Updated: 2021/Apr/14

OPENSSL_LH_COMPFUNC(3)              OpenSSL             OPENSSL_LH_COMPFUNC(3)

       lh_TYPE_free, lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve,
       lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_error - dynamic hash table

       libcrypto, -lcrypto

        #include <openssl/lhash.h>


        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, OPENSSL_LH_DOALL_FUNC func);
        void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
                               TYPE *arg);

        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 *);

       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.

       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

       If your hash table will contain items of some particular type and the
       hash and compare callbacks hash/compare these types, then the
       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:


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

        LHASH_OF(TYPE) *htable;

        htable = lh_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).

       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

       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" */

        /* 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 */

       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 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

        /* 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" */

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

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

       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. It's meaningful only after non-retrieve operations.

       lh_TYPE_free(), lh_TYPE_doall() and lh_TYPE_doall_arg() return no

       The LHASH code is not thread safe. All updating operations, as well as
       lh_TYPE_error call 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"

       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.

       lh_TYPE_insert() returns NULL both for success and error.


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

       Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (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

1.1.1i                            2020-12-10            OPENSSL_LH_COMPFUNC(3)