freebsd-skq/lib/libopenbsd/ohash_init.3

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