1=pod 2 3=head1 NAME 4 5lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error - dynamic hash table 6 7=head1 SYNOPSIS 8 9 #include <openssl/lhash.h> 10 11 LHASH *lh_new(LHASH_HASH_FN_TYPE hash, LHASH_COMP_FN_TYPE compare); 12 void lh_free(LHASH *table); 13 14 void *lh_insert(LHASH *table, void *data); 15 void *lh_delete(LHASH *table, void *data); 16 void *lh_retrieve(LHASH *table, void *data); 17 18 void lh_doall(LHASH *table, LHASH_DOALL_FN_TYPE func); 19 void lh_doall_arg(LHASH *table, LHASH_DOALL_ARG_FN_TYPE func, 20 void *arg); 21 22 int lh_error(LHASH *table); 23 24 typedef int (*LHASH_COMP_FN_TYPE)(const void *, const void *); 25 typedef unsigned long (*LHASH_HASH_FN_TYPE)(const void *); 26 typedef void (*LHASH_DOALL_FN_TYPE)(const void *); 27 typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *); 28 29=head1 DESCRIPTION 30 31This library implements dynamic hash tables. The hash table entries 32can be arbitrary structures. Usually they consist of key and value 33fields. 34 35lh_new() creates a new B<LHASH> structure to store arbitrary data 36entries, and provides the 'hash' and 'compare' callbacks to be used in 37organising the table's entries. The B<hash> callback takes a pointer 38to a table entry as its argument and returns an unsigned long hash 39value for its key field. The hash value is normally truncated to a 40power of 2, so make sure that your hash function returns well mixed 41low order bits. The B<compare> callback takes two arguments (pointers 42to two hash table entries), and returns 0 if their keys are equal, 43non-zero otherwise. If your hash table will contain items of some 44particular type and the B<hash> and B<compare> callbacks hash/compare 45these types, then the B<DECLARE_LHASH_HASH_FN> and 46B<IMPLEMENT_LHASH_COMP_FN> macros can be used to create callback 47wrappers of the prototypes required by lh_new(). These provide 48per-variable casts before calling the type-specific callbacks written 49by the application author. These macros, as well as those used for 50the "doall" callbacks, are defined as; 51 52 #define DECLARE_LHASH_HASH_FN(f_name,o_type) \ 53 unsigned long f_name##_LHASH_HASH(const void *); 54 #define IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \ 55 unsigned long f_name##_LHASH_HASH(const void *arg) { \ 56 o_type a = (o_type)arg; \ 57 return f_name(a); } 58 #define LHASH_HASH_FN(f_name) f_name##_LHASH_HASH 59 60 #define DECLARE_LHASH_COMP_FN(f_name,o_type) \ 61 int f_name##_LHASH_COMP(const void *, const void *); 62 #define IMPLEMENT_LHASH_COMP_FN(f_name,o_type) \ 63 int f_name##_LHASH_COMP(const void *arg1, const void *arg2) { \ 64 o_type a = (o_type)arg1; \ 65 o_type b = (o_type)arg2; \ 66 return f_name(a,b); } 67 #define LHASH_COMP_FN(f_name) f_name##_LHASH_COMP 68 69 #define DECLARE_LHASH_DOALL_FN(f_name,o_type) \ 70 void f_name##_LHASH_DOALL(const void *); 71 #define IMPLEMENT_LHASH_DOALL_FN(f_name,o_type) \ 72 void f_name##_LHASH_DOALL(const void *arg) { \ 73 o_type a = (o_type)arg; \ 74 f_name(a); } 75 #define LHASH_DOALL_FN(f_name) f_name##_LHASH_DOALL 76 77 #define DECLARE_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \ 78 void f_name##_LHASH_DOALL_ARG(const void *, const void *); 79 #define IMPLEMENT_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \ 80 void f_name##_LHASH_DOALL_ARG(const void *arg1, const void *arg2) { \ 81 o_type a = (o_type)arg1; \ 82 a_type b = (a_type)arg2; \ 83 f_name(a,b); } 84 #define LHASH_DOALL_ARG_FN(f_name) f_name##_LHASH_DOALL_ARG 85 86An example of a hash table storing (pointers to) structures of type 'STUFF' 87could be defined as follows; 88 89 /* Calculates the hash value of 'tohash' (implemented elsewhere) */ 90 unsigned long STUFF_hash(const STUFF *tohash); 91 /* Orders 'arg1' and 'arg2' (implemented elsewhere) */ 92 int STUFF_cmp(const STUFF *arg1, const STUFF *arg2); 93 /* Create the type-safe wrapper functions for use in the LHASH internals */ 94 static IMPLEMENT_LHASH_HASH_FN(STUFF_hash, const STUFF *) 95 static IMPLEMENT_LHASH_COMP_FN(STUFF_cmp, const STUFF *); 96 /* ... */ 97 int main(int argc, char *argv[]) { 98 /* Create the new hash table using the hash/compare wrappers */ 99 LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash), 100 LHASH_COMP_FN(STUFF_cmp)); 101 /* ... */ 102 } 103 104lh_free() frees the B<LHASH> structure B<table>. Allocated hash table 105entries will not be freed; consider using lh_doall() to deallocate any 106remaining entries in the hash table (see below). 107 108lh_insert() inserts the structure pointed to by B<data> into B<table>. 