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    Command:

    lhash

    
    
    
    

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

    1.0.1e 2013-02-11 lhash(3)

    
    
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