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 DECLARE_LHASH_OF(<type>); 12 13 LHASH *lh_<type>_new(); 14 void lh_<type>_free(LHASH_OF(<type> *table); 15 16 <type> *lh_<type>_insert(LHASH_OF(<type> *table, <type> *data); 17 <type> *lh_<type>_delete(LHASH_OF(<type> *table, <type> *data); 18 <type> *lh_retrieve(LHASH_OF<type> *table, <type> *data); 19 20 void lh_<type>_doall(LHASH_OF(<type> *table, LHASH_DOALL_FN_TYPE func); 21 void lh_<type>_doall_arg(LHASH_OF(<type> *table, LHASH_DOALL_ARG_FN_TYPE func, 22 <type2>, <type2> *arg); 23 24 int lh_<type>_error(LHASH_OF(<type> *table); 25 26 typedef int (*LHASH_COMP_FN_TYPE)(const void *, const void *); 27 typedef unsigned long (*LHASH_HASH_FN_TYPE)(const void *); 28 typedef void (*LHASH_DOALL_FN_TYPE)(const void *); 29 typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *); 30 31=head1 DESCRIPTION 32 33This library implements type-checked dynamic hash tables. The hash 34table entries can be arbitrary structures. Usually they consist of key 35and value fields. 36 37lh_<type>_new() creates a new B<LHASH_OF(<type>> structure to store 38arbitrary data entries, and provides the 'hash' and 'compare' 39callbacks to be used in organising the table's entries. The B<hash> 40callback takes a pointer to a table entry as its argument and returns 41an unsigned long hash value for its key field. The hash value is 42normally truncated to a power of 2, so make sure that your hash 43function returns well mixed low order bits. The B<compare> callback 44takes two arguments (pointers to two hash table entries), and returns 450 if their keys are equal, non-zero otherwise. If your hash table 46will contain items of some particular type and the B<hash> and 47B<compare> callbacks hash/compare these types, then the 48B<DECLARE_LHASH_HASH_FN> and B<IMPLEMENT_LHASH_COMP_FN> macros can be 49used to create callback wrappers of the prototypes required by 50lh_<type>_new(). These provide per-variable casts before calling the 51type-specific callbacks written by the application author. These 52macros, as well as those used for the "doall" callbacks, are defined 53as; 54 55 #define DECLARE_LHASH_HASH_FN(name, o_type) \ 56 unsigned long name##_LHASH_HASH(const void *); 57 #define IMPLEMENT_LHASH_HASH_FN(name, o_type) \ 58 unsigned long name##_LHASH_HASH(const void *arg) { \ 59 const o_type *a = arg; \ 60 return name##_hash(a); } 61 #define LHASH_HASH_FN(name) name##_LHASH_HASH 62 63 #define DECLARE_LHASH_COMP_FN(name, o_type) \ 64 int name##_LHASH_COMP(const void *, const void *); 65 #define IMPLEMENT_LHASH_COMP_FN(name, o_type) \ 66 int name##_LHASH_COMP(const void *arg1, const void *arg2) { \ 67 const o_type *a = arg1; \ 68 const o_type *b = arg2; \ 69 return name##_cmp(a,b); } 70 #define LHASH_COMP_FN(name) name##_LHASH_COMP 71 72 #define DECLARE_LHASH_DOALL_FN(name, o_type) \ 73 void name##_LHASH_DOALL(void *); 74 #define IMPLEMENT_LHASH_DOALL_FN(name, o_type) \ 75 void name##_LHASH_DOALL(void *arg) { \ 76 o_type *a = arg; \ 77 name##_doall(a); } 78 #define LHASH_DOALL_FN(name) name##_LHASH_DOALL 79 80 #define DECLARE_LHASH_DOALL_ARG_FN(name, o_type, a_type) \ 81 void name##_LHASH_DOALL_ARG(void *, void *); 82 #define IMPLEMENT_LHASH_DOALL_ARG_FN(name, o_type, a_type) \ 83 void name##_LHASH_DOALL_ARG(void *arg1, void *arg2) { \ 84 o_type *a = arg1; \ 85 a_type *b = arg2; \ 86 name##_doall_arg(a, b); } 87 #define LHASH_DOALL_ARG_FN(name) name##_LHASH_DOALL_ARG 88 89 An example of a hash table storing (pointers to) structures of type 'STUFF' 90 could be defined as follows; 91 92 /* Calculates the hash value of 'tohash' (implemented elsewhere) */ 93 unsigned long STUFF_hash(const STUFF *tohash); 94 /* Orders 'arg1' and 'arg2' (implemented elsewhere) */ 95 int stuff_cmp(const STUFF *arg1, const STUFF *arg2); 96 /* Create the type-safe wrapper functions for use in the LHASH internals */ 97 static IMPLEMENT_LHASH_HASH_FN(stuff, STUFF); 98 static IMPLEMENT_LHASH_COMP_FN(stuff, STUFF); 99 /* ... */ 100 int main(int argc, char *argv[]) { 101 /* Create the new hash table using the hash/compare wrappers */ 102 LHASH_OF(STUFF) *hashtable = lh_STUFF_new(LHASH_HASH_FN(STUFF_hash), 103 LHASH_COMP_FN(STUFF_cmp)); 104 /* ... */ 105 } 106 107lh_<type>_free() frees the B<LHASH_OF(<type>> structure 108B<table>. Allocated hash table entries will not be freed; consider 109using lh_<type>_doall() to deallocate any remaining entries in the 110hash table (see below). 111 112lh_<type>_insert() inserts the structure pointed to by B<data> into 113B<table>. If there already is an entry with the same key, the old 114value is replaced. Note that lh_<type>_insert() stores pointers, the 115data are not copied. 