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| 132 | .\" ======================================================================== |
| 133 | .\" |
| 134 | .IX Title "lhash 3" |
| 135 | .TH lhash 3 "2009-01-11" "0.9.8j" "OpenSSL" |
| 136 | .\" For nroff, turn off justification. Always turn off hyphenation; it makes |
| 137 | .\" way too many mistakes in technical documents. |
| 138 | .if n .ad l |
| 139 | .nh |
| 140 | .SH "NAME" |
| 141 | lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error \- dynamic hash table |
| 142 | .SH "SYNOPSIS" |
| 143 | .IX Header "SYNOPSIS" |
| 144 | .Vb 1 |
| 145 | \& #include <openssl/lhash.h> |
| 146 | \& |
| 147 | \& LHASH *lh_new(LHASH_HASH_FN_TYPE hash, LHASH_COMP_FN_TYPE compare); |
| 148 | \& void lh_free(LHASH *table); |
| 149 | \& |
| 150 | \& void *lh_insert(LHASH *table, void *data); |
| 151 | \& void *lh_delete(LHASH *table, void *data); |
| 152 | \& void *lh_retrieve(LHASH *table, void *data); |
| 153 | \& |
| 154 | \& void lh_doall(LHASH *table, LHASH_DOALL_FN_TYPE func); |
| 155 | \& void lh_doall_arg(LHASH *table, LHASH_DOALL_ARG_FN_TYPE func, |
| 156 | \& void *arg); |
| 157 | \& |
| 158 | \& int lh_error(LHASH *table); |
| 159 | \& |
| 160 | \& typedef int (*LHASH_COMP_FN_TYPE)(const void *, const void *); |
| 161 | \& typedef unsigned long (*LHASH_HASH_FN_TYPE)(const void *); |
| 162 | \& typedef void (*LHASH_DOALL_FN_TYPE)(const void *); |
| 163 | \& typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *); |
| 164 | .Ve |
| 165 | .SH "DESCRIPTION" |
| 166 | .IX Header "DESCRIPTION" |
| 167 | This library implements dynamic hash tables. The hash table entries |
| 168 | can be arbitrary structures. Usually they consist of key and value |
| 169 | fields. |
| 170 | .PP |
| 171 | \&\fIlh_new()\fR creates a new \fB\s-1LHASH\s0\fR structure to store arbitrary data |
| 172 | entries, and provides the 'hash' and 'compare' callbacks to be used in |
| 173 | organising the table's entries. The \fBhash\fR callback takes a pointer |
| 174 | to a table entry as its argument and returns an unsigned long hash |
| 175 | value for its key field. The hash value is normally truncated to a |
| 176 | power of 2, so make sure that your hash function returns well mixed |
| 177 | low order bits. The \fBcompare\fR callback takes two arguments (pointers |
| 178 | to two hash table entries), and returns 0 if their keys are equal, |
| 179 | non-zero otherwise. If your hash table will contain items of some |
| 180 | particular type and the \fBhash\fR and \fBcompare\fR callbacks hash/compare |
| 181 | these types, then the \fB\s-1DECLARE_LHASH_HASH_FN\s0\fR and |
| 182 | \&\fB\s-1IMPLEMENT_LHASH_COMP_FN\s0\fR macros can be used to create callback |
| 183 | wrappers of the prototypes required by \fIlh_new()\fR. These provide |
| 184 | per-variable casts before calling the type-specific callbacks written |
| 185 | by the application author. These macros, as well as those used for |
| 186 | the \*(L"doall\*(R" callbacks, are defined as; |
| 187 | .PP |
| 188 | .Vb 7 |
| 189 | \& #define DECLARE_LHASH_HASH_FN(f_name,o_type) \e |
| 190 | \& unsigned long f_name##_LHASH_HASH(const void *); |
| 191 | \& #define IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \e |
| 192 | \& unsigned long f_name##_LHASH_HASH(const void *arg) { \e |
| 193 | \& o_type a = (o_type)arg; \e |
| 194 | \& return f_name(a); } |
| 195 | \& #define LHASH_HASH_FN(f_name) f_name##_LHASH_HASH |
| 196 | \& |
| 197 | \& #define DECLARE_LHASH_COMP_FN(f_name,o_type) \e |
