xref: /openssl/doc/man3/OPENSSL_LH_COMPFUNC.pod (revision 50ef944c) 
1=pod
2
3=head1 NAME
4
5LHASH, LHASH_OF, DEFINE_LHASH_OF_EX, DEFINE_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_doall_arg_thunk,
16OPENSSL_LH_set_thunks, OPENSSL_LH_num_items,
17OPENSSL_LH_get_down_load, OPENSSL_LH_set_down_load, OPENSSL_LH_error
18- dynamic hash table
19
20=head1 SYNOPSIS
21
22=for openssl generic
23
24 #include <openssl/lhash.h>
25
26 LHASH_OF(TYPE)
27
28 DEFINE_LHASH_OF_EX(TYPE);
29
30 LHASH_OF(TYPE) *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
31 void lh_TYPE_free(LHASH_OF(TYPE) *table);
32 void lh_TYPE_flush(LHASH_OF(TYPE) *table);
33 OPENSSL_LHASH *OPENSSL_LH_set_thunks(OPENSSL_LHASH *lh,
34                                      OPENSSL_LH_HASHFUNCTHUNK hw,
35                                      OPENSSL_LH_COMPFUNCTHUNK cw,
36                                      OPENSSL_LH_DOALL_FUNC_THUNK daw,
37                                      OPENSSL_LH_DOALL_FUNCARG_THUNK daaw)
38
39 TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data);
40 TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data);
41 TYPE *lh_TYPE_retrieve(LHASH_OF(TYPE) *table, TYPE *data);
42
43 void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
44 void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
45                        TYPE *arg);
46 void OPENSSL_LH_doall_arg_thunk(OPENSSL_LHASH *lh,
47                                 OPENSSL_LH_DOALL_FUNCARG_THUNK daaw,
48                                 OPENSSL_LH_DOALL_FUNCARG fn, void *arg)
49
50 unsigned long lh_TYPE_num_items(OPENSSL_LHASH *lh);
51 unsigned long lh_TYPE_get_down_load(OPENSSL_LHASH *lh);
52 void lh_TYPE_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);
53
54 int lh_TYPE_error(LHASH_OF(TYPE) *table);
55
56 typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
57 typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
58 typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
59 typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
60
61 OPENSSL_LHASH *OPENSSL_LH_new(OPENSSL_LH_HASHFUNC h, OPENSSL_LH_COMPFUNC c);
62 void OPENSSL_LH_free(OPENSSL_LHASH *lh);
63 void OPENSSL_LH_flush(OPENSSL_LHASH *lh);
64
65 void *OPENSSL_LH_insert(OPENSSL_LHASH *lh, void *data);
66 void *OPENSSL_LH_delete(OPENSSL_LHASH *lh, const void *data);
67 void *OPENSSL_LH_retrieve(OPENSSL_LHASH *lh, const void *data);
68
69 void OPENSSL_LH_doall(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNC func);
70 void OPENSSL_LH_doall_arg(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNCARG func, void *arg);
71
72 unsigned long OPENSSL_LH_num_items(OPENSSL_LHASH *lh);
73 unsigned long OPENSSL_LH_get_down_load(OPENSSL_LHASH *lh);
74 void OPENSSL_LH_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);
75
76 int OPENSSL_LH_error(OPENSSL_LHASH *lh);
77
78 #define LH_LOAD_MULT   /* integer constant */
79
80The following macro is deprecated:
81
82 DEFINE_LHASH_OF(TYPE);
83
84=head1 DESCRIPTION
85
86This library implements type-checked dynamic hash tables. The hash
87table entries can be arbitrary structures. Usually they consist of key
88and value fields.  In the description here, B<I<TYPE>> is used a placeholder
89for any of the OpenSSL datatypes, such as I<SSL_SESSION>.
90
91To define a new type-checked dynamic hash table, use B<DEFINE_LHASH_OF_EX>().
