4519228649
In addition, list more bugs and caveats. Namely, the NetBSD implementation (like the FreeBSD one where this was ported to) requires that the comparison keys are allocated dynamically, and that hdestroy() will try to free(3) each key. This obviously severely limits the portability, given that other implementations (for example, the Linux one) make no such assumptions. Both approaches are legitimate, and thus the real bug is in the ambiguity of the standard.
241 lines
5.3 KiB
Groff
241 lines
5.3 KiB
Groff
.\" $NetBSD: hcreate.3,v 1.8 2010/05/01 06:18:03 jruoho Exp $
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.\"
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.\" Copyright (c) 1999 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Klaus Klein.
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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 NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' 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 FOUNDATION OR CONTRIBUTORS
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.\" BE 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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.Dd May 1, 2010
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.Dt HCREATE 3
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.Os
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.Sh NAME
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.Nm hcreate ,
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.Nm hdestroy ,
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.Nm hsearch
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.Nd manage hash search table
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In search.h
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.Ft int
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.Fn hcreate "size_t nel"
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.Ft void
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.Fn hdestroy "void"
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.Ft ENTRY *
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.Fn hsearch "ENTRY item" "ACTION action"
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.Sh DESCRIPTION
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The
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.Fn hcreate ,
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.Fn hdestroy
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and
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.Fn hsearch
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functions manage hash search tables.
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.Pp
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The
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.Fn hcreate
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function allocates and initializes the table.
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The
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.Fa nel
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argument specifies an estimate of the maximum number of entries to be held
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by the table.
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Unless further memory allocation fails, supplying an insufficient
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.Fa nel
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value will not result in functional harm, although a performance degradation
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may occur.
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Initialization using the
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.Fn hcreate
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function is mandatory prior to any access operations using
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.Fn hsearch .
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.Pp
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The
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.Fn hdestroy
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function destroys a table previously created using
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.Fn hcreate .
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After a call to
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.Fn hdestroy ,
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the data can no longer be accessed.
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.Pp
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The
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.Fn hsearch
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function is used to search to the hash table.
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It returns a pointer into the
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hash table indicating the address of an item.
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The
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.Fa item
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argument is of type
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.Vt ENTRY ,
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defined in the
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.In search.h
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header.
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This is a structure type that contains two pointers:
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.Pp
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.Bl -tag -compact -offset indent -width "void *data "
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.It Fa char *key
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comparison key
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.It Fa void *data
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pointer to data associated with
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.Fa key
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.El
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.Pp
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The key comparison function used by
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.Fn hsearch
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is
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.Xr strcmp 3 .
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.Pp
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The
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.Fa action
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argument is of type
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.Vt ACTION ,
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an enumeration type which defines the following values:
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.Bl -tag -offset indent -width ENTERXX
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.It Dv ENTER
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Insert
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.Fa item
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into the hash table.
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If an existing item with the same key is found, it is not replaced.
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Note that the
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.Fa key
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and
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.Fa data
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elements of
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.Fa item
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are used directly by the new table entry.
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The storage for the
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key must not be modified during the lifetime of the hash table.
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.It Dv FIND
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Search the hash table without inserting
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.Fa item .
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.El
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.Pp
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Note that the comparison
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.Fa key
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must be allocated using
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.Xr malloc 3
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or
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.Xr calloc 3
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if action is
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.Dv ENTER
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and
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.Fn hdestroy
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will be called.
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This is because
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.Fn hdestroy
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will call
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.Xr free 3
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for each comparison
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.Fa key
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(but not
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.Fa data ) .
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Typically the comparison
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.Fa key
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is allocated by using
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.Xr strdup 3 .
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.Sh RETURN VALUES
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If successful, the
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.Fn hcreate
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function returns a non-zero value.
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Otherwise, a value of 0 is returned and
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.Va errno
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is set to indicate the error.
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.Pp
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The
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.Fn hdestroy
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functions
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returns no value.
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.Pp
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If successful, the
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.Fn hsearch
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function returns a pointer to hash table entry matching
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the provided key.
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If the action is
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.Dv FIND
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and the item was not found, or if the action is
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.Dv ENTER
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and the insertion failed,
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.Dv NULL
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is returned and
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.Va errno
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is set to indicate the error.
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If the action is
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.Dv ENTER
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and an entry already existed in the table matching the given
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key, the existing entry is returned and is not replaced.
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.Sh ERRORS
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The
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.Fn hcreate
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and
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.Fn hsearch
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functions will fail if:
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.Bl -tag -width Er
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.It Bq Er ENOMEM
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Insufficient memory is available.
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.El
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.Sh SEE ALSO
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.Xr bsearch 3 ,
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.Xr lsearch 3 ,
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.Xr malloc 3 ,
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.Xr strcmp 3
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.Sh STANDARDS
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The
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.Fn hcreate ,
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.Fn hdestroy
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and
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.Fn hsearch
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functions conform to
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.St -xpg4.2 .
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.Sh HISTORY
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The
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.Fn hcreate ,
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.Fn hdestroy
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and
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.Fn hsearch
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functions first appeared in
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.At V .
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.Sh CAVEATS
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At least the following limitations can be mentioned:
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.Bl -bullet
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.It
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The interface permits the use of only one hash table at a time.
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.It
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Individual hash table entries can be added, but not deleted.
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.It
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The standard is indecipherable about the
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internal memory usage of the functions,
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mentioning only that
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.Do
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.Fn hcreate
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and
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.Fn hsearch
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functions may use
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.Fn malloc
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to allocate space
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.Dc .
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This limits the portability of the functions,
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given that other implementations may not
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.Xr free 3
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the buffer pointed by
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.Fa key .
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.El
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