1lhash(3)                            OpenSSL                           lhash(3)
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4

NAME

6       lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall,
7       lh_doall_arg, lh_error - dynamic hash table
8

SYNOPSIS

10        #include <openssl/lhash.h>
11
12        DECLARE_LHASH_OF(<type>);
13
14        LHASH *lh_<type>_new();
15        void lh_<type>_free(LHASH_OF(<type> *table);
16
17        <type> *lh_<type>_insert(LHASH_OF(<type> *table, <type> *data);
18        <type> *lh_<type>_delete(LHASH_OF(<type> *table, <type> *data);
19        <type> *lh_retrieve(LHASH_OF<type> *table, <type> *data);
20
21        void lh_<type>_doall(LHASH_OF(<type> *table, LHASH_DOALL_FN_TYPE func);
22        void lh_<type>_doall_arg(LHASH_OF(<type> *table, LHASH_DOALL_ARG_FN_TYPE func,
23                 <type2>, <type2> *arg);
24
25        int lh_<type>_error(LHASH_OF(<type> *table);
26
27        typedef int (*LHASH_COMP_FN_TYPE)(const void *, const void *);
28        typedef unsigned long (*LHASH_HASH_FN_TYPE)(const void *);
29        typedef void (*LHASH_DOALL_FN_TYPE)(const void *);
30        typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
31

