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OPENSSL_LH_COMPFUNC(3ossl)          OpenSSL         OPENSSL_LH_COMPFUNC(3ossl)



NAME

       LHASH, LHASH_OF, DEFINE_LHASH_OF_EX, DEFINE_LHASH_OF,
       OPENSSL_LH_COMPFUNC, OPENSSL_LH_HASHFUNC, OPENSSL_LH_DOALL_FUNC,
       LHASH_DOALL_ARG_FN_TYPE, IMPLEMENT_LHASH_HASH_FN,
       IMPLEMENT_LHASH_COMP_FN, lh_TYPE_new, lh_TYPE_free, lh_TYPE_flush,
       lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve, lh_TYPE_doall,
       lh_TYPE_doall_arg, lh_TYPE_num_items, lh_TYPE_get_down_load,
       lh_TYPE_set_down_load, lh_TYPE_error, OPENSSL_LH_new, OPENSSL_LH_free,
       OPENSSL_LH_flush, OPENSSL_LH_insert, OPENSSL_LH_delete,
       OPENSSL_LH_retrieve, OPENSSL_LH_doall, OPENSSL_LH_doall_arg,
       OPENSSL_LH_doall_arg_thunk, OPENSSL_LH_set_thunks,
       OPENSSL_LH_num_items, OPENSSL_LH_get_down_load,
       OPENSSL_LH_set_down_load, OPENSSL_LH_error - dynamic hash table


SYNOPSIS

        #include <openssl/lhash.h>

        LHASH_OF(TYPE)

        DEFINE_LHASH_OF_EX(TYPE);

        LHASH_OF(TYPE) *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
        void lh_TYPE_free(LHASH_OF(TYPE) *table);
        void lh_TYPE_flush(LHASH_OF(TYPE) *table);
        OPENSSL_LHASH *OPENSSL_LH_set_thunks(OPENSSL_LHASH *lh,
                                             OPENSSL_LH_HASHFUNCTHUNK hw,
                                             OPENSSL_LH_COMPFUNCTHUNK cw,
                                             OPENSSL_LH_DOALL_FUNC_THUNK daw,
                                             OPENSSL_LH_DOALL_FUNCARG_THUNK daaw)

        TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data);
        TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data);
        TYPE *lh_TYPE_retrieve(LHASH_OF(TYPE) *table, TYPE *data);

        void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
        void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
                               TYPE *arg);
        void OPENSSL_LH_doall_arg_thunk(OPENSSL_LHASH *lh,
                                        OPENSSL_LH_DOALL_FUNCARG_THUNK daaw,
                                        OPENSSL_LH_DOALL_FUNCARG fn, void *arg)

        unsigned long lh_TYPE_num_items(OPENSSL_LHASH *lh);
        unsigned long lh_TYPE_get_down_load(OPENSSL_LHASH *lh);
        void lh_TYPE_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);

        int lh_TYPE_error(LHASH_OF(TYPE) *table);

        typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
        typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
        typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
        typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);

        OPENSSL_LHASH *OPENSSL_LH_new(OPENSSL_LH_HASHFUNC h, OPENSSL_LH_COMPFUNC c);
        void OPENSSL_LH_free(OPENSSL_LHASH *lh);
        void OPENSSL_LH_flush(OPENSSL_LHASH *lh);

        void *OPENSSL_LH_insert(OPENSSL_LHASH *lh, void *data);
        void *OPENSSL_LH_delete(OPENSSL_LHASH *lh, const void *data);
        void *OPENSSL_LH_retrieve(OPENSSL_LHASH *lh, const void *data);

        void OPENSSL_LH_doall(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNC func);
        void OPENSSL_LH_doall_arg(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNCARG func, void *arg);

        unsigned long OPENSSL_LH_num_items(OPENSSL_LHASH *lh);
        unsigned long OPENSSL_LH_get_down_load(OPENSSL_LHASH *lh);
        void OPENSSL_LH_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);

        int OPENSSL_LH_error(OPENSSL_LHASH *lh);

        #define LH_LOAD_MULT   /* integer constant */

       The following macro is deprecated:

        DEFINE_LHASH_OF(TYPE);


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.  In the description here, TYPE is used a placeholder
       for any of the OpenSSL datatypes, such as SSL_SESSION.

       To define a new type-checked dynamic hash table, use
       DEFINE_LHASH_OF_EX().  DEFINE_LHASH_OF() was previously used for this
       purpose, but is now deprecated. The DEFINE_LHASH_OF_EX() macro provides
       all functionality of DEFINE_LHASH_OF() except for certain deprecated
       statistics functions (see OPENSSL_LH_stats(3)).

       lh_TYPE_new() creates a new LHASH_OF(TYPE) structure to store arbitrary
       data entries, and specifies 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, nonzero
       otherwise.

