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141 .TH lhash 3 "0.9.7a" "2003-02-19" "OpenSSL"
144 lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error \- dynamic hash table
146 .IX Header "SYNOPSIS"
148 \& #include <openssl/lhash.h>
151 \& LHASH *lh_new(LHASH_HASH_FN_TYPE hash, LHASH_COMP_FN_TYPE compare);
152 \& void lh_free(LHASH *table);
155 \& void *lh_insert(LHASH *table, void *data);
156 \& void *lh_delete(LHASH *table, void *data);
157 \& void *lh_retrieve(LHASH *table, void *data);
160 \& void lh_doall(LHASH *table, LHASH_DOALL_FN_TYPE func);
161 \& void lh_doall_arg(LHASH *table, LHASH_DOALL_ARG_FN_TYPE func,
165 \& int lh_error(LHASH *table);
168 \& typedef int (*LHASH_COMP_FN_TYPE)(const void *, const void *);
169 \& typedef unsigned long (*LHASH_HASH_FN_TYPE)(const void *);
170 \& typedef void (*LHASH_DOALL_FN_TYPE)(const void *);
171 \& typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
174 .IX Header "DESCRIPTION"
175 This library implements dynamic hash tables. The hash table entries
176 can be arbitrary structures. Usually they consist of key and value
179 \&\fIlh_new()\fR creates a new \fB\s-1LHASH\s0\fR structure to store arbitrary data
180 entries, and provides the 'hash' and 'compare' callbacks to be used in
181 organising the table's entries. The \fBhash\fR callback takes a pointer
182 to a table entry as its argument and returns an unsigned long hash
183 value for its key field. The hash value is normally truncated to a
184 power of 2, so make sure that your hash function returns well mixed
185 low order bits. The \fBcompare\fR callback takes two arguments (pointers
186 to two hash table entries), and returns 0 if their keys are equal,
187 non-zero otherwise. If your hash table will contain items of some
188 particular type and the \fBhash\fR and \fBcompare\fR callbacks hash/compare
189 these types, then the \fB\s-1DECLARE_LHASH_HASH_FN\s0\fR and
190 \&\fB\s-1IMPLEMENT_LHASH_COMP_FN\s0\fR macros can be used to create callback
191 wrappers of the prototypes required by \fIlh_new()\fR. These provide
192 per-variable casts before calling the type-specific callbacks written
193 by the application author. These macros, as well as those used for
194 the \*(L"doall\*(R" callbacks, are defined as;
197 \& #define DECLARE_LHASH_HASH_FN(f_name,o_type) \e
198 \& unsigned long f_name##_LHASH_HASH(const void *);
199 \& #define IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \e
200 \& unsigned long f_name##_LHASH_HASH(const void *arg) { \e
201 \& o_type a = (o_type)arg; \e
202 \& return f_name(a); }
203 \& #define LHASH_HASH_FN(f_name) f_name##_LHASH_HASH
206 \& #define DECLARE_LHASH_COMP_FN(f_name,o_type) \e
207 \& int f_name##_LHASH_COMP(const void *, const void *);
208 \& #define IMPLEMENT_LHASH_COMP_FN(f_name,o_type) \e
209 \& int f_name##_LHASH_COMP(const void *arg1, const void *arg2) { \e
210 \& o_type a = (o_type)arg1; \e
211 \& o_type b = (o_type)arg2; \e
212 \& return f_name(a,b); }
213 \& #define LHASH_COMP_FN(f_name) f_name##_LHASH_COMP
216 \& #define DECLARE_LHASH_DOALL_FN(f_name,o_type) \e
217 \& void f_name##_LHASH_DOALL(const void *);
218 \& #define IMPLEMENT_LHASH_DOALL_FN(f_name,o_type) \e
219 \& void f_name##_LHASH_DOALL(const void *arg) { \e
220 \& o_type a = (o_type)arg; \e
222 \& #define LHASH_DOALL_FN(f_name) f_name##_LHASH_DOALL
225 \& #define DECLARE_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \e
226 \& void f_name##_LHASH_DOALL_ARG(const void *, const void *);
227 \& #define IMPLEMENT_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \e
228 \& void f_name##_LHASH_DOALL_ARG(const void *arg1, const void *arg2) { \e
229 \& o_type a = (o_type)arg1; \e
230 \& a_type b = (a_type)arg2; \e
232 \& #define LHASH_DOALL_ARG_FN(f_name) f_name##_LHASH_DOALL_ARG
234 An example of a hash table storing (pointers to) structures of type '\s-1STUFF\s0'
235 could be defined as follows;
238 \& /* Calculates the hash value of 'tohash' (implemented elsewhere) */
239 \& unsigned long STUFF_hash(const STUFF *tohash);
240 \& /* Orders 'arg1' and 'arg2' (implemented elsewhere) */
241 \& int STUFF_cmp(const STUFF *arg1, const STUFF *arg2);
242 \& /* Create the type-safe wrapper functions for use in the LHASH internals */
243 \& static IMPLEMENT_LHASH_HASH_FN(STUFF_hash, const STUFF *)
244 \& static IMPLEMENT_LHASH_COMP_FN(STUFF_cmp, const STUFF *);
246 \& int main(int argc, char *argv[]) {
247 \& /* Create the new hash table using the hash/compare wrappers */
248 \& LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash),
249 \& LHASH_COMP_FN(STUFF_cmp));
253 \&\fIlh_free()\fR frees the \fB\s-1LHASH\s0\fR structure \fBtable\fR. Allocated hash table
254 entries will not be freed; consider using \fIlh_doall()\fR to deallocate any
255 remaining entries in the hash table (see below).