116 117lh_<type>_delete() deletes an entry from B<table>. 118 119lh_<type>_retrieve() looks up an entry in B<table>. Normally, B<data> 120is a structure with the key field(s) set; the function will return a 121pointer to a fully populated structure. 122 123lh_<type>_doall() will, for every entry in the hash table, call 124B<func> with the data item as its parameter. For lh_<type>_doall() 125and lh_<type>_doall_arg(), function pointer casting should be avoided 126in the callbacks (see B<NOTE>) - instead use the declare/implement 127macros to create type-checked wrappers that cast variables prior to 128calling your type-specific callbacks. An example of this is 129illustrated here where the callback is used to cleanup resources for 130items in the hash table prior to the hashtable itself being 131deallocated: 132 133 /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */ 134 void STUFF_cleanup_doall(STUFF *a); 135 /* Implement a prototype-compatible wrapper for "STUFF_cleanup" */ 136 IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF) 137 /* ... then later in the code ... */ 138 /* So to run "STUFF_cleanup" against all items in a hash table ... */ 139 lh_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup)); 140 /* Then the hash table itself can be deallocated */ 141 lh_STUFF_free(hashtable); 142 143When doing this, be careful if you delete entries from the hash table 144in your callbacks: the table may decrease in size, moving the item 145that you are currently on down lower in the hash table - this could 146cause some entries to be skipped during the iteration. The second 147best solution to this problem is to set hash-E<gt>down_load=0 before 148you start (which will stop the hash table ever decreasing in size). 149The best solution is probably to avoid deleting items from the hash 150table inside a "doall" callback! 151 152lh_<type>_doall_arg() is the same as lh_<type>_doall() except that 153B<func> will be called with B<arg> as the second argument and B<func> 154should be of type B<LHASH_DOALL_ARG_FN_TYPE> (a callback prototype 155that is passed both the table entry and an extra argument). As with 156lh_doall(), you can instead choose to declare your callback with a 157prototype matching the types you are dealing with and use the 158declare/implement macros to create compatible wrappers that cast 159variables before calling your type-specific callbacks. An example of 160this is demonstrated here (printing all hash table entries to a BIO 161that is provided by the caller): 162 163 /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */ 164 void STUFF_print_doall_arg(const STUFF *a, BIO *output_bio); 165 /* Implement a prototype-compatible wrapper for "STUFF_print" */ 166 static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF, const STUFF, BIO) 167 /* ... then later in the code ... */ 168 /* Print out the entire hashtable to a particular BIO */ 169 lh_STUFF_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), BIO, 170 logging_bio); 171 172lh_<type>_error() can be used to determine if an error occurred in the last 173operation. lh_<type>_error() is a macro. 174 175=head1 RETURN VALUES 176 177lh_<type>_new() returns B<NULL> on error, otherwise a pointer to the new 178B<LHASH> structure. 179 180When a hash table entry is replaced, lh_<type>_insert() returns the value 181being replaced. B<NULL> is returned on normal operation and on error. 182 183lh_<type>_delete() returns the entry being deleted. B<NULL> is returned if 184there is no such value in the hash table. 185 186lh_<type>_retrieve() returns the hash table entry if it has been found, 187B<NULL> otherwise. 188 189lh_<type>_error() returns 1 if an error occurred in the last operation, 0 190otherwise. 191 192lh_<type>_free(), lh_<type>_doall() and lh_<type>_doall_arg() return no values. 193 194=head1 NOTE 195 196The various LHASH macros and callback types exist to make it possible 197to write type-checked code without resorting to function-prototype 198casting - an evil that makes application code much harder to 199audit/verify and also opens the window of opportunity for stack 200corruption and other hard-to-find bugs. It also, apparently, violates 201ANSI-C. 202 203The LHASH code regards table entries as constant data. As such, it 204internally represents lh_insert()'d items with a "const void *" 205pointer type. This is why callbacks such as those used by lh_doall() 206and lh_doall_arg() declare their prototypes with "const", even for the 207parameters that pass back the table items' data pointers - for 208consistency, user-provided data is "const" at all times as far as the 209LHASH code is concerned. However, as callers are themselves providing 210these pointers, they can choose whether they too should be treating 211all such parameters as constant. 