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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)
       { 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
     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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