257 \&\fIlh_insert()\fR inserts the structure pointed to by \fBdata\fR into \fBtable\fR.
258 If there already is an entry with the same key, the old value is
259 replaced. Note that \fIlh_insert()\fR stores pointers, the data are not
262 \&\fIlh_delete()\fR deletes an entry from \fBtable\fR.
264 \&\fIlh_retrieve()\fR looks up an entry in \fBtable\fR. Normally, \fBdata\fR is
265 a structure with the key \fIfield\fR\|(s) set; the function will return a
266 pointer to a fully populated structure.
268 \&\fIlh_doall()\fR will, for every entry in the hash table, call \fBfunc\fR with
269 the data item as its parameter. For \fIlh_doall()\fR and \fIlh_doall_arg()\fR,
270 function pointer casting should be avoided in the callbacks (see
271 \&\fB\s-1NOTE\s0\fR) \- instead, either declare the callbacks to match the
272 prototype required in \fIlh_new()\fR or use the declare/implement macros to
273 create type-safe wrappers that cast variables prior to calling your
274 type-specific callbacks. An example of this is illustrated here where
275 the callback is used to cleanup resources for items in the hash table
276 prior to the hashtable itself being deallocated:
279 \& /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
280 \& void STUFF_cleanup(STUFF *a);
281 \& /* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
282 \& IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF *)
283 \& /* ... then later in the code ... */
284 \& /* So to run "STUFF_cleanup" against all items in a hash table ... */
285 \& lh_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
286 \& /* Then the hash table itself can be deallocated */
287 \& lh_free(hashtable);
289 When doing this, be careful if you delete entries from the hash table
290 in your callbacks: the table may decrease in size, moving the item
291 that you are currently on down lower in the hash table \- this could
292 cause some entries to be skipped during the iteration. The second
293 best solution to this problem is to set hash->down_load=0 before
294 you start (which will stop the hash table ever decreasing in size).
295 The best solution is probably to avoid deleting items from the hash
296 table inside a \*(L"doall\*(R" callback!
298 \&\fIlh_doall_arg()\fR is the same as \fIlh_doall()\fR except that \fBfunc\fR will be
299 called with \fBarg\fR as the second argument and \fBfunc\fR should be of
300 type \fB\s-1LHASH_DOALL_ARG_FN_TYPE\s0\fR (a callback prototype that is passed
301 both the table entry and an extra argument). As with \fIlh_doall()\fR, you
302 can instead choose to declare your callback with a prototype matching
303 the types you are dealing with and use the declare/implement macros to
304 create compatible wrappers that cast variables before calling your
305 type-specific callbacks. An example of this is demonstrated here
306 (printing all hash table entries to a \s-1BIO\s0 that is provided by the
310 \& /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
311 \& void STUFF_print(const STUFF *a, BIO *output_bio);
312 \& /* Implement a prototype-compatible wrapper for "STUFF_print" */
313 \& static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print, const STUFF *, BIO *)
314 \& /* ... then later in the code ... */
315 \& /* Print out the entire hashtable to a particular BIO */
316 \& lh_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), logging_bio);
318 \&\fIlh_error()\fR can be used to determine if an error occurred in the last
319 operation. \fIlh_error()\fR is a macro.
321 .IX Header "RETURN VALUES"
322 \&\fIlh_new()\fR returns \fB\s-1NULL\s0\fR on error, otherwise a pointer to the new
323 \&\fB\s-1LHASH\s0\fR structure.
325 When a hash table entry is replaced, \fIlh_insert()\fR returns the value
326 being replaced. \fB\s-1NULL\s0\fR is returned on normal operation and on error.
328 \&\fIlh_delete()\fR returns the entry being deleted. \fB\s-1NULL\s0\fR is returned if
329 there is no such value in the hash table.
331 \&\fIlh_retrieve()\fR returns the hash table entry if it has been found,
332 \&\fB\s-1NULL\s0\fR otherwise.
334 \&\fIlh_error()\fR returns 1 if an error occurred in the last operation, 0
337 \&\fIlh_free()\fR, \fIlh_doall()\fR and \fIlh_doall_arg()\fR return no values.
