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32 .\" @(#)hash.3 8.6 (Berkeley) 8/18/94
34 .TH HASH 3 2012-04-23 "" "Linux Programmer's Manual"
37 hash \- hash database access method
41 #include <sys/types.h>
47 This page documents interfaces provided in glibc up until version 2.1.
48 Since version 2.2, glibc no longer provides these interfaces.
49 Probably, you are looking for the APIs provided by the
55 is the library interface to database files.
56 One of the supported file formats is hash files.
57 The general description of the database access methods is in
59 this manual page describes only the hash specific information.
61 The hash data structure is an extensible, dynamic hashing scheme.
63 The access method specific data structure provided to
67 include file as follows:
75 unsigned int cachesize;
76 uint32_t (*hash)(const void *, size_t);
82 The elements of this structure are as follows:
85 defines the hash 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.
90 indicates a desired density within the hash table.
91 It is an approximation of the number of keys allowed to accumulate in any
92 one bucket, determining when the hash table grows or shrinks.
93 The default value is 8.
96 is an estimate of the final size of the hash table.
97 If not set or set too low, hash tables will expand gracefully as keys
98 are entered, although a slight performance degradation may be noticed.
99 The default value is 1.
102 is the suggested maximum size, in bytes, of the memory cache.
104 .IR "only advisory" ,
105 and the access method will allocate more memory rather than fail.
108 is a user-defined hash function.
109 Since no hash function performs equally well on all possible data, the
110 user may find that the built-in hash function does poorly on a particular
112 A user-specified hash functions must take two arguments (a pointer to a byte
113 string and a length) and return a 32-bit quantity to be used as the hash
117 is the byte order for integers in the stored database metadata.
118 The number should represent the order as an integer; for example,
119 big endian order would be the number 4,321.
122 is 0 (no order is specified) the current host order is used.
123 If the file already exists, the specified value is ignored and the
124 value specified when the tree was created is used.
126 If the file already exists (and the
128 flag is not specified), the
136 ignored and the values specified when the tree was created are used.
138 If a hash function is specified,
140 will attempt to determine if the hash function specified is the same as
141 the one with which the database was created, and will fail if it is not.
143 Backward-compatible interfaces to the routines described in
147 are provided, however these interfaces are not compatible with
148 previous file formats.
152 access method routines may fail and set
154 for any of the errors specified for the library routine
157 Only big and little endian byte order are supported.
164 .IR "Dynamic Hash Tables" ,
165 Per-Ake Larson, Communications of the ACM, April 1988.
167 .IR "A New Hash Package for UNIX" ,
168 Margo Seltzer, USENIX Proceedings, Winter 1991.