xref: /freebsd/crypto/openssl/doc/man3/OPENSSL_LH_COMPFUNC.pod (revision b3e7694832e81d7a904a10f525f8797b753bf0d3) 
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_error,
12OPENSSL_LH_new, OPENSSL_LH_free,  OPENSSL_LH_flush,
13OPENSSL_LH_insert, OPENSSL_LH_delete, OPENSSL_LH_retrieve,
14OPENSSL_LH_doall, OPENSSL_LH_doall_arg, OPENSSL_LH_error
15- dynamic hash table
16
17=head1 SYNOPSIS
18
19=for openssl generic
20
21 #include <openssl/lhash.h>
22
23 DECLARE_LHASH_OF(TYPE);
24
25 LHASH_OF(TYPE) *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
26 void lh_TYPE_free(LHASH_OF(TYPE) *table);
27 void lh_TYPE_flush(LHASH_OF(TYPE) *table);
28
29 TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data);
30 TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data);
31 TYPE *lh_TYPE_retrieve(LHASH_OF(TYPE) *table, TYPE *data);
32
33 void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
34 void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
35                        TYPE *arg);
36
37 int lh_TYPE_error(LHASH_OF(TYPE) *table);
38
39 typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
40 typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
41 typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
42 typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
43
44 OPENSSL_LHASH *OPENSSL_LH_new(OPENSSL_LH_HASHFUNC h, OPENSSL_LH_COMPFUNC c);
45 void OPENSSL_LH_free(OPENSSL_LHASH *lh);
46 void OPENSSL_LH_flush(OPENSSL_LHASH *lh);
47
48 void *OPENSSL_LH_insert(OPENSSL_LHASH *lh, void *data);
49 void *OPENSSL_LH_delete(OPENSSL_LHASH *lh, const void *data);
50 void *OPENSSL_LH_retrieve(OPENSSL_LHASH *lh, const void *data);
51
52 void OPENSSL_LH_doall(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNC func);
53 void OPENSSL_LH_doall_arg(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNCARG func, void *arg);
54
55 int OPENSSL_LH_error(OPENSSL_LHASH *lh);
56
57=head1 DESCRIPTION
58
59This library implements type-checked dynamic hash tables. The hash
60table entries can be arbitrary structures. Usually they consist of key
61and value fields.  In the description here, B<I<TYPE>> is used a placeholder
62for any of the OpenSSL datatypes, such as I<SSL_SESSION>.
63
64B<lh_I<TYPE>_new>() creates a new B<LHASH_OF>(B<I<TYPE>>) structure to store
65arbitrary data entries, and specifies the 'hash' and 'compare'
66callbacks to be used in organising the table's entries.  The I<hash>
67callback takes a pointer to a table entry as its argument and returns
68an unsigned long hash value for its key field.  The hash value is
69normally truncated to a power of 2, so make sure that your hash
70function returns well mixed low order bits.  The I<compare> callback
71takes two arguments (pointers to two hash table entries), and returns
720 if their keys are equal, nonzero otherwise.
73
74If your hash table
75will contain items of some particular type and the I<hash> and
76I<compare> callbacks hash/compare these types, then the
77B<IMPLEMENT_LHASH_HASH_FN> and B<IMPLEMENT_LHASH_COMP_FN> macros can be
78used to create callback wrappers of the prototypes required by
79B<lh_I<TYPE>_new>() as shown in this example:
80
81 /*
82  * Implement the hash and compare functions; "stuff" can be any word.
83  */
84 static unsigned long stuff_hash(const TYPE *a)
85 {
86     ...
87 }
88 static int stuff_cmp(const TYPE *a, const TYPE *b)
89 {
90     ...
91 }
92
93 /*
94  * Implement the wrapper functions.
