xref: /freebsd/crypto/openssl/doc/man3/OPENSSL_LH_COMPFUNC.pod (revision 53bb5613a8a15363718b6e6de8d965bf9a2c5469) 
1=pod
2
3=head1 NAME
4
5LHASH, DECLARE_LHASH_OF,
6OPENSSL_LH_COMPFUNC, OPENSSL_LH_HASHFUNC, OPENSSL_LH_DOALL_FUNC,
7LHASH_DOALL_ARG_FN_TYPE,
8IMPLEMENT_LHASH_HASH_FN, IMPLEMENT_LHASH_COMP_FN,
9lh_TYPE_new, lh_TYPE_free, lh_TYPE_flush,
10lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve,
11lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_num_items, lh_TYPE_get_down_load,
12lh_TYPE_set_down_load, lh_TYPE_error,
13OPENSSL_LH_new, OPENSSL_LH_free,  OPENSSL_LH_flush,
14OPENSSL_LH_insert, OPENSSL_LH_delete, OPENSSL_LH_retrieve,
15OPENSSL_LH_doall, OPENSSL_LH_doall_arg, OPENSSL_LH_num_items,
16OPENSSL_LH_get_down_load, OPENSSL_LH_set_down_load, OPENSSL_LH_error
17- dynamic hash table
18
19=head1 SYNOPSIS
20
21=for openssl generic
22
23 #include <openssl/lhash.h>
24
25 DECLARE_LHASH_OF(TYPE);
26
27 LHASH_OF(TYPE) *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
28 void lh_TYPE_free(LHASH_OF(TYPE) *table);
29 void lh_TYPE_flush(LHASH_OF(TYPE) *table);
30
31 TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data);
32 TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data);
33 TYPE *lh_TYPE_retrieve(LHASH_OF(TYPE) *table, TYPE *data);
34
35 void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
36 void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
37                        TYPE *arg);
38
39 unsigned long lh_TYPE_num_items(OPENSSL_LHASH *lh);
40 unsigned long lh_TYPE_get_down_load(OPENSSL_LHASH *lh);
41 void lh_TYPE_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);
42
43 int lh_TYPE_error(LHASH_OF(TYPE) *table);
44
45 typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
46 typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
47 typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
48 typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
49
50 OPENSSL_LHASH *OPENSSL_LH_new(OPENSSL_LH_HASHFUNC h, OPENSSL_LH_COMPFUNC c);
51 void OPENSSL_LH_free(OPENSSL_LHASH *lh);
52 void OPENSSL_LH_flush(OPENSSL_LHASH *lh);
53
54 void *OPENSSL_LH_insert(OPENSSL_LHASH *lh, void *data);
55 void *OPENSSL_LH_delete(OPENSSL_LHASH *lh, const void *data);
56 void *OPENSSL_LH_retrieve(OPENSSL_LHASH *lh, const void *data);
57
58 void OPENSSL_LH_doall(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNC func);
59 void OPENSSL_LH_doall_arg(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNCARG func, void *arg);
60
61 unsigned long OPENSSL_LH_num_items(OPENSSL_LHASH *lh);
62 unsigned long OPENSSL_LH_get_down_load(OPENSSL_LHASH *lh);
63 void OPENSSL_LH_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);
64
65 int OPENSSL_LH_error(OPENSSL_LHASH *lh);
66
67 #define LH_LOAD_MULT   /* integer constant */
68
69=head1 DESCRIPTION
70
71This library implements type-checked dynamic hash tables. The hash
72table entries can be arbitrary structures. Usually they consist of key
73and value fields.  In the description here, B<I<TYPE>> is used a placeholder
74for any of the OpenSSL datatypes, such as I<SSL_SESSION>.
75
76B<lh_I<TYPE>_new>() creates a new B<LHASH_OF>(B<I<TYPE>>) structure to store
77arbitrary data entries, and specifies the 'hash' and 'compare'
78callbacks to be used in organising the table's entries.  The I<hash>
79callback takes a pointer to a table entry as its argument and returns
80an unsigned long hash value for its key field.  The hash value is
81normally truncated to a power of 2, so make sure that your hash
82function returns well mixed low order bits.  The I<compare> callback
83takes two arguments (pointers to two hash table entries), and returns
840 if their keys are equal, nonzero otherwise.
85
86If your hash table
87will contain items of some particular type and the I<hash> and
88I<compare> callbacks hash/compare these types, then the
89B<IMPLEMENT_LHASH_HASH_FN> and B<IMPLEMENT_LHASH_COMP_FN> macros can be
90used to create callback wrappers of the prototypes required by
91B<lh_I<TYPE>_new>() as shown in this example:
92
93 /*
94  * Implement the hash and compare functions; "stuff" can be any word.
95  */
96 static unsigned long stuff_hash(const TYPE *a)
97 {
98     ...
99 }
100 static int stuff_cmp(const TYPE *a, const TYPE *b)
101 {
102     ...
