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274 lines
6.7 KiB
Groff
274 lines
6.7 KiB
Groff
.\" $OpenBSD: ohash_init.3,v 1.2 2014/05/13 14:01:41 jmc Exp $
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.\" Copyright (c) 1999 Marc Espie <espie@openbsd.org>
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.\"
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.\" Permission to use, copy, modify, and distribute this software for any
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.\" purpose with or without fee is hereby granted, provided that the above
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.\" copyright notice and this permission notice appear in all copies.
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.\"
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.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd May 12, 2014
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.Dt OHASH_INIT 3
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.Os
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.Sh NAME
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.Nm ohash_init ,
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.Nm ohash_delete ,
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.Nm ohash_lookup_interval ,
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.Nm ohash_lookup_memory ,
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.Nm ohash_find ,
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.Nm ohash_remove ,
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.Nm ohash_insert ,
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.Nm ohash_first ,
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.Nm ohash_next ,
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.Nm ohash_entries
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.Nd light-weight open hashing
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.Sh SYNOPSIS
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.In stdint.h
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.In stddef.h
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.In ohash.h
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.Ft void
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.Fn ohash_init "struct ohash *h" "unsigned int size" "struct ohash_info *info"
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.Ft void
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.Fn ohash_delete "struct ohash *h"
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.Ft "unsigned int"
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.Fn ohash_lookup_interval "struct ohash *h" "const char *start" "const char *end" "uint32_t hv"
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.Ft "unsigned int"
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.Fn ohash_lookup_memory "struct ohash *h" "const char *k" "size_t s" "uint32_t hv"
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.Ft void *
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.Fn ohash_find "struct ohash *h" "unsigned int i"
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.Ft void *
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.Fn ohash_remove "struct ohash *h" "unsigned int i"
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.Ft void *
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.Fn ohash_insert "struct ohash *h" "unsigned int i" "void *p"
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.Ft void *
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.Fn ohash_first "struct ohash *h" "unsigned int *i"
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.Ft void *
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.Fn ohash_next "struct ohash *h" "unsigned int *i"
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.Ft "unsigned int"
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.Fn ohash_entries "struct ohash *h"
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.Sh DESCRIPTION
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These functions have been designed as a fast, extensible alternative to
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the usual hash table functions.
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They provide storage and retrieval of records indexed by keys,
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where a key is a contiguous sequence of bytes at a fixed position in
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each record.
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Keys can either be NUL-terminated strings or fixed-size memory areas.
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All functions take a pointer to an ohash structure as the
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.Fa h
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function argument.
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Storage for this structure should be provided by user code.
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.Pp
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.Fn ohash_init
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initializes the table to store roughly 2 to the power
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.Fa size
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elements.
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.Fa info
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is a pointer to a
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.Fa struct ohash_info .
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.Bd -literal -offset indent
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struct ohash_info {
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ptrdiff_t key_offset;
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void *data; /* user data */
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void *(*calloc)(size_t, size_t, void *);
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void (*free)(void *, void *);
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void *(*alloc)(size_t, void *);
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};
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.Ed
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.Pp
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The
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.Va offset
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field holds the position of the key in each record;
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the
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.Va calloc
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and
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.Va free
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fields are pointers to
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.Xr calloc 3
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and
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.Xr free 3 Ns -like
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functions, used for managing the table internal storage;
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the
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.Va alloc
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field is only used by the utility function
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.Xr ohash_create_entry 3 .
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.Pp
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Each of these functions are called similarly to their standard counterpart,
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but with an extra
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.Ft void *
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parameter corresponding to the content of the field
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.Fa data ,
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which can be used to communicate specific information to the functions.
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.Pp
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.Fn ohash_init
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stores a copy of those fields internally, so
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.Fa info
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can be reclaimed after initialization.
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.Pp
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.Fn ohash_delete
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frees storage internal to
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.Fa h .
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Elements themselves should be freed by the user first, using for instance
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.Fn ohash_first
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and
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.Fn ohash_next .
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.Pp
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.Fn ohash_lookup_interval
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and
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.Fn ohash_lookup_memory
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are the basic look-up element functions.