       If your hash table will contain items of some particular type and the
       hash and compare callbacks hash/compare these types, then the
       IMPLEMENT_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros can be used
       to create callback wrappers of the prototypes required by lh_TYPE_new()
       as shown in this example:

        /*
         * Implement the hash and compare functions; "stuff" can be any word.
         */
        static unsigned long stuff_hash(const TYPE *a)
        {
            ...
        }
        static int stuff_cmp(const TYPE *a, const TYPE *b)
        {
            ...
        }

        /*
         * Implement the wrapper functions.
         */
        static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
        static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)

       If the type is going to be used in several places, the following macros
       can be used in a common header file to declare the function wrappers:

        DECLARE_LHASH_HASH_FN(stuff, TYPE)
        DECLARE_LHASH_COMP_FN(stuff, TYPE)

       Then a hash table of TYPE objects can be created using this:

        LHASH_OF(TYPE) *htable;

        htable = B<lh_I<TYPE>_new>(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));

       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). If the
       argument is NULL, nothing is done.

       lh_TYPE_flush() empties the LHASH_OF(TYPE) structure table. New entries
       can be added to the flushed 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 example:

        /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
        void TYPE_cleanup_doall(TYPE *a);

        /* Implement a prototype-compatible wrapper for "TYPE_cleanup" */
        IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)

        /* Call "TYPE_cleanup" against all items in a hash table. */
        lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));

        /* Then the hash table itself can be deallocated */
        lh_TYPE_free(hashtable);

       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 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 TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);

        /* Implement a prototype-compatible wrapper for "TYPE_print" */
        static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)

        /* Print out the entire hashtable to a particular BIO */
        lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
                          logging_bio);

       Note that it is by default not safe to use lh_TYPE_delete() inside a
       callback passed to lh_TYPE_doall() or lh_TYPE_doall_arg(). The reason
       for this is that deleting an item from the hash table may result in the
       hash table being contracted to a smaller size and rehashed.
       lh_TYPE_doall() and lh_TYPE_doall_arg() are unsafe and will exhibit
       undefined behaviour under these conditions, as these functions assume
       the hash table size and bucket pointers do not change during the call.

       If it is desired to use lh_TYPE_doall() or lh_TYPE_doall_arg() with
       lh_TYPE_delete(), it is essential that you call lh_TYPE_set_down_load()
       with a down_load argument of 0 first. This disables hash table
       contraction and guarantees that it will be safe to delete items from a
       hash table during a call to lh_TYPE_doall() or lh_TYPE_doall_arg().

       It is never safe to call lh_TYPE_insert() during a call to
       lh_TYPE_doall() or lh_TYPE_doall_arg().

       lh_TYPE_error() can be used to determine if an error occurred in the
       last operation.

       lh_TYPE_num_items() returns the number of items in the hash table.

       lh_TYPE_get_down_load() and lh_TYPE_set_down_load() get and set the
       factor used to determine when the hash table is contracted. The factor
       is the load factor at or below which hash table contraction will occur,
       multiplied by LH_LOAD_MULT, where the load factor is the number of
       items divided by the number of nodes. Setting this value to 0 disables
       hash table contraction.

       OPENSSL_LH_new() is the same as the lh_TYPE_new() except that it is not
       type specific. So instead of returning an LHASH_OF(TYPE) value it
       returns a void *. In the same way the functions OPENSSL_LH_free(),
       OPENSSL_LH_flush(), OPENSSL_LH_insert(), OPENSSL_LH_delete(),
       OPENSSL_LH_retrieve(), OPENSSL_LH_doall(), OPENSSL_LH_doall_arg(),
       OPENSSL_LH_num_items(), OPENSSL_LH_get_down_load(),
       OPENSSL_LH_set_down_load() and OPENSSL_LH_error() are equivalent to the
       similarly named lh_TYPE functions except that they return or use a void
       * where the equivalent lh_TYPE function returns or uses a TYPE * or
       LHASH_OF(TYPE) *.  lh_TYPE functions are implemented as type checked
       wrappers around the OPENSSL_LH functions. Most applications should not
       call the OPENSSL_LH functions directly.

       OPENSSL_LH_set_thunks() and OPENSSL_LH_doall_arg_thunk(), while public
       by necessity, are actually internal functions and should not be used.


RETURN VALUES

       lh_TYPE_new() and OPENSSL_LH_new() return NULL on error, otherwise a
       pointer to the new LHASH structure.

       When a hash table entry is replaced, lh_TYPE_insert() or
       OPENSSL_LH_insert() return the value being replaced. NULL is returned
       on normal operation and on error.

       lh_TYPE_delete() and OPENSSL_LH_delete() return the entry being
       deleted.  NULL is returned if there is no such value in the hash table.

       lh_TYPE_retrieve() and OPENSSL_LH_retrieve() return the hash table
       entry if it has been found, NULL otherwise.

       lh_TYPE_error() and OPENSSL_LH_error() return 1 if an error occurred in
       the last operation, 0 otherwise. It's meaningful only after
       non-retrieve operations.

       lh_TYPE_free(), OPENSSL_LH_free(), lh_TYPE_flush(), OPENSSL_LH_flush(),
       lh_TYPE_doall() OPENSSL_LH_doall(), lh_TYPE_doall_arg() and
       OPENSSL_LH_doall_arg() return no values.


NOTE

       The LHASH code is not thread safe. All updating operations, as well as
       lh_TYPE_error() or OPENSSL_LH_error() calls must be performed under a
       write lock. All retrieve operations should be performed under a read
       lock, unless accurate usage statistics are desired. In which case, a
       write lock should be used for retrieve operations as well. For output
       of the usage statistics, using the functions from OPENSSL_LH_stats(3),
       a read lock suffices.

       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, 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 (i.e. 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
       "TYPE_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() and OPENSSL_LH_insert() return NULL both for success
       and error.


SEE ALSO

       OPENSSL_LH_stats(3)


HISTORY

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

       In OpenSSL 3.1, DEFINE_LHASH_OF_EX() was introduced and
       DEFINE_LHASH_OF() was deprecated.


COPYRIGHT

       Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at
       <https://www.openssl.org/source/license.html>.

3.3.2                             2024-09-04        OPENSSL_LH_COMPFUNC(3ossl)

openssl 3.3.2 - Generated Sun Sep 8 15:22:09 CDT 2024
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