109If there already is an entry with the same key, the old value is 110replaced. Note that lh_insert() stores pointers, the data are not 111copied. 112 113lh_delete() deletes an entry from B<table>. 114 115lh_retrieve() looks up an entry in B<table>. Normally, B<data> is 116a structure with the key field(s) set; the function will return a 117pointer to a fully populated structure. 118 119lh_doall() will, for every entry in the hash table, call B<func> with 120the data item as its parameter. For lh_doall() and lh_doall_arg(), 121function pointer casting should be avoided in the callbacks (see 122B<NOTE>) - instead, either declare the callbacks to match the 123prototype required in lh_new() or use the declare/implement macros to 124create type-safe wrappers that cast variables prior to calling your 125type-specific callbacks. An example of this is illustrated here where 126the callback is used to cleanup resources for items in the hash table 127prior to the hashtable itself being deallocated: 128 129 /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */ 130 void STUFF_cleanup(STUFF *a); 131 /* Implement a prototype-compatible wrapper for "STUFF_cleanup" */ 132 IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF *) 133 /* ... then later in the code ... */ 134 /* So to run "STUFF_cleanup" against all items in a hash table ... */ 135 lh_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup)); 136 /* Then the hash table itself can be deallocated */ 137 lh_free(hashtable); 138 139When doing this, be careful if you delete entries from the hash table 140in your callbacks: the table may decrease in size, moving the item 141that you are currently on down lower in the hash table - this could 142cause some entries to be skipped during the iteration. The second 143best solution to this problem is to set hash-E<gt>down_load=0 before 144you start (which will stop the hash table ever decreasing in size). 145The best solution is probably to avoid deleting items from the hash 146table inside a "doall" callback! 147 148lh_doall_arg() is the same as lh_doall() except that B<func> will be 149called with B<arg> as the second argument and B<func> should be of 150type B<LHASH_DOALL_ARG_FN_TYPE> (a callback prototype that is passed 151both the table entry and an extra argument). As with lh_doall(), you 152can instead choose to declare your callback with a prototype matching 153the types you are dealing with and use the declare/implement macros to 154create compatible wrappers that cast variables before calling your 155type-specific callbacks. An example of this is demonstrated here 156(printing all hash table entries to a BIO that is provided by the 157caller): 158 159 /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */ 160 void STUFF_print(const STUFF *a, BIO *output_bio); 161 /* Implement a prototype-compatible wrapper for "STUFF_print" */ 162 static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print, const STUFF *, BIO *) 163 /* ... then later in the code ... */ 164 /* Print out the entire hashtable to a particular BIO */ 165 lh_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), logging_bio); 166 167lh_error() can be used to determine if an error occurred in the last 168operation. lh_error() is a macro. 169 170=head1 RETURN VALUES 171 172lh_new() returns B<NULL> on error, otherwise a pointer to the new 173B<LHASH> structure. 174 175When a hash table entry is replaced, lh_insert() returns the value 176being replaced. B<NULL> is returned on normal operation and on error. 177 178lh_delete() returns the entry being deleted. B<NULL> is returned if 179there is no such value in the hash table. 180 181lh_retrieve() returns the hash table entry if it has been found, 182B<NULL> otherwise. 183 184lh_error() returns 1 if an error occurred in the last operation, 0 185otherwise. 186 187lh_free(), lh_doall() and lh_doall_arg() return no values. 188 189=head1 NOTE 190 191The various LHASH macros and callback types exist to make it possible 192to write type-safe code without resorting to function-prototype 193casting - an evil that makes application code much harder to 194audit/verify and also opens the window of opportunity for stack 195corruption and other hard-to-find bugs. It also, apparently, violates 196ANSI-C. 197 198The LHASH code regards table entries as constant data. As such, it 199internally represents lh_insert()'d items with a "const void *" 200pointer type. This is why callbacks such as those used by lh_doall() 201and lh_doall_arg() declare their prototypes with "const", even for the 202parameters that pass back the table items' data pointers - for 203consistency, user-provided data is "const" at all times as far as the 204LHASH code is concerned. However, as callers are themselves providing 205these pointers, they can choose whether they too should be treating 206all such parameters as constant. 