MirOS Manual: lhash(3), lh_delete(3), lh_doall(3), lh_doall_arg(3), lh_error(3), lh_free(3), lh_insert(3), lh_new(3), lh_retrieve(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>

      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' call-
     backs to be used in organising the table's entries.  The
     hash callback takes a pointer to a table entry as its argu-
     ment 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 com-
     pare 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;

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

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     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 proto-
     type that is passed both the table entry and an extra

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LHASH(3)                     OpenSSL                     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 cal-
     ling 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 opera-
     tion, 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
     back the table items' data pointers - for consistency,

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LHASH(3)                     OpenSSL                     LHASH(3)

     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 ques-
     tion, then they may well wish to make modifications to table
     item passed back in the lh_doall() or lh_doall_arg() call-
     backs (see the "STUFF_cleanup" example above).  If so, the
     caller can either cast the "const" away (if they're provid-
     ing 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 index-
     ing in a table, yet declare callbacks without constant types
     (or cast the "const" away themselves), are therefore creat-
     ing 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 documenta-
     tion:

     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 redis-
     tributed 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

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LHASH(3)                     OpenSSL                     LHASH(3)

     (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 struc-
     tures, 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 func-
     tion 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.

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