92B<DEFINE_LHASH_OF>() was previously used for this purpose, but is now
93deprecated. The B<DEFINE_LHASH_OF_EX>() macro provides all functionality of
94B<DEFINE_LHASH_OF>() except for certain deprecated statistics functions (see
95OPENSSL_LH_stats(3)).
96
97B<lh_I<TYPE>_new>() creates a new B<LHASH_OF>(B<I<TYPE>>) structure to store
98arbitrary data entries, and specifies the 'hash' and 'compare'
99callbacks to be used in organising the table's entries.  The I<hash>
100callback takes a pointer to a table entry as its argument and returns
101an unsigned long hash value for its key field.  The hash value is
102normally truncated to a power of 2, so make sure that your hash
103function returns well mixed low order bits.  The I<compare> callback
104takes two arguments (pointers to two hash table entries), and returns
1050 if their keys are equal, nonzero otherwise.
106
107If your hash table
108will contain items of some particular type and the I<hash> and
109I<compare> callbacks hash/compare these types, then the
110B<IMPLEMENT_LHASH_HASH_FN> and B<IMPLEMENT_LHASH_COMP_FN> macros can be
111used to create callback wrappers of the prototypes required by
112B<lh_I<TYPE>_new>() as shown in this example:
113
114 /*
115  * Implement the hash and compare functions; "stuff" can be any word.
116  */
117 static unsigned long stuff_hash(const TYPE *a)
118 {
119     ...
120 }
121 static int stuff_cmp(const TYPE *a, const TYPE *b)
122 {
123     ...
124 }
125
126 /*
127  * Implement the wrapper functions.
128  */
129 static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
130 static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)
131
132If the type is going to be used in several places, the following macros
133can be used in a common header file to declare the function wrappers:
134
135 DECLARE_LHASH_HASH_FN(stuff, TYPE)
136 DECLARE_LHASH_COMP_FN(stuff, TYPE)
137
138Then a hash table of B<I<TYPE>> objects can be created using this:
139
140 LHASH_OF(TYPE) *htable;
141
142 htable = B<lh_I<TYPE>_new>(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
143
144B<lh_I<TYPE>_free>() frees the B<LHASH_OF>(B<I<TYPE>>) structure
145I<table>. Allocated hash table entries will not be freed; consider
146using B<lh_I<TYPE>_doall>() to deallocate any remaining entries in the
147hash table (see below). If the argument is NULL, nothing is done.
148
149B<lh_I<TYPE>_flush>() empties the B<LHASH_OF>(B<I<TYPE>>) structure I<table>. New
150entries can be added to the flushed table.  Allocated hash table entries
151will not be freed; consider using B<lh_I<TYPE>_doall>() to deallocate any
152remaining entries in the hash table (see below).
153
154B<lh_I<TYPE>_insert>() inserts the structure pointed to by I<data> into
155I<table>.  If there already is an entry with the same key, the old
156value is replaced. Note that B<lh_I<TYPE>_insert>() stores pointers, the
157data are not copied.
158
159B<lh_I<TYPE>_delete>() deletes an entry from I<table>.
160
161B<lh_I<TYPE>_retrieve>() looks up an entry in I<table>. Normally, I<data>
162is a structure with the key field(s) set; the function will return a
163pointer to a fully populated structure.
164
165B<lh_I<TYPE>_doall>() will, for every entry in the hash table, call
166I<func> with the data item as its parameter.