| 198 | \& int f_name##_LHASH_COMP(const void *, const void *); |
| 199 | \& #define IMPLEMENT_LHASH_COMP_FN(f_name,o_type) \e |
| 200 | \& int f_name##_LHASH_COMP(const void *arg1, const void *arg2) { \e |
| 201 | \& o_type a = (o_type)arg1; \e |
| 202 | \& o_type b = (o_type)arg2; \e |
| 203 | \& return f_name(a,b); } |
| 204 | \& #define LHASH_COMP_FN(f_name) f_name##_LHASH_COMP |
| 205 | \& |
| 206 | \& #define DECLARE_LHASH_DOALL_FN(f_name,o_type) \e |
| 207 | \& void f_name##_LHASH_DOALL(const void *); |
| 208 | \& #define IMPLEMENT_LHASH_DOALL_FN(f_name,o_type) \e |
| 209 | \& void f_name##_LHASH_DOALL(const void *arg) { \e |
| 210 | \& o_type a = (o_type)arg; \e |
| 211 | \& f_name(a); } |
| 212 | \& #define LHASH_DOALL_FN(f_name) f_name##_LHASH_DOALL |
| 213 | \& |
| 214 | \& #define DECLARE_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \e |
| 215 | \& void f_name##_LHASH_DOALL_ARG(const void *, const void *); |
| 216 | \& #define IMPLEMENT_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \e |
| 217 | \& void f_name##_LHASH_DOALL_ARG(const void *arg1, const void *arg2) { \e |
| 218 | \& o_type a = (o_type)arg1; \e |
| 219 | \& a_type b = (a_type)arg2; \e |
| 220 | \& f_name(a,b); } |
| 221 | \& #define LHASH_DOALL_ARG_FN(f_name) f_name##_LHASH_DOALL_ARG |
| 222 | .Ve |
| 223 | .PP |
| 224 | An example of a hash table storing (pointers to) structures of type '\s-1STUFF\s0' |
| 225 | could be defined as follows; |
| 226 | .PP |
| 227 | .Vb 10 |
| 228 | \& /* Calculates the hash value of \*(Aqtohash\*(Aq (implemented elsewhere) */ |
| 229 | \& unsigned long STUFF_hash(const STUFF *tohash); |
| 230 | \& /* Orders \*(Aqarg1\*(Aq and \*(Aqarg2\*(Aq (implemented elsewhere) */ |
| 231 | \& int STUFF_cmp(const STUFF *arg1, const STUFF *arg2); |
| 232 | \& /* Create the type\-safe wrapper functions for use in the LHASH internals */ |
| 233 | \& static IMPLEMENT_LHASH_HASH_FN(STUFF_hash, const STUFF *) |
| 234 | \& static IMPLEMENT_LHASH_COMP_FN(STUFF_cmp, const STUFF *); |
| 235 | \& /* ... */ |
| 236 | \& int main(int argc, char *argv[]) { |
| 237 | \& /* Create the new hash table using the hash/compare wrappers */ |
| 238 | \& LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash), |
| 239 | \& LHASH_COMP_FN(STUFF_cmp)); |
| 240 | \& /* ... */ |
| 241 | \& } |
| 242 | .Ve |
| 243 | .PP |
| 244 | \&\fIlh_free()\fR frees the \fB\s-1LHASH\s0\fR structure \fBtable\fR. Allocated hash table |
| 245 | entries will not be freed; consider using \fIlh_doall()\fR to deallocate any |
| 246 | remaining entries in the hash table (see below). |
| 247 | .PP |
| 248 | \&\fIlh_insert()\fR inserts the structure pointed to by \fBdata\fR into \fBtable\fR. |
| 249 | If there already is an entry with the same key, the old value is |
| 250 | replaced. Note that \fIlh_insert()\fR stores pointers, the data are not |
| 251 | copied. |
| 252 | .PP |
| 253 | \&\fIlh_delete()\fR deletes an entry from \fBtable\fR. |
| 254 | .PP |
| 255 | \&\fIlh_retrieve()\fR looks up an entry in \fBtable\fR. Normally, \fBdata\fR is |
| 256 | a structure with the key field(s) set; the function will return a |
| 257 | pointer to a fully populated structure. |
| 258 | .PP |
| 259 | \&\fIlh_doall()\fR will, for every entry in the hash table, call \fBfunc\fR with |
| 260 | the data item as its parameter. For \fIlh_doall()\fR and \fIlh_doall_arg()\fR, |
| 261 | function pointer casting should be avoided in the callbacks (see |
| 262 | \&\fB\s-1NOTE\s0\fR) \- instead, either declare the callbacks to match the |
