consequences.
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