lhash(3)

     lhash(3)               0.9.6h (2000-09-19)               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(unsigned long (*hash)(/*void *a*/),
                    int (*compare)(/*void *a,void *b*/));
           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, void (*func)(/*void *b*/));
           void lh_doall_arg(LHASH *table, void (*func)(/*void *a,void *b*/),
                    void *arg);

           int lh_error(LHASH *table);

     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. hash takes a pointer
          to the structure and returns an unsigned long hash value of
          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. compare takes two arguments, and
          returns 0 if their keys are equal, non-zero otherwise.

          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.

          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 parameters.  This function can be
          quite useful when used as follows:
           void cleanup(STUFF *a)

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            { STUFF_free(a); }
           lh_doall(hash,cleanup);
           lh_free(hash); This can be used to free all the entries.
          lh_free() then cleans up the 'buckets' that point to
          nothing. When doing this, be careful if you delete entries
          from the hash table in func: the table may decrease in size,
          moving item that you are currently on down lower in the hash
          table.  This could cause some entries to be skipped.  The
          best solution to this problem is to set hash->down_load=0
          before you start.  This will stop the hash table ever being
          decreased in size.

          lh_doall_arg() is the same as lh_doall() except that func
          will be called with arg as the second argument.

          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 values.

     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
          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

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          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 you 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
          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.

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