| 263 | prototype required in \fIlh_new()\fR or use the declare/implement macros to |
| 264 | create type-safe wrappers that cast variables prior to calling your |
| 265 | type-specific callbacks. An example of this is illustrated here where |
| 266 | the callback is used to cleanup resources for items in the hash table |
| 267 | prior to the hashtable itself being deallocated: |
| 268 | .PP |
| 269 | .Vb 9 |
| 270 | \& /* Cleans up resources belonging to \*(Aqa\*(Aq (this is implemented elsewhere) */ |
| 271 | \& void STUFF_cleanup(STUFF *a); |
| 272 | \& /* Implement a prototype\-compatible wrapper for "STUFF_cleanup" */ |
| 273 | \& IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF *) |
| 274 | \& /* ... then later in the code ... */ |
| 275 | \& /* So to run "STUFF_cleanup" against all items in a hash table ... */ |
| 276 | \& lh_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup)); |
| 277 | \& /* Then the hash table itself can be deallocated */ |
| 278 | \& lh_free(hashtable); |
| 279 | .Ve |
| 280 | .PP |
| 281 | When doing this, be careful if you delete entries from the hash table |
| 282 | in your callbacks: the table may decrease in size, moving the item |
| 283 | that you are currently on down lower in the hash table \- this could |
| 284 | cause some entries to be skipped during the iteration. The second |
| 285 | best solution to this problem is to set hash\->down_load=0 before |
| 286 | you start (which will stop the hash table ever decreasing in size). |
| 287 | The best solution is probably to avoid deleting items from the hash |
| 288 | table inside a \*(L"doall\*(R" callback! |
| 289 | .PP |
| 290 | \&\fIlh_doall_arg()\fR is the same as \fIlh_doall()\fR except that \fBfunc\fR will be |
| 291 | called with \fBarg\fR as the second argument and \fBfunc\fR should be of |
| 292 | type \fB\s-1LHASH_DOALL_ARG_FN_TYPE\s0\fR (a callback prototype that is passed |
| 293 | both the table entry and an extra argument). As with \fIlh_doall()\fR, you |
| 294 | can instead choose to declare your callback with a prototype matching |
| 295 | the types you are dealing with and use the declare/implement macros to |
| 296 | create compatible wrappers that cast variables before calling your |
| 297 | type-specific callbacks. An example of this is demonstrated here |
| 298 | (printing all hash table entries to a \s-1BIO\s0 that is provided by the |
| 299 | caller): |
| 300 | .PP |
| 301 | .Vb 7 |
| 302 | \& /* Prints item \*(Aqa\*(Aq to \*(Aqoutput_bio\*(Aq (this is implemented elsewhere) */ |
| 303 | \& void STUFF_print(const STUFF *a, BIO *output_bio); |
| 304 | \& /* Implement a prototype\-compatible wrapper for "STUFF_print" */ |
| 305 | \& static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print, const STUFF *, BIO *) |
| 306 | \& /* ... then later in the code ... */ |
| 307 | \& /* Print out the entire hashtable to a particular BIO */ |
| 308 | \& lh_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), logging_bio); |
| 309 | .Ve |
| 310 | .PP |
| 311 | \&\fIlh_error()\fR can be used to determine if an error occurred in the last |
| 312 | operation. \fIlh_error()\fR is a macro. |
| 313 | .SH "RETURN VALUES" |
| 314 | .IX Header "RETURN VALUES" |
| 315 | \&\fIlh_new()\fR returns \fB\s-1NULL\s0\fR on error, otherwise a pointer to the new |
| 316 | \&\fB\s-1LHASH\s0\fR structure. |
| 317 | .PP |
| 318 | When a hash table entry is replaced, \fIlh_insert()\fR returns the value |
