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     /xlv3/openssl/0.9.7e-sgipl1/work/0.9.7e-sgipl1/openssl-
     0.9.7e/doc/crypto

     Page 1					    (printed 10/20/05)

     lhash(3)		   18/Jul/2002 (0.9.7e)		      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 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 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;

     Page 1					    (printed 10/20/05)

     lhash(3)		   18/Jul/2002 (0.9.7e)		      lhash(3)

	   #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));
		   /* ... */
	   }

     Page 2					    (printed 10/20/05)

     lhash(3)		   18/Jul/2002 (0.9.7e)		      lhash(3)

	  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
	  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(), 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" */
	   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 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

     Page 3					    (printed 10/20/05)

     lhash(3)		   18/Jul/2002 (0.9.7e)		      lhash(3)

	  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 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 otherwise.

	  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

     Page 4					    (printed 10/20/05)

     lhash(3)		   18/Jul/2002 (0.9.7e)		      lhash(3)

	  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"
	  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 instances of
	  DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide
	  types without 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
	  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

     Page 5					    (printed 10/20/05)

     lhash(3)		   18/Jul/2002 (0.9.7e)		      lhash(3)

	  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
	  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 pointers 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.

     Page 6					    (printed 10/20/05)

     lhash(3)		   18/Jul/2002 (0.9.7e)		      lhash(3)

     Page 7					    (printed 10/20/05)

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