--- /dev/null
+=pod
+
+=head1 NAME
+
+lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error - dynamic hash table
+
+=head1 SYNOPSIS
+
+ #include <openssl/lhash.h>
+
+ DECLARE_LHASH_OF(<type>);
+
+ LHASH *lh_<type>_new();
+ void lh_<type>_free(LHASH_OF(<type> *table);
+
+ <type> *lh_<type>_insert(LHASH_OF(<type> *table, <type> *data);
+ <type> *lh_<type>_delete(LHASH_OF(<type> *table, <type> *data);
+ <type> *lh_retrieve(LHASH_OF<type> *table, <type> *data);
+
+ void lh_<type>_doall(LHASH_OF(<type> *table, LHASH_DOALL_FN_TYPE func);
+ void lh_<type>_doall_arg(LHASH_OF(<type> *table, LHASH_DOALL_ARG_FN_TYPE func,
+ <type2>, <type2> *arg);
+
+ int lh_<type>_error(LHASH_OF(<type> *table);
+
+ typedef int (*LHASH_COMP_FN_TYPE)(const void *, const void *);
+ typedef unsigned long (*LHASH_HASH_FN_TYPE)(const void *);
+ typedef void (*LHASH_DOALL_FN_TYPE)(const void *);
+ typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
+
+=head1 DESCRIPTION
+
+This library implements type-checked dynamic hash tables. The hash
+table entries can be arbitrary structures. Usually they consist of key
+and value fields.
+
+lh_<type>_new() creates a new B<LHASH_OF(<type>> structure to store
+arbitrary data entries, and provides the 'hash' and 'compare'
+callbacks to be used in organising the table's entries. The B<hash>
+callback takes a pointer to a table entry as its argument and returns
+an unsigned long hash value for its key field. The hash value is
+normally truncated to a power of 2, so make sure that your hash
+function returns well mixed low order bits. The B<compare> callback
+takes two arguments (pointers to two hash table entries), and returns
+0 if their keys are equal, non-zero otherwise. If your hash table
+will contain items of some particular type and the B<hash> and
+B<compare> callbacks hash/compare these types, then the
+B<DECLARE_LHASH_HASH_FN> and B<IMPLEMENT_LHASH_COMP_FN> macros can be
+used to create callback wrappers of the prototypes required by
+lh_<type>_new(). These provide per-variable casts before calling the
+type-specific callbacks written by the application author. These
+macros, as well as those used for the "doall" callbacks, are defined
+as;
+
+ #define DECLARE_LHASH_HASH_FN(name, o_type) \
+ unsigned long name##_LHASH_HASH(const void *);
+ #define IMPLEMENT_LHASH_HASH_FN(name, o_type) \
+ unsigned long name##_LHASH_HASH(const void *arg) { \
+ const o_type *a = arg; \
+ return name##_hash(a); }
+ #define LHASH_HASH_FN(name) name##_LHASH_HASH
+
+ #define DECLARE_LHASH_COMP_FN(name, o_type) \
+ int name##_LHASH_COMP(const void *, const void *);
+ #define IMPLEMENT_LHASH_COMP_FN(name, o_type) \
+ int name##_LHASH_COMP(const void *arg1, const void *arg2) { \
+ const o_type *a = arg1; \
+ const o_type *b = arg2; \
+ return name##_cmp(a,b); }
+ #define LHASH_COMP_FN(name) name##_LHASH_COMP
+
+ #define DECLARE_LHASH_DOALL_FN(name, o_type) \
+ void name##_LHASH_DOALL(void *);
+ #define IMPLEMENT_LHASH_DOALL_FN(name, o_type) \
+ void name##_LHASH_DOALL(void *arg) { \
+ o_type *a = arg; \
+ name##_doall(a); }
+ #define LHASH_DOALL_FN(name) name##_LHASH_DOALL
+
+ #define DECLARE_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
+ void name##_LHASH_DOALL_ARG(void *, void *);
+ #define IMPLEMENT_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
+ void name##_LHASH_DOALL_ARG(void *arg1, void *arg2) { \
+ o_type *a = arg1; \
+ a_type *b = arg2; \
+ name##_doall_arg(a, b); }
+ #define LHASH_DOALL_ARG_FN(name) name##_LHASH_DOALL_ARG
+
+ An example of a hash table storing (pointers to) structures of type 'STUFF'
+ could be defined as follows;
+
+ /* Calculates the hash value of 'tohash' (implemented elsewhere) */
+ unsigned long STUFF_hash(const STUFF *tohash);
+ /* Orders 'arg1' and 'arg2' (implemented elsewhere) */
+ int stuff_cmp(const STUFF *arg1, const STUFF *arg2);
+ /* Create the type-safe wrapper functions for use in the LHASH internals */
+ static IMPLEMENT_LHASH_HASH_FN(stuff, STUFF);
+ static IMPLEMENT_LHASH_COMP_FN(stuff, STUFF);
+ /* ... */
+ int main(int argc, char *argv[]) {
+ /* Create the new hash table using the hash/compare wrappers */
+ LHASH_OF(STUFF) *hashtable = lh_STUFF_new(LHASH_HASH_FN(STUFF_hash),
+ LHASH_COMP_FN(STUFF_cmp));
+ /* ... */
+ }
+
+lh_<type>_free() frees the B<LHASH_OF(<type>> structure
+B<table>. Allocated hash table entries will not be freed; consider
+using lh_<type>_doall() to deallocate any remaining entries in the
+hash table (see below).