| 319 | being replaced. \fB\s-1NULL\s0\fR is returned on normal operation and on error. |
| 320 | .PP |
| 321 | \&\fIlh_delete()\fR returns the entry being deleted. \fB\s-1NULL\s0\fR is returned if |
| 322 | there is no such value in the hash table. |
| 323 | .PP |
| 324 | \&\fIlh_retrieve()\fR returns the hash table entry if it has been found, |
| 325 | \&\fB\s-1NULL\s0\fR otherwise. |
| 326 | .PP |
| 327 | \&\fIlh_error()\fR returns 1 if an error occurred in the last operation, 0 |
| 328 | otherwise. |
| 329 | .PP |
| 330 | \&\fIlh_free()\fR, \fIlh_doall()\fR and \fIlh_doall_arg()\fR return no values. |
| 331 | .SH "NOTE" |
| 332 | .IX Header "NOTE" |
| 333 | The various \s-1LHASH\s0 macros and callback types exist to make it possible |
| 334 | to write type-safe code without resorting to function-prototype |
| 335 | casting \- an evil that makes application code much harder to |
| 336 | audit/verify and also opens the window of opportunity for stack |
| 337 | corruption and other hard-to-find bugs. It also, apparently, violates |
| 338 | ANSI-C. |
| 339 | .PP |
| 340 | The \s-1LHASH\s0 code regards table entries as constant data. As such, it |
| 341 | internally represents \fIlh_insert()\fR'd items with a \*(L"const void *\*(R" |
| 342 | pointer type. This is why callbacks such as those used by \fIlh_doall()\fR |
| 343 | and \fIlh_doall_arg()\fR declare their prototypes with \*(L"const\*(R", even for the |
| 344 | parameters that pass back the table items' data pointers \- for |
| 345 | consistency, user-provided data is \*(L"const\*(R" at all times as far as the |
| 346 | \&\s-1LHASH\s0 code is concerned. However, as callers are themselves providing |
| 347 | these pointers, they can choose whether they too should be treating |
| 348 | all such parameters as constant. |
| 349 | .PP |
| 350 | As an example, a hash table may be maintained by code that, for |
| 351 | reasons of encapsulation, has only \*(L"const\*(R" access to the data being |
| 352 | indexed in the hash table (ie. it is returned as \*(L"const\*(R" from |
| 353 | elsewhere in their code) \- in this case the \s-1LHASH\s0 prototypes are |
| 354 | appropriate as-is. Conversely, if the caller is responsible for the |
| 355 | life-time of the data in question, then they may well wish to make |
| 356 | modifications to table item passed back in the \fIlh_doall()\fR or |
| 357 | \&\fIlh_doall_arg()\fR callbacks (see the \*(L"STUFF_cleanup\*(R" example above). If |
| 358 | so, the caller can either cast the \*(L"const\*(R" away (if they're providing |
| 359 | the raw callbacks themselves) or use the macros to declare/implement |
| 360 | the wrapper functions without \*(L"const\*(R" types. |
| 361 | .PP |
| 362 | Callers that only have \*(L"const\*(R" access to data they're indexing in a |
| 363 | table, yet declare callbacks without constant types (or cast the |
| 364 | \&\*(L"const\*(R" away themselves), are therefore creating their own risks/bugs |
| 365 | without being encouraged to do so by the \s-1API\s0. On a related note, |
| 366 | those auditing code should pay special attention to any instances of |
| 367 | DECLARE/IMPLEMENT_LHASH_DOALL_[\s-1ARG_\s0]_FN macros that provide types |
| 368 | without any \*(L"const\*(R" qualifiers. |
| 369 | .SH "BUGS" |
| 370 | .IX Header "BUGS" |
| 371 | \&\fIlh_insert()\fR returns \fB\s-1NULL\s0\fR both for success and error. |
| 372 | .SH "INTERNALS" |
| 373 | .IX Header "INTERNALS" |
| 374 | The following description is based on the SSLeay documentation: |
| 375 | .PP |