103 }
104
105 /*
106  * Implement the wrapper functions.
107  */
108 static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
109 static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)
110
111If the type is going to be used in several places, the following macros
112can be used in a common header file to declare the function wrappers:
113
114 DECLARE_LHASH_HASH_FN(stuff, TYPE)
115 DECLARE_LHASH_COMP_FN(stuff, TYPE)
116
117Then a hash table of B<I<TYPE>> objects can be created using this:
118
119 LHASH_OF(TYPE) *htable;
120
121 htable = B<lh_I<TYPE>_new>(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
122
123B<lh_I<TYPE>_free>() frees the B<LHASH_OF>(B<I<TYPE>>) structure
124I<table>. Allocated hash table entries will not be freed; consider
125using B<lh_I<TYPE>_doall>() to deallocate any remaining entries in the
126hash table (see below). If the argument is NULL, nothing is done.
127
128B<lh_I<TYPE>_flush>() empties the B<LHASH_OF>(B<I<TYPE>>) structure I<table>. New
129entries can be added to the flushed table.  Allocated hash table entries
130will not be freed; consider using B<lh_I<TYPE>_doall>() to deallocate any
131remaining entries in the hash table (see below).
132
133B<lh_I<TYPE>_insert>() inserts the structure pointed to by I<data> into
134I<table>.  If there already is an entry with the same key, the old
135value is replaced. Note that B<lh_I<TYPE>_insert>() stores pointers, the
136data are not copied.
137
138B<lh_I<TYPE>_delete>() deletes an entry from I<table>.
139
140B<lh_I<TYPE>_retrieve>() looks up an entry in I<table>. Normally, I<data>
141is a structure with the key field(s) set; the function will return a
142pointer to a fully populated structure.
143
144B<lh_I<TYPE>_doall>() will, for every entry in the hash table, call
145I<func> with the data item as its parameter.
146For example:
147
148 /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
149 void TYPE_cleanup_doall(TYPE *a);
150
151 /* Implement a prototype-compatible wrapper for "TYPE_cleanup" */
152 IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)
153
154 /* Call "TYPE_cleanup" against all items in a hash table. */
155 lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));
156
157 /* Then the hash table itself can be deallocated */
158 lh_TYPE_free(hashtable);
159
160B<lh_I<TYPE>_doall_arg>() is the same as B<lh_I<TYPE>_doall>() except that
161I<func> will be called with I<arg> as the second argument and I<func>
162should be of type B<LHASH_DOALL_ARG_FN>(B<I<TYPE>>) (a callback prototype
163that is passed both the table entry and an extra argument).  As with
164lh_doall(), you can instead choose to declare your callback with a
165prototype matching the types you are dealing with and use the
166declare/implement macros to create compatible wrappers that cast
167variables before calling your type-specific callbacks.  An example of
168this is demonstrated here (printing all hash table entries to a BIO
169that is provided by the caller):
170
171 /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
172 void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
173
174 /* Implement a prototype-compatible wrapper for "TYPE_print" */
175 static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)
176
177 /* Print out the entire hashtable to a particular BIO */
178 lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
179                   logging_bio);
180
181Note that it is by default B<not> safe to use B<lh_I<TYPE>_delete>() inside a
182callback passed to B<lh_I<TYPE>_doall>() or B<lh_I<TYPE>_doall_arg>(). The
183reason for this is that deleting an item from the hash table may result in the
184hash table being contracted to a smaller size and rehashed.
185B<lh_I<TYPE>_doall>() and B<lh_I<TYPE>_doall_arg>() are unsafe and will exhibit
186undefined behaviour under these conditions, as these functions assume the hash
187table size and bucket pointers do not change during the call.
188
189If it is desired to use B<lh_I<TYPE>_doall>() or B<lh_I<TYPE>_doall_arg>() with
190B<lh_I<TYPE>_delete>(), it is essential that you call
191B<lh_I<TYPE>_set_down_load>() with a I<down_load> argument of 0 first. This
192disables hash table contraction and guarantees that it will be safe to delete
193items from a hash table during a call to B<lh_I<TYPE>_doall>() or
194B<lh_I<TYPE>_doall_arg>().
195
196It is never safe to call B<lh_I<TYPE>_insert>() during a call to
197B<lh_I<TYPE>_doall>() or B<lh_I<TYPE>_doall_arg>().
198
199B<lh_I<TYPE>_error>() can be used to determine if an error occurred in the last
200operation.
201
202B<lh_I<TYPE>_num_items>() returns the number of items in the hash table.
203
204B<lh_I<TYPE>_get_down_load>() and B<lh_I<TYPE>_set_down_load>() get and set the
205factor used to determine when the hash table is contracted. The factor is the
206load factor at or below which hash table contraction will occur, multiplied by
207B<LH_LOAD_MULT>, where the load factor is the number of items divided by the
208number of nodes. Setting this value to 0 disables hash table contraction.