DESCRIPTION

33       This library implements type-checked dynamic hash tables. The hash
34       table entries can be arbitrary structures. Usually they consist of key
35       and value fields.
36
37       lh_<type>_new() creates a new LHASH_OF(<type> structure to store
38       arbitrary data entries, and provides the 'hash' and 'compare' callbacks
39       to be used in organising the table's entries.  The hash callback takes
40       a pointer to a table entry as its argument and returns an unsigned long
41       hash value for its key field.  The hash value is normally truncated to
42       a power of 2, so make sure that your hash function returns well mixed
43       low order bits.  The compare callback takes two arguments (pointers to
44       two hash table entries), and returns 0 if their keys are equal, non-
45       zero otherwise.  If your hash table will contain items of some
46       particular type and the hash and compare callbacks hash/compare these
47       types, then the DECLARE_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN
48       macros can be used to create callback wrappers of the prototypes
49       required by lh_<type>_new().  These provide per-variable casts before
50       calling the type-specific callbacks written by the application author.
51       These macros, as well as those used for the "doall" callbacks, are
52       defined as;
53
54        #define DECLARE_LHASH_HASH_FN(name, o_type) \
55                unsigned long name##_LHASH_HASH(const void *);
56        #define IMPLEMENT_LHASH_HASH_FN(name, o_type) \
57                unsigned long name##_LHASH_HASH(const void *arg) { \
58                        const o_type *a = arg; \
59                        return name##_hash(a); }
60        #define LHASH_HASH_FN(name) name##_LHASH_HASH
61
62        #define DECLARE_LHASH_COMP_FN(name, o_type) \
63                int name##_LHASH_COMP(const void *, const void *);
64        #define IMPLEMENT_LHASH_COMP_FN(name, o_type) \
65                int name##_LHASH_COMP(const void *arg1, const void *arg2) { \
66                        const o_type *a = arg1;                    \
67                        const o_type *b = arg2; \
68                        return name##_cmp(a,b); }
69        #define LHASH_COMP_FN(name) name##_LHASH_COMP
70
71        #define DECLARE_LHASH_DOALL_FN(name, o_type) \
72                void name##_LHASH_DOALL(void *);
73        #define IMPLEMENT_LHASH_DOALL_FN(name, o_type) \
74                void name##_LHASH_DOALL(void *arg) { \
75                        o_type *a = arg; \
76                        name##_doall(a); }
77        #define LHASH_DOALL_FN(name) name##_LHASH_DOALL
78
79        #define DECLARE_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
80                void name##_LHASH_DOALL_ARG(void *, void *);
81        #define IMPLEMENT_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
82                void name##_LHASH_DOALL_ARG(void *arg1, void *arg2) { \
83                        o_type *a = arg1; \
84                        a_type *b = arg2; \
85                        name##_doall_arg(a, b); }
86        #define LHASH_DOALL_ARG_FN(name) name##_LHASH_DOALL_ARG
87
88        An example of a hash table storing (pointers to) structures of type 'STUFF'
89        could be defined as follows;
90
91        /* Calculates the hash value of 'tohash' (implemented elsewhere) */
92        unsigned long STUFF_hash(const STUFF *tohash);
93        /* Orders 'arg1' and 'arg2' (implemented elsewhere) */
94        int stuff_cmp(const STUFF *arg1, const STUFF *arg2);
95        /* Create the type-safe wrapper functions for use in the LHASH internals */
96        static IMPLEMENT_LHASH_HASH_FN(stuff, STUFF);
97        static IMPLEMENT_LHASH_COMP_FN(stuff, STUFF);
98        /* ... */
99        int main(int argc, char *argv[]) {
100                /* Create the new hash table using the hash/compare wrappers */
101                LHASH_OF(STUFF) *hashtable = lh_STUFF_new(LHASH_HASH_FN(STUFF_hash),
102                                          LHASH_COMP_FN(STUFF_cmp));
103                /* ... */
104        }
105
106       lh_<type>_free() frees the LHASH_OF(<type> structure table. Allocated
107       hash table entries will not be freed; consider using lh_<type>_doall()
108       to deallocate any remaining entries in the hash table (see below).
109
110       lh_<type>_insert() inserts the structure pointed to by data into table.
111       If there already is an entry with the same key, the old value is
112       replaced. Note that lh_<type>_insert() stores pointers, the data are
113       not copied.
114
115       lh_<type>_delete() deletes an entry from table.
116
117       lh_<type>_retrieve() looks up an entry in table. Normally, data is a
118       structure with the key field(s) set; the function will return a pointer
119       to a fully populated structure.
120
121       lh_<type>_doall() will, for every entry in the hash table, call func
122       with the data item as its parameter.  For lh_<type>_doall() and
123       lh_<type>_doall_arg(), function pointer casting should be avoided in
124       the callbacks (see NOTE) - instead use the declare/implement macros to
125       create type-checked wrappers that cast variables prior to calling your
126       type-specific callbacks.  An example of this is illustrated here where
127       the callback is used to cleanup resources for items in the hash table
128       prior to the hashtable itself being deallocated:
129
130        /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
131        void STUFF_cleanup_doall(STUFF *a);
132        /* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
133        IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF)
134                /* ... then later in the code ... */
135        /* So to run "STUFF_cleanup" against all items in a hash table ... */
136        lh_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
137        /* Then the hash table itself can be deallocated */
138        lh_STUFF_free(hashtable);
139
140       When doing this, be careful if you delete entries from the hash table
141       in your callbacks: the table may decrease in size, moving the item that
142       you are currently on down lower in the hash table - this could cause
143       some entries to be skipped during the iteration.  The second best
144       solution to this problem is to set hash->down_load=0 before you start
145       (which will stop the hash table ever decreasing in size).  The best
146       solution is probably to avoid deleting items from the hash table inside
147       a "doall" callback!
148
149       lh_<type>_doall_arg() is the same as lh_<type>_doall() except that func
150       will be called with arg as the second argument and func should be of
151       type LHASH_DOALL_ARG_FN_TYPE (a callback prototype that is passed both
152       the table entry and an extra argument).  As with lh_doall(), you can
153       instead choose to declare your callback with a prototype matching the
154       types you are dealing with and use the declare/implement macros to
155       create compatible wrappers that cast variables before calling your
156       type-specific callbacks.  An example of this is demonstrated here
157       (printing all hash table entries to a BIO that is provided by the
158       caller):
159
160        /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
161        void STUFF_print_doall_arg(const STUFF *a, BIO *output_bio);
162        /* Implement a prototype-compatible wrapper for "STUFF_print" */
163        static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF, const STUFF, BIO)
164                /* ... then later in the code ... */
165        /* Print out the entire hashtable to a particular BIO */
166        lh_STUFF_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), BIO,
167                           logging_bio);
168
169       lh_<type>_error() can be used to determine if an error occurred in the
170       last operation. lh_<type>_error() is a macro.
171

RETURN VALUES

173       lh_<type>_new() returns NULL on error, otherwise a pointer to the new
174       LHASH structure.
175
176       When a hash table entry is replaced, lh_<type>_insert() returns the
177       value being replaced. NULL is returned on normal operation and on
178       error.
179
180       lh_<type>_delete() returns the entry being deleted.  NULL is returned
181       if there is no such value in the hash table.
182
183       lh_<type>_retrieve() returns the hash table entry if it has been found,
184       NULL otherwise.
185
186       lh_<type>_error() returns 1 if an error occurred in the last operation,
187       0 otherwise.
188
189       lh_<type>_free(), lh_<type>_doall() and lh_<type>_doall_arg() return no
190       values.
191

