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, 10lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve, 11lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_error - dynamic hash table 12 13=head1 SYNOPSIS 14 15=for comment generic 16 17 #include <openssl/lhash.h> 18 19 DECLARE_LHASH_OF(TYPE); 20 21 LHASH *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare); 22 void lh_TYPE_free(LHASH_OF(TYPE) *table); 23 24 TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data); 25 TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data); 26 TYPE *lh_retrieve(LHASH_OF(TYPE) *table, TYPE *data); 27 28 void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func); 29 void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func, 30 TYPE *arg); 31 32 int lh_TYPE_error(LHASH_OF(TYPE) *table); 33 34 typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *); 35 typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *); 36 typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *); 37 typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *); 38 39=head1 DESCRIPTION 40 41This library implements type-checked dynamic hash tables. The hash 42table entries can be arbitrary structures. Usually they consist of key 43and value fields. In the description here, I<TYPE> is used a placeholder 44for any of the OpenSSL datatypes, such as I<SSL_SESSION>. 45 46lh_TYPE_new() creates a new B<LHASH_OF(TYPE)> structure to store 47arbitrary data entries, and specifies the 'hash' and 'compare' 48callbacks to be used in organising the table's entries. The B<hash> 49callback takes a pointer to a table entry as its argument and returns 50an unsigned long hash value for its key field. The hash value is 51normally truncated to a power of 2, so make sure that your hash 52function returns well mixed low order bits. The B<compare> callback 53takes two arguments (pointers to two hash table entries), and returns 540 if their keys are equal, nonzero otherwise. 55 56If your hash table 57will contain items of some particular type and the B<hash> and 58B<compare> callbacks hash/compare these types, then the 59B<IMPLEMENT_LHASH_HASH_FN> and B<IMPLEMENT_LHASH_COMP_FN> macros can be 60used to create callback wrappers of the prototypes required by 61lh_TYPE_new() as shown in this example: 62 63 /* 64 * Implement the hash and compare functions; "stuff" can be any word. 65 */ 66 static unsigned long stuff_hash(const TYPE *a) 67 { 68 ... 69 } 70 static int stuff_cmp(const TYPE *a, const TYPE *b) 71 { 72 ... 73 } 74 75 /* 76 * Implement the wrapper functions. 77 */ 78 static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE) 79 static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE) 80 81If the type is going to be used in several places, the following macros 82can be used in a common header file to declare the function wrappers: 83 84 DECLARE_LHASH_HASH_FN(stuff, TYPE) 85 DECLARE_LHASH_COMP_FN(stuff, TYPE) 86 87Then a hash table of TYPE objects can be created using this: 88 89 LHASH_OF(TYPE) *htable; 90 91 htable = lh_TYPE_new(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff)); 92 93lh_TYPE_free() frees the B<LHASH_OF(TYPE)> structure 94B<table>. Allocated hash table entries will not be freed; consider 95using lh_TYPE_doall() to deallocate any remaining entries in the 96hash table (see below). 97 98lh_TYPE_insert() inserts the structure pointed to by B<data> into 99B<table>. If there already is an entry with the same key, the old 100value is replaced. Note that lh_TYPE_insert() stores pointers, the 101data are not copied. 102 103lh_TYPE_delete() deletes an entry from B<table>. 104 105lh_TYPE_retrieve() looks up an entry in B<table>. Normally, B<data> 106is a structure with the key field(s) set; the function will return a 107pointer to a fully populated structure. 108 109lh_TYPE_doall() will, for every entry in the hash table, call 110B<func> with the data item as its parameter. 111For example: 112 113 /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */ 114 void TYPE_cleanup_doall(TYPE *a); 115 116 /* Implement a prototype-compatible wrapper for "TYPE_cleanup" */ 117 IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE) 118 119 /* Call "TYPE_cleanup" against all items in a hash table. */ 120 lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup)); 121 122 /* Then the hash table itself can be deallocated */ 123 lh_TYPE_free(hashtable); 124 125When doing this, be careful if you delete entries from the hash table 126in your callbacks: the table may decrease in size, moving the item 127that you are currently on down lower in the hash table - this could 128cause some entries to be skipped during the iteration. The second 129best solution to this problem is to set hash-E<gt>down_load=0 before 130you start (which will stop the hash table ever decreasing in size). 