db/man/FreeBSD/hash.3

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  32 .\"     @(#)hash.3      8.6 (Berkeley) 8/18/94
  33 .\" $FreeBSD: src/lib/libc/db/man/hash.3,v 1.8 2003/09/08 19:57:13 ru Exp $
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  35 .Dd August 18, 1994
  36 .Dt HASH 3
  37 .Os
  38 .Sh NAME
  39 .Nm hash
  40 .Nd "hash database access method"
  41 .Sh SYNOPSIS
  42 .In sys/types.h
  43 .In db.h
  44 .Sh DESCRIPTION
  45 The routine
  46 .Fn dbopen
  47 is the library interface to database files.
  48 One of the supported file formats is
  49 .Nm
  50 files.
  51 The general description of the database access methods is in
  52 .Xr dbopen 3 ,
  53 this manual page describes only the
  54 .Nm
  55 specific information.
  56 .Pp
  57 The
  58 .Nm
  59 data structure is an extensible, dynamic hashing scheme.
  60 .Pp
  61 The access method specific data structure provided to
  62 .Fn dbopen
  63 is defined in the
  64 .In db.h
  65 include file as follows:
  66 .Bd -literal
  67 typedef struct {
  68         u_int bsize;
  69         u_int ffactor;
  70         u_int nelem;
  71         u_int cachesize;
  72         u_int32_t (*hash)(const void *, size_t);
  73         int lorder;
  74 } HASHINFO;
  75 .Ed
  76 .Pp
  77 The elements of this structure are as follows:
  78 .Bl -tag -width indent
  79 .It Va bsize
  80 The
  81 .Va bsize
  82 element
  83 defines the
  84 .Nm
  85 table bucket size, and is, by default, 256 bytes.
  86 It may be preferable to increase the page size for disk-resident tables
  87 and tables with large data items.
  88 .It Va ffactor
  89 The
  90 .Va ffactor
  91 element
  92 indicates a desired density within the
  93 .Nm
  94 table.
  95 It is an approximation of the number of keys allowed to accumulate in any
  96 one bucket, determining when the
  97 .Nm
  98 table grows or shrinks.
  99 The default value is 8.
 100 .It Va nelem
 101 The
 102 .Va nelem
 103 element
 104 is an estimate of the final size of the
 105 .Nm
 106 table.
 107 If not set or set too low,
 108 .Nm
 109 tables will expand gracefully as keys
 110 are entered, although a slight performance degradation may be noticed.
 111 The default value is 1.
 112 .It Va cachesize
 113 A suggested maximum size, in bytes, of the memory cache.
 114 This value is
 115 .Em only
 116 advisory, and the access method will allocate more memory rather
 117 than fail.
 118 .It Va hash
 119 The
 120 .Va hash
 121 element
 122 is a user defined
 123 .Nm
 124 function.
 125 Since no
 126 .Nm
 127 function performs equally well on all possible data, the
 128 user may find that the built-in
 129 .Nm
 130 function does poorly on a particular
 131 data set.
 132 User specified
 133 .Nm
 134 functions must take two arguments (a pointer to a byte
 135 string and a length) and return a 32-bit quantity to be used as the
 136 .Nm
 137 value.
 138 .It Va lorder
 139 The byte order for integers in the stored database metadata.
 140 The number should represent the order as an integer; for example,
 141 big endian order would be the number 4,321.
 142 If
 143 .Va lorder
 144 is 0 (no order is specified) the current host order is used.
 145 If the file already exists, the specified value is ignored and the
 146 value specified when the tree was created is used.
 147 .El
 148 .Pp
 149 If the file already exists (and the
 150 .Dv O_TRUNC
 151 flag is not specified), the
 152 values specified for the
 153 .Va bsize , ffactor , lorder
 154 and
 155 .Va nelem
 156 arguments
 157 are
 158 ignored and the values specified when the tree was created are used.
 159 .Pp
 160 If a
 161 .Nm
 162 function is specified,
 163 .Fn hash_open
 164 will attempt to determine if the
 165 .Nm
 166 function specified is the same as
 167 the one with which the database was created, and will fail if it is not.
 168 .Pp
 169 Backward compatible interfaces to the older
 170 .Em dbm
 171 and
 172 .Em ndbm
 173 routines are provided, however these interfaces are not compatible with
 174 previous file formats.
 175 .Sh ERRORS
 176 The
 177 .Nm
 178 access method routines may fail and set
 179 .Va errno
 180 for any of the errors specified for the library routine
 181 .Xr dbopen 3 .
 182 .Sh SEE ALSO
 183 .Xr btree 3 ,
 184 .Xr dbopen 3 ,
 185 .Xr mpool 3 ,
 186 .Xr recno 3
 187 .Rs
 188 .%T "Dynamic Hash Tables"
 189 .%A Per-Ake Larson
 190 .%R "Communications of the ACM"
 191 .%D April 1988
 192 .Re
 193 .Rs
 194 .%T "A New Hash Package for UNIX"
 195 .%A Margo Seltzer
 196 .%R "USENIX Proceedings"
 197 .%D Winter 1991
 198 .Re
 199 .Sh BUGS
 200 Only big and little endian byte order is supported.