db/man/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 1.6 2001/10/01 16:08:50 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 .Aq Pa 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 .Va Bsize
  81 defines the
  82 .Nm
  83 table bucket size, and is, by default, 256 bytes.
  84 It may be preferable to increase the page size for disk-resident tables
  85 and tables with large data items.
  86 .It Va ffactor
  87 .Va Ffactor
  88 indicates a desired density within the
  89 .Nm
  90 table.
  91 It is an approximation of the number of keys allowed to accumulate in any
  92 one bucket, determining when the
  93 .Nm
  94 table grows or shrinks.
  95 The default value is 8.
  96 .It Va nelem
  97 .Va Nelem
  98 is an estimate of the final size of the
  99 .Nm
 100 table.
 101 If not set or set too low,
 102 .Nm
 103 tables will expand gracefully as keys
 104 are entered, although a slight performance degradation may be noticed.
 105 The default value is 1.
 106 .It Va cachesize
 107 A suggested maximum size, in bytes, of the memory cache.
 108 This value is
 109 .Em only
 110 advisory, and the access method will allocate more memory rather
 111 than fail.
 112 .It Va hash
 113 .Va Hash
 114 is a user defined
 115 .Nm
 116 function.
 117 Since no
 118 .Nm
 119 function performs equally well on all possible data, the
 120 user may find that the built-in
 121 .Nm
 122 function does poorly on a particular
 123 data set.
 124 User specified
 125 .Nm
 126 functions must take two arguments (a pointer to a byte
 127 string and a length) and return a 32-bit quantity to be used as the
 128 .Nm
 129 value.
 130 .It Va lorder
 131 The byte order for integers in the stored database metadata.
 132 The number should represent the order as an integer; for example,
 133 big endian order would be the number 4,321.
 134 If
 135 .Va lorder
 136 is 0 (no order is specified) the current host order is used.
 137 If the file already exists, the specified value is ignored and the
 138 value specified when the tree was created is used.
 139 .El
 140 .Pp
 141 If the file already exists (and the
 142 .Dv O_TRUNC
 143 flag is not specified), the
 144 values specified for the parameters
 145 .Va bsize , ffactor , lorder
 146 and
 147 .Va nelem
 148 are
 149 ignored and the values specified when the tree was created are used.
 150 .Pp
 151 If a
 152 .Nm
 153 function is specified,
 154 .Fn hash_open
 155 will attempt to determine if the
 156 .Nm
 157 function specified is the same as
 158 the one with which the database was created, and will fail if it is not.
 159 .Pp
 160 Backward compatible interfaces to the older
 161 .Em dbm
 162 and
 163 .Em ndbm
 164 routines are provided, however these interfaces are not compatible with
 165 previous file formats.
 166 .Sh ERRORS
 167 The
 168 .Nm
 169 access method routines may fail and set
 170 .Va errno
 171 for any of the errors specified for the library routine
 172 .Xr dbopen 3 .
 173 .Sh SEE ALSO
 174 .Xr btree 3 ,
 175 .Xr dbopen 3 ,
 176 .Xr mpool 3 ,
 177 .Xr recno 3
 178 .Rs
 179 .%T "Dynamic Hash Tables"
 180 .%A Per-Ake Larson
 181 .%R "Communications of the ACM"
 182 .%D April 1988
 183 .Re
 184 .Rs
 185 .%T "A New Hash Package for UNIX"
 186 .%A Margo Seltzer
 187 .%R "USENIX Proceedings"
 188 .%D Winter 1991
 189 .Re
 190 .Sh BUGS
 191 Only big and little endian byte order is supported.