+
+lh_<type>_insert() inserts the structure pointed to by B<data> into
+B<table>. If there already is an entry with the same key, the old
+value is replaced. Note that lh_<type>_insert() stores pointers, the
+data are not copied.
+
+lh_<type>_delete() deletes an entry from B<table>.
+
+lh_<type>_retrieve() looks up an entry in B<table>. Normally, B<data>
+is a structure with the key field(s) set; the function will return a
+pointer to a fully populated structure.
+
+lh_<type>_doall() will, for every entry in the hash table, call
+B<func> with the data item as its parameter. For lh_<type>_doall()
+and lh_<type>_doall_arg(), function pointer casting should be avoided
+in the callbacks (see B<NOTE>) - instead use the declare/implement
+macros to create type-checked wrappers that cast variables prior to
+calling your type-specific callbacks. An example of this is
+illustrated here where the callback is used to cleanup resources for
+items in the hash table prior to the hashtable itself being
+deallocated:
+
+ /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
+ void STUFF_cleanup_doall(STUFF *a);
+ /* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
+ IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF)
+ /* ... then later in the code ... */
+ /* So to run "STUFF_cleanup" against all items in a hash table ... */
+ lh_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
+ /* Then the hash table itself can be deallocated */
+ lh_STUFF_free(hashtable);
+
+When doing this, be careful if you delete entries from the hash table
+in your callbacks: the table may decrease in size, moving the item
+that you are currently on down lower in the hash table - this could
+cause some entries to be skipped during the iteration. The second
+best solution to this problem is to set hash-E<gt>down_load=0 before
+you start (which will stop the hash table ever decreasing in size).
+The best solution is probably to avoid deleting items from the hash
+table inside a "doall" callback!
+
+lh_<type>_doall_arg() is the same as lh_<type>_doall() except that
+B<func> will be called with B<arg> as the second argument and B<func>
+should be of type B<LHASH_DOALL_ARG_FN_TYPE> (a callback prototype
+that is passed both the table entry and an extra argument). As with
+lh_doall(), you can instead choose to declare your callback with a
+prototype matching the types you are dealing with and use the
+declare/implement macros to create compatible wrappers that cast
+variables before calling your type-specific callbacks. An example of
+this is demonstrated here (printing all hash table entries to a BIO
+that is provided by the caller):
+
+ /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
+ void STUFF_print_doall_arg(const STUFF *a, BIO *output_bio);
+ /* Implement a prototype-compatible wrapper for "STUFF_print" */
+ static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF, const STUFF, BIO)
+ /* ... then later in the code ... */
+ /* Print out the entire hashtable to a particular BIO */
+ lh_STUFF_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), BIO,
+ logging_bio);
+
+lh_<type>_error() can be used to determine if an error occurred in the last
+operation. lh_<type>_error() is a macro.
+
+=head1 RETURN VALUES
+
+lh_<type>_new() returns B<NULL> on error, otherwise a pointer to the new
+B<LHASH> structure.
+
+When a hash table entry is replaced, lh_<type>_insert() returns the value
+being replaced. B<NULL> is returned on normal operation and on error.
+
+lh_<type>_delete() returns the entry being deleted. B<NULL> is returned if
+there is no such value in the hash table.
+
+lh_<type>_retrieve() returns the hash table entry if it has been found,
+B<NULL> otherwise.
+
+lh_<type>_error() returns 1 if an error occurred in the last operation, 0
+otherwise.
+
+lh_<type>_free(), lh_<type>_doall() and lh_<type>_doall_arg() return no values.
+
+=head1 NOTE
+
+The various LHASH macros and callback types exist to make it possible
+to write type-checked code without resorting to function-prototype
+casting - an evil that makes application code much harder to
+audit/verify and also opens the window of opportunity for stack
+corruption and other hard-to-find bugs. It also, apparently, violates
+ANSI-C.