131The best solution is probably to avoid deleting items from the hash 132table inside a "doall" callback! 133 134lh_TYPE_doall_arg() is the same as lh_TYPE_doall() except that 135B<func> will be called with B<arg> as the second argument and B<func> 136should be of type B<LHASH_DOALL_ARG_FN_TYPE> (a callback prototype 137that is passed both the table entry and an extra argument). As with 138lh_doall(), you can instead choose to declare your callback with a 139prototype matching the types you are dealing with and use the 140declare/implement macros to create compatible wrappers that cast 141variables before calling your type-specific callbacks. An example of 142this is demonstrated here (printing all hash table entries to a BIO 143that is provided by the caller): 144 145 /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */ 146 void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio); 147 148 /* Implement a prototype-compatible wrapper for "TYPE_print" */ 149 static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO) 150 151 /* Print out the entire hashtable to a particular BIO */ 152 lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO, 153 logging_bio); 154 155 156lh_TYPE_error() can be used to determine if an error occurred in the last 157operation. 158 159=head1 RETURN VALUES 160 161lh_TYPE_new() returns B<NULL> on error, otherwise a pointer to the new 162B<LHASH> structure. 163 164When a hash table entry is replaced, lh_TYPE_insert() returns the value 165being replaced. B<NULL> is returned on normal operation and on error. 166 167lh_TYPE_delete() returns the entry being deleted. B<NULL> is returned if 168there is no such value in the hash table. 169 170lh_TYPE_retrieve() returns the hash table entry if it has been found, 171B<NULL> otherwise. 172 173lh_TYPE_error() returns 1 if an error occurred in the last operation, 0 174otherwise. It's meaningful only after non-retrieve operations. 175 176lh_TYPE_free(), lh_TYPE_doall() and lh_TYPE_doall_arg() return no values. 177 178=head1 NOTE 179 180The LHASH code is not thread safe. All updating operations, as well as 181lh_TYPE_error call must be performed under a write lock. All retrieve 182operations should be performed under a read lock, I<unless> accurate 183usage statistics are desired. In which case, a write lock should be used 184for retrieve operations as well. For output of the usage statistics, 185using the functions from L<OPENSSL_LH_stats(3)>, a read lock suffices. 186 187The LHASH code regards table entries as constant data. As such, it 188internally represents lh_insert()'d items with a "const void *" 189pointer type. This is why callbacks such as those used by lh_doall() 190and lh_doall_arg() declare their prototypes with "const", even for the 191parameters that pass back the table items' data pointers - for 192consistency, user-provided data is "const" at all times as far as the 193LHASH code is concerned. However, as callers are themselves providing 194these pointers, they can choose whether they too should be treating 195all such parameters as constant. 196 197As an example, a hash table may be maintained by code that, for 198reasons of encapsulation, has only "const" access to the data being 199indexed in the hash table (i.e. it is returned as "const" from 200elsewhere in their code) - in this case the LHASH prototypes are 201appropriate as-is. Conversely, if the caller is responsible for the 202life-time of the data in question, then they may well wish to make 203modifications to table item passed back in the lh_doall() or 204lh_doall_arg() callbacks (see the "TYPE_cleanup" example above). If 205so, the caller can either cast the "const" away (if they're providing 206the raw callbacks themselves) or use the macros to declare/implement 207the wrapper functions without "const" types. 208 209Callers that only have "const" access to data they're indexing in a 210table, yet declare callbacks without constant types (or cast the 211"const" away themselves), are therefore creating their own risks/bugs 212without being encouraged to do so by the API. On a related note, 213those auditing code should pay special attention to any instances of 214DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types 215without any "const" qualifiers. 216 217=head1 BUGS 218 219lh_TYPE_insert() returns B<NULL> both for success and error. 220 221=head1 SEE ALSO 222 223L<OPENSSL_LH_stats(3)> 224 225=head1 HISTORY 226 227In OpenSSL 1.0.0, the lhash interface was revamped for better 228type checking. 229 230=head1 COPYRIGHT 231 232Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved. 233 234Licensed under the OpenSSL license (the "License"). You may not use 235this file except in compliance with the License. You can obtain a copy 236in the file LICENSE in the source distribution or at 237L<https://www.openssl.org/source/license.html>. 238 239=cut 240