209
210OPENSSL_LH_new() is the same as the B<lh_I<TYPE>_new>() except that it is not
211type specific. So instead of returning an B<LHASH_OF(I<TYPE>)> value it returns
212a B<void *>. In the same way the functions OPENSSL_LH_free(),
213OPENSSL_LH_flush(), OPENSSL_LH_insert(), OPENSSL_LH_delete(),
214OPENSSL_LH_retrieve(), OPENSSL_LH_doall(), OPENSSL_LH_doall_arg(),
215OPENSSL_LH_num_items(), OPENSSL_LH_get_down_load(), OPENSSL_LH_set_down_load()
216and OPENSSL_LH_error() are equivalent to the similarly named B<lh_I<TYPE>>
217functions except that they return or use a B<void *> where the equivalent
218B<lh_I<TYPE>> function returns or uses a B<I<TYPE> *> or B<LHASH_OF(I<TYPE>) *>.
219B<lh_I<TYPE>> functions are implemented as type checked wrappers around the
220B<OPENSSL_LH> functions. Most applications should not call the B<OPENSSL_LH>
221functions directly.
222
223=head1 RETURN VALUES
224
225B<lh_I<TYPE>_new>() and OPENSSL_LH_new() return NULL on error, otherwise a
226pointer to the new B<LHASH> structure.
227
228When a hash table entry is replaced, B<lh_I<TYPE>_insert>() or
229OPENSSL_LH_insert() return the value being replaced. NULL is returned on normal
230operation and on error.
231
232B<lh_I<TYPE>_delete>() and OPENSSL_LH_delete() return the entry being deleted.
233NULL is returned if there is no such value in the hash table.
234
235B<lh_I<TYPE>_retrieve>() and OPENSSL_LH_retrieve() return the hash table entry
236if it has been found, NULL otherwise.
237
238B<lh_I<TYPE>_error>() and OPENSSL_LH_error() return 1 if an error occurred in
239the last operation, 0 otherwise. It's meaningful only after non-retrieve
240operations.
241
242B<lh_I<TYPE>_free>(), OPENSSL_LH_free(), B<lh_I<TYPE>_flush>(),
243OPENSSL_LH_flush(), B<lh_I<TYPE>_doall>() OPENSSL_LH_doall(),
244B<lh_I<TYPE>_doall_arg>() and OPENSSL_LH_doall_arg() return no values.
245
246=head1 NOTE
247
248The LHASH code is not thread safe. All updating operations, as well as
249B<lh_I<TYPE>_error>() or OPENSSL_LH_error() calls must be performed under
250a write lock. All retrieve operations should be performed under a read lock,
251I<unless> accurate usage statistics are desired. In which case, a write lock
252should be used for retrieve operations as well. For output of the usage
253statistics, using the functions from L<OPENSSL_LH_stats(3)>, a read lock
254suffices.
255
256The LHASH code regards table entries as constant data.  As such, it
257internally represents lh_insert()'d items with a "const void *"
258pointer type.  This is why callbacks such as those used by lh_doall()
259and lh_doall_arg() declare their prototypes with "const", even for the
260parameters that pass back the table items' data pointers - for
261consistency, user-provided data is "const" at all times as far as the
262LHASH code is concerned.  However, as callers are themselves providing
263these pointers, they can choose whether they too should be treating
264all such parameters as constant.
265
266As an example, a hash table may be maintained by code that, for
267reasons of encapsulation, has only "const" access to the data being
268indexed in the hash table (i.e. it is returned as "const" from
269elsewhere in their code) - in this case the LHASH prototypes are
270appropriate as-is.  Conversely, if the caller is responsible for the
271life-time of the data in question, then they may well wish to make
272modifications to table item passed back in the lh_doall() or
273lh_doall_arg() callbacks (see the "TYPE_cleanup" example above).  If
274so, the caller can either cast the "const" away (if they're providing
275the raw callbacks themselves) or use the macros to declare/implement
276the wrapper functions without "const" types.
277
278Callers that only have "const" access to data they're indexing in a
279table, yet declare callbacks without constant types (or cast the
280"const" away themselves), are therefore creating their own risks/bugs
281without being encouraged to do so by the API.  On a related note,
282those auditing code should pay special attention to any instances of
283DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
284without any "const" qualifiers.
285
286=head1 BUGS
287
288B<lh_I<TYPE>_insert>() and OPENSSL_LH_insert() return NULL both for success
289and error.
290
291=head1 SEE ALSO
292
293L<OPENSSL_LH_stats(3)>
294
295=head1 HISTORY
296
297In OpenSSL 1.0.0, the lhash interface was revamped for better
298type checking.
299
300=head1 COPYRIGHT
301
302Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.
303
304Licensed under the Apache License 2.0 (the "License").  You may not use
305this file except in compliance with the License.  You can obtain a copy
306in the file LICENSE in the source distribution or at
307L<https://www.openssl.org/source/license.html>.
308
309=cut
310