lhash(3)                    OpenSSL                    lhash(3)





NAME
       lh_new, lh_free, lh_insert, lh_delete, lh_retrieve,
       lh_doall, lh_doall_arg, lh_error - dynamic hash table

SYNOPSIS
        #include <openssl/lhash.h>

        LHASH *lh_new(LHASH_HASH_FN_TYPE hash, LHASH_COMP_FN_TYPE compare);
        void lh_free(LHASH *table);

        void *lh_insert(LHASH *table, void *data);
        void *lh_delete(LHASH *table, void *data);
        void *lh_retrieve(LHASH *table, void *data);

        void lh_doall(LHASH *table, LHASH_DOALL_FN_TYPE func);
        void lh_doall_arg(LHASH *table, LHASH_DOALL_ARG_FN_TYPE func,
                 void *arg);

        int lh_error(LHASH *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 *);

DESCRIPTION
       This library implements dynamic hash tables. The hash
       table entries can be arbitrary structures. Usually they
       consist of key and value fields.

       lh_new() creates a new LHASH structure to store arbi-
       trary data entries, and provides the 'hash' and 'com-
       pare' callbacks to be used in organising the table's
       entries.  The 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 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 hash and compare call-
       backs hash/compare these types, then the
       DECLARE_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros
       can be used to create callback wrappers of the proto-
       types required by lh_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(f_name,o_type) \
                unsigned long f_name##_LHASH_HASH(const void *);
        #define IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \
                unsigned long f_name##_LHASH_HASH(const void *arg) { \
                        o_type a = (o_type)arg; \
                        return f_name(a); }
        #define LHASH_HASH_FN(f_name) f_name##_LHASH_HASH







        #define DECLARE_LHASH_COMP_FN(f_name,o_type) \
                int f_name##_LHASH_COMP(const void *, const void *);
        #define IMPLEMENT_LHASH_COMP_FN(f_name,o_type) \
                int f_name##_LHASH_COMP(const void *arg1, const void *arg2) { \
                        o_type a = (o_type)arg1; \
                        o_type b = (o_type)arg2; \
                        return f_name(a,b); }
        #define LHASH_COMP_FN(f_name) f_name##_LHASH_COMP

        #define DECLARE_LHASH_DOALL_FN(f_name,o_type) \
                void f_name##_LHASH_DOALL(const void *);
        #define IMPLEMENT_LHASH_DOALL_FN(f_name,o_type) \
                void f_name##_LHASH_DOALL(const void *arg) { \
                        o_type a = (o_type)arg; \
                        f_name(a); }
        #define LHASH_DOALL_FN(f_name) f_name##_LHASH_DOALL

        #define DECLARE_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \
                void f_name##_LHASH_DOALL_ARG(const void *, const void *);
        #define IMPLEMENT_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \
                void f_name##_LHASH_DOALL_ARG(const void *arg1, const void *arg2) { \
                        o_type a = (o_type)arg1; \
                        a_type b = (a_type)arg2; \
                        f_name(a,b); }
        #define LHASH_DOALL_ARG_FN(f_name) f_name##_LHASH_DOALL_ARG

       An example of a hash table storing (pointers to) struc-
       tures 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_hash, const STUFF *)
        static IMPLEMENT_LHASH_COMP_FN(STUFF_cmp, const STUFF *);
        /* ... */
        int main(int argc, char *argv[]) {
                /* Create the new hash table using the hash/compare wrappers */
                LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash),
                                          LHASH_COMP_FN(STUFF_cmp));
                /* ... */
        }

       lh_free() frees the LHASH structure table. Allocated
       hash table entries will not be freed; consider using
       lh_doall() to deallocate any remaining entries in the
       hash table (see below).

       lh_insert() inserts the structure pointed to by data
       into table.  If there already is an entry with the same
       key, the old value is replaced. Note that lh_insert()
       stores pointers, the data are not copied.

       lh_delete() deletes an entry from table.

       lh_retrieve() looks up an entry in table. Normally, data
       is a structure with the key field(s) set; the function
       will return a pointer to a fully populated structure.

       lh_doall() will, for every entry in the hash table, call
       func with the data item as its parameter.  For
       lh_doall() and lh_doall_arg(), function pointer casting
       should be avoided in the callbacks (see NOTE) - instead,
       either declare the callbacks to match the prototype
       required in lh_new() or use the declare/implement macros
       to create type-safe 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(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_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
        /* Then the hash table itself can be deallocated */
        lh_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->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_doall_arg() is the same as lh_doall() except that
       func will be called with arg as the second argument and
       func should be of type LHASH_DOALL_ARG_FN_TYPE (a call-
       back 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 match-
       ing 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 pro-
       vided by the caller):

        /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
        void STUFF_print(const STUFF *a, BIO *output_bio);
        /* Implement a prototype-compatible wrapper for "STUFF_print" */
        static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print, const STUFF *, BIO *)
                /* ... then later in the code ... */
        /* Print out the entire hashtable to a particular BIO */
        lh_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), logging_bio);

       lh_error() can be used to determine if an error occurred
       in the last operation. lh_error() is a macro.

RETURN VALUES
       lh_new() returns NULL on error, otherwise a pointer to
       the new LHASH structure.

       When a hash table entry is replaced, lh_insert() returns
       the value being replaced. NULL is returned on normal
       operation and on error.

       lh_delete() returns the entry being deleted.  NULL is
       returned if there is no such value in the hash table.

       lh_retrieve() returns the hash table entry if it has
       been found, NULL otherwise.

       lh_error() returns 1 if an error occurred in the last
       operation, 0 otherwise.

       lh_free(), lh_doall() and lh_doall_arg() return no val-
       ues.

NOTE
       The various LHASH macros and callback types exist to
       make it possible to write type-safe 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 corrup-
       tion 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 call-
       backs 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 con-
       cerned.  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 con-
       stant 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" quali-
       fiers.

BUGS
       lh_insert() returns NULL both for success and error.

INTERNALS
       The following description is based on the SSLeay docu-
       mentation:

       The lhash library implements a hash table described in
       the 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 'buck-
       ets', one bucket is split.  So when an 'expand' is done,
       there is only a minimal cost to redistribute some val-
       ues.  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
       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 <
       load) => expand.  if (hash->down_load > load) => con-
       tract.  The up_load has a default value of 1 and
       down_load has a default value of 2.  These numbers can
       be modified by the application by just playing with the
       up_load and down_load variables.  The 'load' is kept in
       a form which is multiplied by 256.  So
       hash->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->num_comp_calls is incre-
       mented.  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 val-
       ues.  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
       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 LHASH routines would normally be passed struc-
       tures, this routine would not normally be passed to
       lh_new(), rather it would be used in the function passed
       to lh_new().

SEE ALSO
       lh_stats(3)

HISTORY
       The 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.



0.9.7c                     2002-07-18                  lhash(3)