+
+The LHASH code regards table entries as constant data. As such, it
+internally represents lh_insert()'d items with a "const void *"
+pointer type. This is why callbacks such as those used by lh_doall()
+and lh_doall_arg() declare their prototypes with "const", even for the
+parameters that pass back the table items' data pointers - for
+consistency, user-provided data is "const" at all times as far as the
+LHASH code is concerned. However, as callers are themselves providing
+these pointers, they can choose whether they too should be treating
+all such parameters as constant.
+
+As an example, a hash table may be maintained by code that, for
+reasons of encapsulation, has only "const" access to the data being
+indexed in the hash table (ie. it is returned as "const" from
+elsewhere in their code) - in this case the LHASH prototypes are
+appropriate as-is. Conversely, if the caller is responsible for the
+life-time of the data in question, then they may well wish to make
+modifications to table item passed back in the lh_doall() or
+lh_doall_arg() callbacks (see the "STUFF_cleanup" example above). If
+so, the caller can either cast the "const" away (if they're providing
+the raw callbacks themselves) or use the macros to declare/implement
+the wrapper functions without "const" types.
+
+Callers that only have "const" access to data they're indexing in a
+table, yet declare callbacks without constant types (or cast the
+"const" away themselves), are therefore creating their own risks/bugs
+without being encouraged to do so by the API. On a related note,
+those auditing code should pay special attention to any instances of
+DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
+without any "const" qualifiers.
+
+=head1 BUGS
+
+lh_<type>_insert() returns B<NULL> both for success and error.
+
+=head1 INTERNALS
+
+The following description is based on the SSLeay documentation:
+
+The B<lhash> library implements a hash table described in the
+I<Communications of the ACM> in 1991. What makes this hash table
+different is that as the table fills, the hash table is increased (or
+decreased) in size via OPENSSL_realloc(). When a 'resize' is done, instead of
+all hashes being redistributed over twice as many 'buckets', one
+bucket is split. So when an 'expand' is done, there is only a minimal
+cost to redistribute some values. Subsequent inserts will cause more
+single 'bucket' redistributions but there will never be a sudden large
+cost due to redistributing all the 'buckets'.
+
+The state for a particular hash table is kept in the B<LHASH> structure.
+The decision to increase or decrease the hash table size is made
+depending on the 'load' of the hash table. The load is the number of
+items in the hash table divided by the size of the hash table. The
+default values are as follows. If (hash->up_load E<lt> load) =E<gt>
+expand. if (hash-E<gt>down_load E<gt> load) =E<gt> contract. The
+B<up_load> has a default value of 1 and B<down_load> has a default value
+of 2. These numbers can be modified by the application by just
+playing with the B<up_load> and B<down_load> variables. The 'load' is
+kept in a form which is multiplied by 256. So
+hash-E<gt>up_load=8*256; will cause a load of 8 to be set.
+
+If you are interested in performance the field to watch is
+num_comp_calls. The hash library keeps track of the 'hash' value for
+each item so when a lookup is done, the 'hashes' are compared, if
+there is a match, then a full compare is done, and
+hash-E<gt>num_comp_calls is incremented. If num_comp_calls is not equal
+to num_delete plus num_retrieve it means that your hash function is
+generating hashes that are the same for different values. It is
+probably worth changing your hash function if this is the case because
+even if your hash table has 10 items in a 'bucket', it can be searched
+with 10 B<unsigned long> compares and 10 linked list traverses. This
+will be much less expensive that 10 calls to your compare function.
+
+lh_strhash() is a demo string hashing function:
+
+ unsigned long lh_strhash(const char *c);
+
+Since the B<LHASH> routines would normally be passed structures, this
+routine would not normally be passed to lh_<type>_new(), rather it would be
+used in the function passed to lh_<type>_new().
+
+=head1 SEE ALSO
+
+L<lh_stats(3)|lh_stats(3)>
+
+=head1 HISTORY
+
+The B<lhash> library is available in all versions of SSLeay and OpenSSL.
+lh_error() was added in SSLeay 0.9.1b.
+
+This manpage is derived from the SSLeay documentation.
+
+In OpenSSL 0.9.7, all lhash functions that were passed function pointers
+were changed for better type safety, and the function types LHASH_COMP_FN_TYPE,
+LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE and LHASH_DOALL_ARG_FN_TYPE
+became available.
+
+In OpenSSL 1.0.0, the lhash interface was revamped for even better
+type checking.
+
+=cut
projects / cassiopeia.git / blobdiff