340 The various \s-1LHASH\s0 macros and callback types exist to make it possible
341 to write type-safe code without resorting to function-prototype
342 casting \- an evil that makes application code much harder to
343 audit/verify and also opens the window of opportunity for stack
344 corruption and other hard-to-find bugs. It also, apparently, violates
347 The \s-1LHASH\s0 code regards table entries as constant data. As such, it
348 internally represents \fIlh_insert()\fR'd items with a \*(L"const void *\*(R"
349 pointer type. This is why callbacks such as those used by \fIlh_doall()\fR
350 and \fIlh_doall_arg()\fR declare their prototypes with \*(L"const\*(R", even for the
351 parameters that pass back the table items' data pointers \- for
352 consistency, user-provided data is \*(L"const\*(R" at all times as far as the
353 \&\s-1LHASH\s0 code is concerned. However, as callers are themselves providing
354 these pointers, they can choose whether they too should be treating
355 all such parameters as constant.
357 As an example, a hash table may be maintained by code that, for
358 reasons of encapsulation, has only \*(L"const\*(R" access to the data being
359 indexed in the hash table (ie. it is returned as \*(L"const\*(R" from
360 elsewhere in their code) \- in this case the \s-1LHASH\s0 prototypes are
361 appropriate as-is. Conversely, if the caller is responsible for the
362 life-time of the data in question, then they may well wish to make
363 modifications to table item passed back in the \fIlh_doall()\fR or
364 \&\fIlh_doall_arg()\fR callbacks (see the \*(L"STUFF_cleanup\*(R" example above). If
365 so, the caller can either cast the \*(L"const\*(R" away (if they're providing
366 the raw callbacks themselves) or use the macros to declare/implement
367 the wrapper functions without \*(L"const\*(R" types.
369 Callers that only have \*(L"const\*(R" access to data they're indexing in a
370 table, yet declare callbacks without constant types (or cast the
371 \&\*(L"const\*(R" away themselves), are therefore creating their own risks/bugs
372 without being encouraged to do so by the \s-1API\s0. On a related note,
373 those auditing code should pay special attention to any instances of
374 DECLARE/IMPLEMENT_LHASH_DOALL_[\s-1ARG_\s0]_FN macros that provide types
375 without any \*(L"const\*(R" qualifiers.
378 \&\fIlh_insert()\fR returns \fB\s-1NULL\s0\fR both for success and error.
380 .IX Header "INTERNALS"
381 The following description is based on the SSLeay documentation:
383 The \fBlhash\fR library implements a hash table described in the
384 \&\fICommunications of the \s-1ACM\s0\fR in 1991. What makes this hash table
385 different is that as the table fills, the hash table is increased (or
386 decreased) in size via \fIOPENSSL_realloc()\fR. When a 'resize' is done, instead of
387 all hashes being redistributed over twice as many 'buckets', one
388 bucket is split. So when an 'expand' is done, there is only a minimal
389 cost to redistribute some values. Subsequent inserts will cause more
390 single 'bucket' redistributions but there will never be a sudden large
391 cost due to redistributing all the 'buckets'.
393 The state for a particular hash table is kept in the \fB\s-1LHASH\s0\fR structure.
394 The decision to increase or decrease the hash table size is made
395 depending on the 'load' of the hash table. The load is the number of
396 items in the hash table divided by the size of the hash table. The
397 default values are as follows. If (hash->up_load < load) =>
398 expand. if (hash->down_load > load) => contract. The
399 \&\fBup_load\fR has a default value of 1 and \fBdown_load\fR has a default value
400 of 2. These numbers can be modified by the application by just
401 playing with the \fBup_load\fR and \fBdown_load\fR variables. The 'load' is
402 kept in a form which is multiplied by 256. So
403 hash->up_load=8*256; will cause a load of 8 to be set.
405 If you are interested in performance the field to watch is
406 num_comp_calls. The hash library keeps track of the 'hash' value for
407 each item so when a lookup is done, the 'hashes' are compared, if
408 there is a match, then a full compare is done, and
409 hash->num_comp_calls is incremented. If num_comp_calls is not equal
410 to num_delete plus num_retrieve it means that your hash function is
411 generating hashes that are the same for different values. It is
412 probably worth changing your hash function if this is the case because
413 even if your hash table has 10 items in a 'bucket', it can be searched
414 with 10 \fBunsigned long\fR compares and 10 linked list traverses. This
415 will be much less expensive that 10 calls to your compare function.
417 \&\fIlh_strhash()\fR is a demo string hashing function:
420 \& unsigned long lh_strhash(const char *c);
422 Since the \fB\s-1LHASH\s0\fR routines would normally be passed structures, this
423 routine would not normally be passed to \fIlh_new()\fR, rather it would be
424 used in the function passed to \fIlh_new()\fR.
426 .IX Header "SEE ALSO"
430 The \fBlhash\fR library is available in all versions of SSLeay and OpenSSL.
431 \&\fIlh_error()\fR was added in SSLeay 0.9.1b.
433 This manpage is derived from the SSLeay documentation.
435 In OpenSSL 0.9.7, all lhash functions that were passed function pointers
436 were changed for better type safety, and the function types \s-1LHASH_COMP_FN_TYPE\s0,
437 \&\s-1LHASH_HASH_FN_TYPE\s0, \s-1LHASH_DOALL_FN_TYPE\s0 and \s-1LHASH_DOALL_ARG_FN_TYPE\s0
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