167For example:
168
169 /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
170 void TYPE_cleanup_doall(TYPE *a);
171
172 /* Implement a prototype-compatible wrapper for "TYPE_cleanup" */
173 IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)
174
175 /* Call "TYPE_cleanup" against all items in a hash table. */
176 lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));
177
178 /* Then the hash table itself can be deallocated */
179 lh_TYPE_free(hashtable);
180
181B<lh_I<TYPE>_doall_arg>() is the same as B<lh_I<TYPE>_doall>() except that
182I<func> will be called with I<arg> as the second argument and I<func>
183should be of type B<LHASH_DOALL_ARG_FN>(B<I<TYPE>>) (a callback prototype
184that is passed both the table entry and an extra argument).  As with
185lh_doall(), you can instead choose to declare your callback with a
186prototype matching the types you are dealing with and use the
187declare/implement macros to create compatible wrappers that cast
188variables before calling your type-specific callbacks.  An example of
189this is demonstrated here (printing all hash table entries to a BIO
190that is provided by the caller):
191
192 /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
193 void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
194
195 /* Implement a prototype-compatible wrapper for "TYPE_print" */
196 static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)
197
198 /* Print out the entire hashtable to a particular BIO */
199 lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
200                   logging_bio);
201
202Note that it is by default B<not> safe to use B<lh_I<TYPE>_delete>() inside a
203callback passed to B<lh_I<TYPE>_doall>() or B<lh_I<TYPE>_doall_arg>(). The
204reason for this is that deleting an item from the hash table may result in the
205hash table being contracted to a smaller size and rehashed.
206B<lh_I<TYPE>_doall>() and B<lh_I<TYPE>_doall_arg>() are unsafe and will exhibit
207undefined behaviour under these conditions, as these functions assume the hash
208table size and bucket pointers do not change during the call.
209
210If it is desired to use B<lh_I<TYPE>_doall>() or B<lh_I<TYPE>_doall_arg>() with
211B<lh_I<TYPE>_delete>(), it is essential that you call
212B<lh_I<TYPE>_set_down_load>() with a I<down_load> argument of 0 first. This
213disables hash table contraction and guarantees that it will be safe to delete
214items from a hash table during a call to B<lh_I<TYPE>_doall>() or
215B<lh_I<TYPE>_doall_arg>().
216
217It is never safe to call B<lh_I<TYPE>_insert>() during a call to
218B<lh_I<TYPE>_doall>() or B<lh_I<TYPE>_doall_arg>().
219
220B<lh_I<TYPE>_error>() can be used to determine if an error occurred in the last
221operation.
222
223B<lh_I<TYPE>_num_items>() returns the number of items in the hash table.
224
225B<lh_I<TYPE>_get_down_load>() and B<lh_I<TYPE>_set_down_load>() get and set the
226factor used to determine when the hash table is contracted. The factor is the
227load factor at or below which hash table contraction will occur, multiplied by
228B<LH_LOAD_MULT>, where the load factor is the number of items divided by the
229number of nodes. Setting this value to 0 disables hash table contraction.
230
231OPENSSL_LH_new() is the same as the B<lh_I<TYPE>_new>() except that it is not
232type specific. So instead of returning an B<LHASH_OF(I<TYPE>)> value it returns
233a B<void *>. In the same way the functions OPENSSL_LH_free(),
234OPENSSL_LH_flush(), OPENSSL_LH_insert(), OPENSSL_LH_delete(),
235OPENSSL_LH_retrieve(), OPENSSL_LH_doall(), OPENSSL_LH_doall_arg(),
236OPENSSL_LH_num_items(), OPENSSL_LH_get_down_load(), OPENSSL_LH_set_down_load()
237and OPENSSL_LH_error() are equivalent to the similarly named B<lh_I<TYPE>>
238functions except that they return or use a B<void *> where the equivalent
239B<lh_I<TYPE>> function returns or uses a B<I<TYPE> *> or B<LHASH_OF(I<TYPE>) *>.
240B<lh_I<TYPE>> functions are implemented as type checked wrappers around the
241B<OPENSSL_LH> functions. Most applications should not call the B<OPENSSL_LH>
242functions directly.
243
244OPENSSL_LH_set_thunks() and OPENSSL_LH_doall_arg_thunk(), while public by
245necessity, are actually internal functions and should not be used.
246
247=head1 RETURN VALUES
248
249B<lh_I<TYPE>_new>() and OPENSSL_LH_new() return NULL on error, otherwise a
250pointer to the new B<LHASH> structure.