207 208As an example, a hash table may be maintained by code that, for 209reasons of encapsulation, has only "const" access to the data being 210indexed in the hash table (ie. it is returned as "const" from 211elsewhere in their code) - in this case the LHASH prototypes are 212appropriate as-is. Conversely, if the caller is responsible for the 213life-time of the data in question, then they may well wish to make 214modifications to table item passed back in the lh_doall() or 215lh_doall_arg() callbacks (see the "STUFF_cleanup" example above). If 216so, the caller can either cast the "const" away (if they're providing 217the raw callbacks themselves) or use the macros to declare/implement 218the wrapper functions without "const" types. 219 220Callers that only have "const" access to data they're indexing in a 221table, yet declare callbacks without constant types (or cast the 222"const" away themselves), are therefore creating their own risks/bugs 223without being encouraged to do so by the API. On a related note, 224those auditing code should pay special attention to any instances of 225DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types 226without any "const" qualifiers. 227 228=head1 BUGS 229 230lh_insert() returns B<NULL> both for success and error. 231 232=head1 INTERNALS 233 234The following description is based on the SSLeay documentation: 235 236The B<lhash> library implements a hash table described in the 237I<Communications of the ACM> in 1991. What makes this hash table 238different is that as the table fills, the hash table is increased (or 239decreased) in size via OPENSSL_realloc(). When a 'resize' is done, instead of 240all hashes being redistributed over twice as many 'buckets', one 241bucket is split. So when an 'expand' is done, there is only a minimal 242cost to redistribute some values. Subsequent inserts will cause more 243single 'bucket' redistributions but there will never be a sudden large 244cost due to redistributing all the 'buckets'. 245 246The state for a particular hash table is kept in the B<LHASH> structure. 247The decision to increase or decrease the hash table size is made 248depending on the 'load' of the hash table. The load is the number of 249items in the hash table divided by the size of the hash table. The 250default values are as follows. If (hash->up_load E<lt> load) =E<gt> 251expand. if (hash-E<gt>down_load E<gt> load) =E<gt> contract. The 252B<up_load> has a default value of 1 and B<down_load> has a default value 253of 2. These numbers can be modified by the application by just 254playing with the B<up_load> and B<down_load> variables. The 'load' is 255kept in a form which is multiplied by 256. So 256hash-E<gt>up_load=8*256; will cause a load of 8 to be set. 257 258If you are interested in performance the field to watch is 259num_comp_calls. The hash library keeps track of the 'hash' value for 260each item so when a lookup is done, the 'hashes' are compared, if 261there is a match, then a full compare is done, and 262hash-E<gt>num_comp_calls is incremented. If num_comp_calls is not equal 263to num_delete plus num_retrieve it means that your hash function is 264generating hashes that are the same for different values. It is 265probably worth changing your hash function if this is the case because 266even if your hash table has 10 items in a 'bucket', it can be searched 267with 10 B<unsigned long> compares and 10 linked list traverses. This 268will be much less expensive that 10 calls to your compare function. 269 270lh_strhash() is a demo string hashing function: 271 272 unsigned long lh_strhash(const char *c); 273 274Since the B<LHASH> routines would normally be passed structures, this 275routine would not normally be passed to lh_new(), rather it would be 276used in the function passed to lh_new(). 277 278=head1 SEE ALSO 279 280L<lh_stats(3)|lh_stats(3)> 281 282=head1 HISTORY 283 284The B<lhash> library is available in all versions of SSLeay and OpenSSL. 285lh_error() was added in SSLeay 0.9.1b. 286 287This manpage is derived from the SSLeay documentation. 288 289In OpenSSL 0.9.7, all lhash functions that were passed function pointers 290were changed for better type safety, and the function types LHASH_COMP_FN_TYPE, 291LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE and LHASH_DOALL_ARG_FN_TYPE 292became available. 293 294=cut 295