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lhash(3)




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>

      DECLARE_LHASH_OF(<type>);

      LHASH *lh_<type>_new();
      void lh_<type>_free(LHASH_OF(<type> *table);

      <type> *lh_<type>_insert(LHASH_OF(<type> *table, <type> *data);
      <type> *lh_<type>_delete(LHASH_OF(<type> *table, <type> *data);
      <type> *lh_retrieve(LHASH_OF<type> *table, <type> *data);

      void lh_<type>_doall(LHASH_OF(<type> *table, LHASH_DOALL_FN_TYPE func);
      void lh_<type>_doall_arg(LHASH_OF(<type> *table, LHASH_DOALL_ARG_FN_TYPE func,
               <type2>, <type2> *arg);

      int lh_<type>_error(LHASH_OF(<type> *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 type-checked dynamic hash tables.
     The hash table entries can be arbitrary structures. Usually
     they consist of key and value fields.

     lh_<type>_new() creates a new LHASH_OF(<type> 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_<type>_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;

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lhash(3)                     OpenSSL                     lhash(3)

      #define DECLARE_LHASH_HASH_FN(name, o_type) \
              unsigned long name##_LHASH_HASH(const void *);
      #define IMPLEMENT_LHASH_HASH_FN(name, o_type) \
              unsigned long name##_LHASH_HASH(const void *arg) { \
                      const o_type *a = arg; \
                      return name##_hash(a); }
      #define LHASH_HASH_FN(name) name##_LHASH_HASH

      #define DECLARE_LHASH_COMP_FN(name, o_type) \
              int name##_LHASH_COMP(const void *, const void *);
      #define IMPLEMENT_LHASH_COMP_FN(name, o_type) \
              int name##_LHASH_COMP(const void *arg1, const void *arg2) { \
                      const o_type *a = arg1;                    \
                      const o_type *b = arg2; \
                      return name##_cmp(a,b); }
      #define LHASH_COMP_FN(name) name##_LHASH_COMP

      #define DECLARE_LHASH_DOALL_FN(name, o_type) \
              void name##_LHASH_DOALL(void *);
      #define IMPLEMENT_LHASH_DOALL_FN(name, o_type) \
              void name##_LHASH_DOALL(void *arg) { \
                      o_type *a = arg; \
                      name##_doall(a); }
      #define LHASH_DOALL_FN(name) name##_LHASH_DOALL

      #define DECLARE_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
              void name##_LHASH_DOALL_ARG(void *, void *);
      #define IMPLEMENT_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
              void name##_LHASH_DOALL_ARG(void *arg1, void *arg2) { \
                      o_type *a = arg1; \
                      a_type *b = arg2; \
                      name##_doall_arg(a, b); }
      #define LHASH_DOALL_ARG_FN(name) 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, STUFF);
      static IMPLEMENT_LHASH_COMP_FN(stuff, STUFF);
      /* ... */
      int main(int argc, char *argv[]) {
              /* Create the new hash table using the hash/compare wrappers */
              LHASH_OF(STUFF) *hashtable = lh_STUFF_new(LHASH_HASH_FN(STUFF_hash),
                                        LHASH_COMP_FN(STUFF_cmp));
              /* ... */
      }

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lhash(3)                     OpenSSL                     lhash(3)

     lh_<type>_free() frees the LHASH_OF(<type> structure table.
     Allocated hash table entries will not be freed; consider
     using lh_<type>_doall() to deallocate any remaining entries
     in the hash table (see below).

     lh_<type>_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_<type>_insert()
     stores pointers, the data are not copied.

     lh_<type>_delete() deletes an entry from table.

     lh_<type>_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_<type>_doall() will, for every entry in the hash table,
     call func with the data item as its parameter.  For
     lh_<type>_doall() and lh_<type>_doall_arg(), function
     pointer casting should be avoided in the callbacks (see
     NOTE) - instead use the declare/implement macros to create
     type-checked 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_doall(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_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
      /* Then the hash table itself can be deallocated */
      lh_STUFF_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_<type>_doall_arg() is the same as lh_<type>_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

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lhash(3)                     OpenSSL                     lhash(3)

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

     lh_<type>_error() can be used to determine if an error
     occurred in the last operation. lh_<type>_error() is a
     macro.


RETURN VALUES

     lh_<type>_new() returns NULL on error, otherwise a pointer
     to the new LHASH structure.

     When a hash table entry is replaced, lh_<type>_insert()
     returns the value being replaced. NULL is returned on normal
     operation and on error.

     lh_<type>_delete() returns the entry being deleted.  NULL is
     returned if there is no such value in the hash table.

     lh_<type>_retrieve() returns the hash table entry if it has
     been found, NULL otherwise.

     lh_<type>_error() returns 1 if an error occurred in the last
     operation, 0 otherwise.

     lh_<type>_free(), lh_<type>_doall() and
     lh_<type>_doall_arg() return no values.


NOTE

     The various LHASH macros and callback types exist to make it
     possible to write type-checked 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

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lhash(3)                     OpenSSL                     lhash(3)

     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 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_<type>_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

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lhash(3)                     OpenSSL                     lhash(3)

     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_<type>_new(), rather it would be used in the function
     passed to lh_<type>_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.

     In OpenSSL 1.0.0, the lhash interface was revamped for even
     better type checking.

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