NOTE

193       The various LHASH macros and callback types exist to make it possible
194       to write type-checked code without resorting to function-prototype
195       casting - an evil that makes application code much harder to
196       audit/verify and also opens the window of opportunity for stack
197       corruption and other hard-to-find bugs.  It also, apparently, violates
198       ANSI-C.
199
200       The LHASH code regards table entries as constant data.  As such, it
201       internally represents lh_insert()'d items with a "const void *" pointer
202       type.  This is why callbacks such as those used by lh_doall() and
203       lh_doall_arg() declare their prototypes with "const", even for the
204       parameters that pass back the table items' data pointers - for
205       consistency, user-provided data is "const" at all times as far as the
206       LHASH code is concerned.  However, as callers are themselves providing
207       these pointers, they can choose whether they too should be treating all
208       such parameters as constant.
209
210       As an example, a hash table may be maintained by code that, for reasons
211       of encapsulation, has only "const" access to the data being indexed in
212       the hash table (ie. it is returned as "const" from elsewhere in their
213       code) - in this case the LHASH prototypes are appropriate as-is.
214       Conversely, if the caller is responsible for the life-time of the data
215       in question, then they may well wish to make modifications to table
216       item passed back in the lh_doall() or lh_doall_arg() callbacks (see the
217       "STUFF_cleanup" example above).  If so, the caller can either cast the
218       "const" away (if they're providing the raw callbacks themselves) or use
219       the macros to declare/implement the wrapper functions without "const"
220       types.
221
222       Callers that only have "const" access to data they're indexing in a
223       table, yet declare callbacks without constant types (or cast the
224       "const" away themselves), are therefore creating their own risks/bugs
225       without being encouraged to do so by the API.  On a related note, those
226       auditing code should pay special attention to any instances of
227       DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
228       without any "const" qualifiers.
229

BUGS

231       lh_<type>_insert() returns NULL both for success and error.
232

INTERNALS

234       The following description is based on the SSLeay documentation:
235
236       The lhash library implements a hash table described in the
237       Communications of the ACM in 1991.  What makes this hash table
238       different is that as the table fills, the hash table is increased (or
239       decreased) in size via OPENSSL_realloc().  When a 'resize' is done,
240       instead of all hashes being redistributed over twice as many 'buckets',
241       one bucket is split.  So when an 'expand' is done, there is only a
242       minimal cost to redistribute some values.  Subsequent inserts will
243       cause more single 'bucket' redistributions but there will never be a
244       sudden large cost due to redistributing all the 'buckets'.
245
246       The state for a particular hash table is kept in the LHASH structure.
247       The decision to increase or decrease the hash table size is made
248       depending on the 'load' of the hash table.  The load is the number of
249       items in the hash table divided by the size of the hash table.  The
250       default values are as follows.  If (hash->up_load < load) => expand.
251       if (hash->down_load > load) => contract.  The up_load has a default
252       value of 1 and down_load has a default value of 2.  These numbers can
253       be modified by the application by just playing with the up_load and
254       down_load variables.  The 'load' is kept in a form which is multiplied
255       by 256.  So hash->up_load=8*256; will cause a load of 8 to be set.
256
257       If you are interested in performance the field to watch is
258       num_comp_calls.  The hash library keeps track of the 'hash' value for
259       each item so when a lookup is done, the 'hashes' are compared, if there
260       is a match, then a full compare is done, and hash->num_comp_calls is
261       incremented.  If num_comp_calls is not equal to num_delete plus
262       num_retrieve it means that your hash function is generating hashes that
263       are the same for different values.  It is probably worth changing your
264       hash function if this is the case because even if your hash table has
265       10 items in a 'bucket', it can be searched with 10 unsigned long
266       compares and 10 linked list traverses.  This will be much less
267       expensive that 10 calls to your compare function.
268
269       lh_strhash() is a demo string hashing function:
270
271        unsigned long lh_strhash(const char *c);
272
273       Since the LHASH routines would normally be passed structures, this
274       routine would not normally be passed to lh_<type>_new(), rather it
275       would be used in the function passed to lh_<type>_new().
276

SEE ALSO

278       lh_stats(3)
279

HISTORY

281       The lhash library is available in all versions of SSLeay and OpenSSL.
282       lh_error() was added in SSLeay 0.9.1b.
283
284       This manpage is derived from the SSLeay documentation.
285
286       In OpenSSL 0.9.7, all lhash functions that were passed function
287       pointers were changed for better type safety, and the function types
288       LHASH_COMP_FN_TYPE, LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE and
289       LHASH_DOALL_ARG_FN_TYPE became available.
290
291       In OpenSSL 1.0.0, the lhash interface was revamped for even better type
292       checking.
293
294
295
2961.0.2o                            2020-01-28                          lhash(3)
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