| 376 | The \fBlhash\fR library implements a hash table described in the |
| 377 | \&\fICommunications of the \s-1ACM\s0\fR in 1991. What makes this hash table |
| 378 | different is that as the table fills, the hash table is increased (or |
| 379 | decreased) in size via \fIOPENSSL_realloc()\fR. When a 'resize' is done, instead of |
| 380 | all hashes being redistributed over twice as many 'buckets', one |
| 381 | bucket is split. So when an 'expand' is done, there is only a minimal |
| 382 | cost to redistribute some values. Subsequent inserts will cause more |
| 383 | single 'bucket' redistributions but there will never be a sudden large |
| 384 | cost due to redistributing all the 'buckets'. |
| 385 | .PP |
| 386 | The state for a particular hash table is kept in the \fB\s-1LHASH\s0\fR structure. |
| 387 | The decision to increase or decrease the hash table size is made |
| 388 | depending on the 'load' of the hash table. The load is the number of |
| 389 | items in the hash table divided by the size of the hash table. The |
| 390 | default values are as follows. If (hash\->up_load < load) => |
| 391 | expand. if (hash\->down_load > load) => contract. The |
| 392 | \&\fBup_load\fR has a default value of 1 and \fBdown_load\fR has a default value |
| 393 | of 2. These numbers can be modified by the application by just |
| 394 | playing with the \fBup_load\fR and \fBdown_load\fR variables. The 'load' is |
| 395 | kept in a form which is multiplied by 256. So |
| 396 | hash\->up_load=8*256; will cause a load of 8 to be set. |
| 397 | .PP |
| 398 | If you are interested in performance the field to watch is |
| 399 | num_comp_calls. The hash library keeps track of the 'hash' value for |
| 400 | each item so when a lookup is done, the 'hashes' are compared, if |
| 401 | there is a match, then a full compare is done, and |
| 402 | hash\->num_comp_calls is incremented. If num_comp_calls is not equal |
| 403 | to num_delete plus num_retrieve it means that your hash function is |
| 404 | generating hashes that are the same for different values. It is |
| 405 | probably worth changing your hash function if this is the case because |
| 406 | even if your hash table has 10 items in a 'bucket', it can be searched |
| 407 | with 10 \fBunsigned long\fR compares and 10 linked list traverses. This |
| 408 | will be much less expensive that 10 calls to your compare function. |
| 409 | .PP |
| 410 | \&\fIlh_strhash()\fR is a demo string hashing function: |
| 411 | .PP |
| 412 | .Vb 1 |
| 413 | \& unsigned long lh_strhash(const char *c); |
| 414 | .Ve |
| 415 | .PP |
| 416 | Since the \fB\s-1LHASH\s0\fR routines would normally be passed structures, this |
| 417 | routine would not normally be passed to \fIlh_new()\fR, rather it would be |
| 418 | used in the function passed to \fIlh_new()\fR. |
| 419 | .SH "SEE ALSO" |
| 420 | .IX Header "SEE ALSO" |
| 421 | \&\fIlh_stats\fR\|(3) |
| 422 | .SH "HISTORY" |
| 423 | .IX Header "HISTORY" |
| 424 | The \fBlhash\fR library is available in all versions of SSLeay and OpenSSL. |
| 425 | \&\fIlh_error()\fR was added in SSLeay 0.9.1b. |
| 426 | .PP |
| 427 | This manpage is derived from the SSLeay documentation. |
| 428 | .PP |
| 429 | In OpenSSL 0.9.7, all lhash functions that were passed function pointers |
| 430 | were changed for better type safety, and the function types \s-1LHASH_COMP_FN_TYPE\s0, |
| 431 | \&\s-1LHASH_HASH_FN_TYPE\s0, \s-1LHASH_DOALL_FN_TYPE\s0 and \s-1LHASH_DOALL_ARG_FN_TYPE\s0 |
| 432 | became available. |