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The hashing function result is provided by the user as
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.Fa hv .
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These return a
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.Qq slot
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in the ohash table
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.Fa h ,
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to be used with
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.Fn ohash_find ,
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.Fn ohash_insert ,
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or
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.Fn ohash_remove .
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This slot is only valid up to the next call to
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.Fn ohash_insert
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or
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.Fn ohash_remove .
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.Pp
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.Fn ohash_lookup_interval
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handles string-like keys.
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.Fn ohash_lookup_interval
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assumes the key is the interval between
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.Fa start
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and
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.Fa end ,
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exclusive,
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though the actual elements stored in the table should only contain
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NUL-terminated keys.
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.Pp
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.Fn ohash_lookup_memory
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assumes the key is the memory area starting at
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.Fa k
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of size
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.Fa s .
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All bytes are significant in key comparison.
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.Pp
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.Fn ohash_find
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retrieves an element from a slot
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.Fa i
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returned by the
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.Fn ohash_lookup*
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functions.
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It returns
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.Dv NULL
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if the slot is empty.
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.Pp
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.Fn ohash_insert
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inserts a new element
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.Fa p
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at slot
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.Fa i .
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Slot
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.Fa i
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must be empty and element
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.Fa p
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must have a key corresponding to the
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.Fn ohash_lookup*
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call.
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.Pp
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.Fn ohash_remove
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removes the element at slot
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.Fa i .
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It returns the removed element, for user code to dispose of, or
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.Dv NULL
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if the slot was empty.
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.Pp
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.Fn ohash_first
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and
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.Fn ohash_next
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can be used to access all elements in an ohash table, like this:
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.Bd -literal -offset indent
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for (n = ohash_first(h, &i); n != NULL; n = ohash_next(h, &i))
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do_something_with(n);
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.Ed
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.Pp
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.Fa i
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points to an auxiliary unsigned integer used to record the current position
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in the ohash table.
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Those functions are safe to use even while entries are added to/removed
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from the table, but in such a case they don't guarantee that new entries
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will be returned.
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As a special case, they can safely be used to free elements in the table.
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.Pp
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.Fn ohash_entries
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returns the number of elements in the hash table.
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.Sh STORAGE HANDLING
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Only
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.Fn ohash_init ,
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.Fn ohash_insert ,
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.Fn ohash_remove
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and
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.Fn ohash_delete
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may call the user-supplied memory functions:
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.Bd -literal -offset indent
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p = (*info->calloc)(n, sizeof_record, info->data);
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/* copy data from old to p */
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(*info->free)(old, info->data);
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.Ed
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.Pp
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It is the responsibility of the user memory allocation code to verify
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that those calls did not fail.
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.Pp
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If memory allocation fails,
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.Fn ohash_init
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returns a useless hash table.
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.Fn ohash_insert
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and
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.Fn ohash_remove
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still perform the requested operation, but the returned table should be
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considered read-only.
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It can still be accessed by
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.Fn ohash_lookup* ,
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.Fn ohash_find ,
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.Fn ohash_first
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and
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.Fn ohash_next
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to dump relevant information to disk before aborting.
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.Sh THREAD SAFETY
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The open hashing functions are not thread-safe by design.
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In particular, in a threaded environment, there is no guarantee that a
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.Qq slot
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will not move between a
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.Fn ohash_lookup*
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and a
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.Fn ohash_find ,
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.Fn ohash_insert
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or
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.Fn ohash_remove
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call.
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.Pp
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Multi-threaded applications should explicitly protect ohash table access.
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.Sh SEE ALSO
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.Xr hcreate 3 ,
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.Xr ohash_interval 3
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.Rs
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.%A Donald E. Knuth
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.%B The Art of Computer Programming
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.%V Vol. 3
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.%P pp 506-550
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.%D 1973
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.Re
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.Sh STANDARDS
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Those functions are completely non-standard and should be avoided in
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portable programs.
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.Sh HISTORY
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Those functions were designed and written for
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.Ox
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.Xr make 1
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by Marc Espie in 1999.
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