Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
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 arbitrary data entries,
       and provides the 'hash' and 'compare' callbacks to be used in organis-
       ing 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 types, then the
       DECLARE_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros can be used to
       create callback wrappers of the prototypes 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) 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_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 remain-
       ing 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(), func-
       tion 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 call-
       backs.  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 solu-
       tion 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 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 cre-
       ate 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(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 oth-
       erwise.

       lh_free(), lh_doall() and lh_doall_arg() return no values.

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 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 consis-
       tency, 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.  Con-
       versely, 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 ta-
       ble, 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 audit-
       ing code should pay special attention to any instances of
       DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types with-
       out any "const" qualifiers.

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

INTERNALS
       The following description is based on the SSLeay documentation:

       The lhash library implements a hash table described in the Communica-
       tions 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 redis-
       tribute 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 com-
       pares 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 rou-
       tine 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 point-
       ers 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.

0.9.7d                            2005-02-24                          lhash(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | NOTE | BUGS | INTERNALS | SEE ALSO | HISTORY

Want to link to this manual page? Use this URL:
<http://www.freebsd.org/cgi/man.cgi?query=lhash&sektion=3&manpath=FreeBSD+6.2-RELEASE>

home | help