251
252When a hash table entry is replaced, B<lh_I<TYPE>_insert>() or
253OPENSSL_LH_insert() return the value being replaced. NULL is returned on normal
254operation and on error.
255
256B<lh_I<TYPE>_delete>() and OPENSSL_LH_delete() return the entry being deleted.
257NULL is returned if there is no such value in the hash table.
258
259B<lh_I<TYPE>_retrieve>() and OPENSSL_LH_retrieve() return the hash table entry
260if it has been found, NULL otherwise.
261
262B<lh_I<TYPE>_error>() and OPENSSL_LH_error() return 1 if an error occurred in
263the last operation, 0 otherwise. It's meaningful only after non-retrieve
264operations.
265
266B<lh_I<TYPE>_free>(), OPENSSL_LH_free(), B<lh_I<TYPE>_flush>(),
267OPENSSL_LH_flush(), B<lh_I<TYPE>_doall>() OPENSSL_LH_doall(),
268B<lh_I<TYPE>_doall_arg>() and OPENSSL_LH_doall_arg() return no values.
269
270=head1 NOTE
271
272The LHASH code is not thread safe. All updating operations, as well as
273B<lh_I<TYPE>_error>() or OPENSSL_LH_error() calls must be performed under
274a write lock. All retrieve operations should be performed under a read lock,
275I<unless> accurate usage statistics are desired. In which case, a write lock
276should be used for retrieve operations as well. For output of the usage
277statistics, using the functions from L<OPENSSL_LH_stats(3)>, a read lock
278suffices.
279
280The LHASH code regards table entries as constant data.  As such, it
281internally represents lh_insert()'d items with a "const void *"
282pointer type.  This is why callbacks such as those used by lh_doall()
283and lh_doall_arg() declare their prototypes with "const", even for the
284parameters that pass back the table items' data pointers - for
285consistency, user-provided data is "const" at all times as far as the
286LHASH code is concerned.  However, as callers are themselves providing
287these pointers, they can choose whether they too should be treating
288all such parameters as constant.
289
290As an example, a hash table may be maintained by code that, for
291reasons of encapsulation, has only "const" access to the data being
292indexed in the hash table (i.e. it is returned as "const" from
293elsewhere in their code) - in this case the LHASH prototypes are
294appropriate as-is.  Conversely, if the caller is responsible for the
295life-time of the data in question, then they may well wish to make
296modifications to table item passed back in the lh_doall() or
297lh_doall_arg() callbacks (see the "TYPE_cleanup" example above).  If
298so, the caller can either cast the "const" away (if they're providing
299the raw callbacks themselves) or use the macros to declare/implement
300the wrapper functions without "const" types.
301
302Callers that only have "const" access to data they're indexing in a
303table, yet declare callbacks without constant types (or cast the
304"const" away themselves), are therefore creating their own risks/bugs
305without being encouraged to do so by the API.  On a related note,
306those auditing code should pay special attention to any instances of
307DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
308without any "const" qualifiers.
309
310=head1 BUGS
311
312B<lh_I<TYPE>_insert>() and OPENSSL_LH_insert() return NULL both for success
313and error.
314
315=head1 SEE ALSO
316
317L<OPENSSL_LH_stats(3)>
318
319=head1 HISTORY
320
321In OpenSSL 1.0.0, the lhash interface was revamped for better
322type checking.
323
324In OpenSSL 3.1, B<DEFINE_LHASH_OF_EX>() was introduced and B<DEFINE_LHASH_OF>()
325was deprecated.
326
327OPENSSL_LH_doall_arg_thunk(), OPENSSL_LH_set_thunks() were added in
328OpenSSL 3.3.
329
330=head1 COPYRIGHT
331
332Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.
333
334Licensed under the Apache License 2.0 (the "License").  You may not use
335this file except in compliance with the License.  You can obtain a copy
336in the file LICENSE in the source distribution or at
337L<https://www.openssl.org/source/license.html>.
338
339=cut
340