man3/hash.3

   1 .\" Copyright (c) 1990, 1993
   2 .\"     The Regents of the University of California.  All rights reserved.
   3 .\"
   4 .\" SPDX-License-Identifier: BSD-4-Clause-UC
   5 .\"
   6 .\"     @(#)hash.3      8.6 (Berkeley) 8/18/94
   7 .\"
   8 .TH HASH 3 2017-09-15 "" "Linux Programmer's Manual"
   9 .UC 7
  10 .SH NAME
  11 hash \- hash database access method
  12 .SH LIBRARY
  13 Standard C library
  14 .RI ( libc ", " \-lc )
  15 .SH SYNOPSIS
  16 .nf
  17 .ft B
  18 #include <sys/types.h>
  19 #include <db.h>
  20 .ft R
  21 .fi
  22 .SH DESCRIPTION
  23 .IR "Note well" :
  24 This page documents interfaces provided in glibc up until version 2.1.
  25 Since version 2.2, glibc no longer provides these interfaces.
  26 Probably, you are looking for the APIs provided by the
  27 .I libdb
  28 library instead.
  29 .PP
  30 The routine
  31 .BR dbopen (3)
  32 is the library interface to database files.
  33 One of the supported file formats is hash files.
  34 The general description of the database access methods is in
  35 .BR dbopen (3),
  36 this manual page describes only the hash-specific information.
  37 .PP
  38 The hash data structure is an extensible, dynamic hashing scheme.
  39 .PP
  40 The access-method-specific data structure provided to
  41 .BR dbopen (3)
  42 is defined in the
  43 .I <db.h>
  44 include file as follows:
  45 .PP
  46 .in +4n
  47 .EX
  48 typedef struct {
  49     unsigned int       bsize;
  50     unsigned int       ffactor;
  51     unsigned int       nelem;
  52     unsigned int       cachesize;
  53     uint32_t         (*hash)(const void *, size_t);
  54     int         lorder;
  55 } HASHINFO;
  56 .EE
  57 .in
  58 .PP
  59 The elements of this structure are as follows:
  60 .TP 10
  61 .I bsize
  62 defines the hash table bucket size, and is, by default, 256 bytes.
  63 It may be preferable to increase the page size for disk-resident tables
  64 and tables with large data items.
  65 .TP
  66 .I ffactor
  67 indicates a desired density within the hash table.
  68 It is an approximation of the number of keys allowed to accumulate in any
  69 one bucket, determining when the hash table grows or shrinks.
  70 The default value is 8.
  71 .TP
  72 .I nelem
  73 is an estimate of the final size of the hash table.
  74 If not set or set too low, hash tables will expand gracefully as keys
  75 are entered, although a slight performance degradation may be noticed.
  76 The default value is 1.
  77 .TP
  78 .I cachesize
  79 is the suggested maximum size, in bytes, of the memory cache.
  80 This value is
  81 .IR "only advisory" ,
  82 and the access method will allocate more memory rather than fail.
  83 .TP
  84 .I hash
  85 is a user-defined hash function.
  86 Since no hash function performs equally well on all possible data, the
  87 user may find that the built-in hash function does poorly on a particular
  88 data set.
  89 A user-specified hash functions must take two arguments (a pointer to a byte
  90 string and a length) and return a 32-bit quantity to be used as the hash
  91 value.
  92 .TP
  93 .I lorder
  94 is the byte order for integers in the stored database metadata.
  95 The number should represent the order as an integer; for example,
  96 big endian order would be the number 4,321.
  97 If
  98 .I lorder
  99 is 0 (no order is specified), the current host order is used.
 100 If the file already exists, the specified value is ignored and the
 101 value specified when the tree was created is used.
 102 .PP
 103 If the file already exists (and the
 104 .B O_TRUNC
 105 flag is not specified), the
 106 values specified for
 107 .IR bsize ,
 108 .IR ffactor ,
 109 .IR lorder ,
 110 and
 111 .I nelem
 112 are
 113 ignored and the values specified when the tree was created are used.
 114 .PP
 115 If a hash function is specified,
 116 .I hash_open
 117 attempts to determine if the hash function specified is the same as
 118 the one with which the database was created, and fails if it is not.
 119 .PP
 120 Backward-compatible interfaces to the routines described in
 121 .BR dbm (3),
 122 and
 123 .BR ndbm (3)
 124 are provided, however these interfaces are not compatible with
 125 previous file formats.
 126 .SH ERRORS
 127 The
 128 .I hash
 129 access method routines may fail and set
 130 .I errno
 131 for any of the errors specified for the library routine
 132 .BR dbopen (3).
 133 .SH BUGS
 134 Only big and little endian byte order are supported.
 135 .SH SEE ALSO
 136 .BR btree (3),
 137 .BR dbopen (3),
 138 .BR mpool (3),
 139 .BR recno (3)
 140 .PP
 141 .IR "Dynamic Hash Tables" ,
 142 Per-Ake Larson, Communications of the ACM, April 1988.
 143 .PP
 144 .IR "A New Hash Package for UNIX" ,
 145 Margo Seltzer, USENIX Proceedings, Winter 1991.