*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->OpenBSD man pages -> lhash (3)              



NAME    [Toc]    [Back]

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

SYNOPSIS    [Toc]    [Back]

        #include <openssl/lhash.h>

        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
                 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    [Toc]    [Back]

       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 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
       types, then the DECLARE_LHASH_HASH_FN and IMPLE-
       MENT_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)

       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_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

       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" */
                /* ... 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 */

       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 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(const STUFF *a, BIO *output_bio);
        /*   Implement   a   prototype-compatible   wrapper   for
"STUFF_print" */
        static  IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print,   const
                /* ... then later in the code ... */
        /* Print out the entire hashtable to a particular BIO */
        lh_doall_arg(hashtable,  LHASH_DOALL_ARG_FN(STUFF_print),

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

RETURN VALUES    [Toc]    [Back]

       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 values.

NOTE    [Toc]    [Back]

       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 hardto-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
       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
       macros that provide types without any "const"  qualifiers.

BUGS    [Toc]    [Back]

       lh_insert() returns NULL both for success and error.

INTERNALS    [Toc]    [Back]

       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_new(), rather it would be used in the function passed
       to lh_new().

SEE ALSO    [Toc]    [Back]


HISTORY    [Toc]    [Back]

       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,

OpenBSD 3.6                 2002-09-10                          6
[ Back ]
 Similar pages
Name OS Title
hsearch Linux hash table management
hcreate NetBSD manage hash search table
hcreate FreeBSD manage hash search table
hsearch FreeBSD manage hash search table
hdestroy OpenBSD manage hash search table
hdestroy FreeBSD manage hash search table
hdestroy NetBSD manage hash search table
hsearch NetBSD manage hash search table
hsearch OpenBSD manage hash search table
hcreate OpenBSD manage hash search table
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service