95  */
96 static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
97 static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)
98
99If the type is going to be used in several places, the following macros
100can be used in a common header file to declare the function wrappers:
101
102 DECLARE_LHASH_HASH_FN(stuff, TYPE)
103 DECLARE_LHASH_COMP_FN(stuff, TYPE)
104
105Then a hash table of B<I<TYPE>> objects can be created using this:
106
107 LHASH_OF(TYPE) *htable;
108
109 htable = B<lh_I<TYPE>_new>(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
110
111B<lh_I<TYPE>_free>() frees the B<LHASH_OF>(B<I<TYPE>>) structure
112I<table>. Allocated hash table entries will not be freed; consider
113using B<lh_I<TYPE>_doall>() to deallocate any remaining entries in the
114hash table (see below).
115
116B<lh_I<TYPE>_flush>() empties the B<LHASH_OF>(B<I<TYPE>>) structure I<table>. New
117entries can be added to the flushed table.  Allocated hash table entries
118will not be freed; consider using B<lh_I<TYPE>_doall>() to deallocate any
119remaining entries in the hash table (see below).
120
121B<lh_I<TYPE>_insert>() inserts the structure pointed to by I<data> into
122I<table>.  If there already is an entry with the same key, the old
123value is replaced. Note that B<lh_I<TYPE>_insert>() stores pointers, the
124data are not copied.
125
126B<lh_I<TYPE>_delete>() deletes an entry from I<table>.
127
128B<lh_I<TYPE>_retrieve>() looks up an entry in I<table>. Normally, I<data>
129is a structure with the key field(s) set; the function will return a
130pointer to a fully populated structure.
131
132B<lh_I<TYPE>_doall>() will, for every entry in the hash table, call
133I<func> with the data item as its parameter.
134For example:
135
136 /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
137 void TYPE_cleanup_doall(TYPE *a);
138
139 /* Implement a prototype-compatible wrapper for "TYPE_cleanup" */
140 IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)
141
142 /* Call "TYPE_cleanup" against all items in a hash table. */
143 lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));
144
145 /* Then the hash table itself can be deallocated */
146 lh_TYPE_free(hashtable);
147
148When doing this, be careful if you delete entries from the hash table
149in your callbacks: the table may decrease in size, moving the item
150that you are currently on down lower in the hash table - this could
151cause some entries to be skipped during the iteration.  The second
152best solution to this problem is to set hash-E<gt>down_load=0 before
153you start (which will stop the hash table ever decreasing in size).
154The best solution is probably to avoid deleting items from the hash
155table inside a "doall" callback!
156
157B<lh_I<TYPE>_doall_arg>() is the same as B<lh_I<TYPE>_doall>() except that
158I<func> will be called with I<arg> as the second argument and I<func>
159should be of type B<LHASH_DOALL_ARG_FN>(B<I<TYPE>>) (a callback prototype
160that is passed both the table entry and an extra argument).  As with
161lh_doall(), you can instead choose to declare your callback with a
162prototype matching the types you are dealing with and use the
163declare/implement macros to create compatible wrappers that cast
164variables before calling your type-specific callbacks.  An example of
165this is demonstrated here (printing all hash table entries to a BIO
166that is provided by the caller):
167
168 /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
169 void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
170
171 /* Implement a prototype-compatible wrapper for "TYPE_print" */
172 static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)
173
174 /* Print out the entire hashtable to a particular BIO */
175 lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
176                   logging_bio);
177
178
179B<lh_I<TYPE>_error>() can be used to determine if an error occurred in the last
180operation.
181
182OPENSSL_LH_new() is the same as the B<lh_I<TYPE>_new>() except that it is not
183type specific. So instead of returning an B<LHASH_OF(I<TYPE>)> value it returns
184a B<void *>. In the same way the functions OPENSSL_LH_free(),
185OPENSSL_LH_flush(), OPENSSL_LH_insert(), OPENSSL_LH_delete(),
186OPENSSL_LH_retrieve(), OPENSSL_LH_doall(), OPENSSL_LH_doall_arg(), and
187OPENSSL_LH_error() are equivalent to the similarly named B<lh_I<TYPE>> functions
188except that they return or use a B<void *> where the equivalent B<lh_I<TYPE>>
189function returns or uses a B<I<TYPE> *> or B<LHASH_OF(I<TYPE>) *>. B<lh_I<TYPE>>
190functions are implemented as type checked wrappers around the B<OPENSSL_LH>
191functions. Most applications should not call the B<OPENSSL_LH> functions
192directly.