212 213As an example, a hash table may be maintained by code that, for 214reasons of encapsulation, has only "const" access to the data being 215indexed in the hash table (ie. it is returned as "const" from 216elsewhere in their code) - in this case the LHASH prototypes are 217appropriate as-is. Conversely, if the caller is responsible for the 218life-time of the data in question, then they may well wish to make 219modifications to table item passed back in the lh_doall() or 220lh_doall_arg() callbacks (see the "STUFF_cleanup" example above). If 221so, the caller can either cast the "const" away (if they're providing 222the raw callbacks themselves) or use the macros to declare/implement 223the wrapper functions without "const" types. 224 225Callers that only have "const" access to data they're indexing in a 226table, yet declare callbacks without constant types (or cast the 227"const" away themselves), are therefore creating their own risks/bugs 228without being encouraged to do so by the API. On a related note, 229those auditing code should pay special attention to any instances of 230DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types 231without any "const" qualifiers. 232 233=head1 BUGS 234 235lh_<type>_insert() returns B<NULL> both for success and error. 236 237=head1 INTERNALS 238 239The following description is based on the SSLeay documentation: 240 241The B<lhash> library implements a hash table described in the 242I<Communications of the ACM> in 1991. What makes this hash table 243different is that as the table fills, the hash table is increased (or 244decreased) in size via OPENSSL_realloc(). When a 'resize' is done, instead of 245all hashes being redistributed over twice as many 'buckets', one 246bucket is split. So when an 'expand' is done, there is only a minimal 247cost to redistribute some values. Subsequent inserts will cause more 248single 'bucket' redistributions but there will never be a sudden large 249cost due to redistributing all the 'buckets'. 250 251The state for a particular hash table is kept in the B<LHASH> structure. 252The decision to increase or decrease the hash table size is made 253depending on the 'load' of the hash table. The load is the number of 254items in the hash table divided by the size of the hash table. The 255default values are as follows. If (hash->up_load E<lt> load) =E<gt> 256expand. if (hash-E<gt>down_load E<gt> load) =E<gt> contract. The 257B<up_load> has a default value of 1 and B<down_load> has a default value 258of 2. These numbers can be modified by the application by just 259playing with the B<up_load> and B<down_load> variables. The 'load' is 260kept in a form which is multiplied by 256. So 261hash-E<gt>up_load=8*256; will cause a load of 8 to be set. 262 263If you are interested in performance the field to watch is 264num_comp_calls. The hash library keeps track of the 'hash' value for 265each item so when a lookup is done, the 'hashes' are compared, if 266there is a match, then a full compare is done, and 267hash-E<gt>num_comp_calls is incremented. If num_comp_calls is not equal 268to num_delete plus num_retrieve it means that your hash function is 269generating hashes that are the same for different values. It is 270probably worth changing your hash function if this is the case because 271even if your hash table has 10 items in a 'bucket', it can be searched 272with 10 B<unsigned long> compares and 10 linked list traverses. This 273will be much less expensive that 10 calls to your compare function. 274 275lh_strhash() is a demo string hashing function: 276 277 unsigned long lh_strhash(const char *c); 278 279Since the B<LHASH> routines would normally be passed structures, this 280routine would not normally be passed to lh_<type>_new(), rather it would be 281used in the function passed to lh_<type>_new(). 282 283=head1 SEE ALSO 284 285L<lh_stats(3)|lh_stats(3)> 286 287=head1 HISTORY 288 289The B<lhash> library is available in all versions of SSLeay and OpenSSL. 290lh_error() was added in SSLeay 0.9.1b. 291 292This manpage is derived from the SSLeay documentation. 293 294In OpenSSL 0.9.7, all lhash functions that were passed function pointers 295were changed for better type safety, and the function types LHASH_COMP_FN_TYPE, 296LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE and LHASH_DOALL_ARG_FN_TYPE 297became available. 298 299In OpenSSL 1.0.0, the lhash interface was revamped for even better 300type checking. 301 302=cut 303