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32 .\" @(#)hash.3 8.6 (Berkeley) 8/18/94
33 .\"
34 .TH HASH 3 "August 18, 1994"
35 .UC 7
36 .SH NAME
37 hash \- hash database access method
38 .SH SYNOPSIS
39 .nf
40 .ft B
41 #include <sys/types.h>
42 #include <db.h>
43 .ft R
44 .fi
45 .SH DESCRIPTION
46 The routine
47 .IR dbopen
48 is the library interface to database files.
49 One of the supported file formats is hash files.
50 The general description of the database access methods is in
51 .IR dbopen (3),
52 this manual page describes only the hash specific information.
53 .PP
54 The hash data structure is an extensible, dynamic hashing scheme.
55 .PP
56 The access method specific data structure provided to
57 .I dbopen
58 is defined in the <db.h> include file as follows:
59 .sp
60 typedef struct {
61 .RS
62 u_int bsize;
63 .br
64 u_int ffactor;
65 .br
66 u_int nelem;
67 .br
68 u_int cachesize;
69 .br
70 u_int32_t (*hash)(const void *, size_t);
71 .br
72 int lorder;
73 .RE
74 } HASHINFO;
75 .PP
76 The elements of this structure are as follows:
77 .TP
78 bsize
79 .I Bsize
80 defines the hash table bucket size, and is, by default, 256 bytes.
81 It may be preferable to increase the page size for disk-resident tables
82 and tables with large data items.
83 .TP
84 ffactor
85 .I Ffactor
86 indicates a desired density within the hash table.
87 It is an approximation of the number of keys allowed to accumulate in any
88 one bucket, determining when the hash table grows or shrinks.
89 The default value is 8.
90 .TP
91 nelem
92 .I Nelem
93 is an estimate of the final size of the hash table.
94 If not set or set too low, hash tables will expand gracefully as keys
95 are entered, although a slight performance degradation may be noticed.
96 The default value is 1.
97 .TP
98 cachesize
99 A suggested maximum size, in bytes, of the memory cache.
100 This value is
101 .B only
102 advisory, and the access method will allocate more memory rather
103 than fail.
104 .TP
105 hash
106 .I Hash
107 is a user defined hash function.
108 Since no hash function performs equally well on all possible data, the
109 user may find that the built-in hash function does poorly on a particular
110 data set.
111 User specified hash functions must take two arguments (a pointer to a byte
112 string and a length) and return a 32-bit quantity to be used as the hash
113 value.
114 .TP
115 lorder
116 The byte order for integers in the stored database metadata.
117 The number should represent the order as an integer; for example,
118 big endian order would be the number 4,321.
119 If
120 .I lorder
121 is 0 (no order is specified) the current host order is used.
122 If the file already exists, the specified value is ignored and the
123 value specified when the tree was created is used.
124 .PP
125 If the file already exists (and the O_TRUNC flag is not specified), the
126 values specified for the parameters bsize, ffactor, lorder and nelem are
127 ignored and the values specified when the tree was created are used.
128 .PP
129 If a hash function is specified,
130 .I hash_open
131 will attempt to determine if the hash function specified is the same as
132 the one with which the database was created, and will fail if it is not.
133 .PP
134 Backward compatible interfaces to the routines described in
135 .IR dbm (3),
136 and
137 .IR ndbm (3)
138 are provided, however these interfaces are not compatible with
139 previous file formats.
140 .SH ERRORS
141 The
142 .I hash
143 access method routines may fail and set
144 .I errno
145 for any of the errors specified for the library routine
146 .IR dbopen (3).
147 .SH "SEE ALSO"
148 .IR btree (3),
149 .IR dbopen (3),
150 .IR mpool (3),
151 .IR recno (3)
152 .sp
153 .IR "Dynamic Hash Tables" ,
154 Per-Ake Larson, Communications of the ACM, April 1988.
155 .sp
156 .IR "A New Hash Package for UNIX" ,
157 Margo Seltzer, USENIX Proceedings, Winter 1991.
158 .SH BUGS
159 Only big and little endian byte order is supported.