193
194=head1 RETURN VALUES
195
196B<lh_I<TYPE>_new>() and OPENSSL_LH_new() return NULL on error, otherwise a
197pointer to the new B<LHASH> structure.
198
199When a hash table entry is replaced, B<lh_I<TYPE>_insert>() or
200OPENSSL_LH_insert() return the value being replaced. NULL is returned on normal
201operation and on error.
202
203B<lh_I<TYPE>_delete>() and OPENSSL_LH_delete() return the entry being deleted.
204NULL is returned if there is no such value in the hash table.
205
206B<lh_I<TYPE>_retrieve>() and OPENSSL_LH_retrieve() return the hash table entry
207if it has been found, NULL otherwise.
208
209B<lh_I<TYPE>_error>() and OPENSSL_LH_error() return 1 if an error occurred in
210the last operation, 0 otherwise. It's meaningful only after non-retrieve
211operations.
212
213B<lh_I<TYPE>_free>(), OPENSSL_LH_free(), B<lh_I<TYPE>_flush>(),
214OPENSSL_LH_flush(), B<lh_I<TYPE>_doall>() OPENSSL_LH_doall(),
215B<lh_I<TYPE>_doall_arg>() and OPENSSL_LH_doall_arg() return no values.
216
217=head1 NOTE
218
219The LHASH code is not thread safe. All updating operations, as well as
220B<lh_I<TYPE>_error>() or OPENSSL_LH_error() calls must be performed under
221a write lock. All retrieve operations should be performed under a read lock,
222I<unless> accurate usage statistics are desired. In which case, a write lock
223should be used for retrieve operations as well. For output of the usage
224statistics, using the functions from L<OPENSSL_LH_stats(3)>, a read lock
225suffices.
226
227The LHASH code regards table entries as constant data.  As such, it
228internally represents lh_insert()'d items with a "const void *"
229pointer type.  This is why callbacks such as those used by lh_doall()
230and lh_doall_arg() declare their prototypes with "const", even for the
231parameters that pass back the table items' data pointers - for
232consistency, user-provided data is "const" at all times as far as the
233LHASH code is concerned.  However, as callers are themselves providing
234these pointers, they can choose whether they too should be treating
235all such parameters as constant.
236
237As an example, a hash table may be maintained by code that, for
238reasons of encapsulation, has only "const" access to the data being
239indexed in the hash table (i.e. it is returned as "const" from
240elsewhere in their code) - in this case the LHASH prototypes are
241appropriate as-is.  Conversely, if the caller is responsible for the
242life-time of the data in question, then they may well wish to make
243modifications to table item passed back in the lh_doall() or
244lh_doall_arg() callbacks (see the "TYPE_cleanup" example above).  If
245so, the caller can either cast the "const" away (if they're providing
246the raw callbacks themselves) or use the macros to declare/implement
247the wrapper functions without "const" types.
248
249Callers that only have "const" access to data they're indexing in a
250table, yet declare callbacks without constant types (or cast the
251"const" away themselves), are therefore creating their own risks/bugs
252without being encouraged to do so by the API.  On a related note,
253those auditing code should pay special attention to any instances of
254DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
255without any "const" qualifiers.
256
257=head1 BUGS
258
259B<lh_I<TYPE>_insert>() and OPENSSL_LH_insert() return NULL both for success
260and error.
261
262=head1 SEE ALSO
263
264L<OPENSSL_LH_stats(3)>
265
266=head1 HISTORY
267
268In OpenSSL 1.0.0, the lhash interface was revamped for better
269type checking.
270
271=head1 COPYRIGHT
272
273Copyright 2000-2022 The OpenSSL Project Authors. All Rights Reserved.
274
275Licensed under the Apache License 2.0 (the "License").  You may not use
276this file except in compliance with the License.  You can obtain a copy
277in the file LICENSE in the source distribution or at
278L<https://www.openssl.org/source/license.html>.
279
280=cut
281