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lhash(3)                            OpenSSL                           lhash(3)

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

        #include <openssl/lhash.h>


        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 *);

       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 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 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 callbacks hash/compare these
       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),
                /* ... */

       lh_<type>_free() frees the LHASH_OF(<type> structure 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 data into 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 table.

       lh_<type>_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_<type>_doall() will, for every entry in the hash table, call 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 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" */
                /* ... 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 */

       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_<type>_doall_arg() is the same as lh_<type>_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 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

        /* 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" */
                /* ... 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,

       lh_<type>_error() can be used to determine if an error occurred in the
       last operation. lh_<type>_error() is a macro.

       lh_<type>_new() returns NULL on error, otherwise a pointer to the new
       LHASH structure.

       When a hash table entry is replaced, lh_<type>_insert() returns the
       value being replaced. NULL is returned on normal operation and on

       lh_<type>_delete() returns the entry being deleted.  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,
       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

       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

       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"

       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.

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

       The following description is based on the SSLeay documentation:

       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 '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 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) => contract.  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
       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 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 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().


       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_DOALL_ARG_FN_TYPE became available.

       In OpenSSL 1.0.0, the lhash interface was revamped for even better type

1.0.1e                            2013-02-11                          lhash(3)


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