lhash, lh_new, lh_free, lh_insert, lh_delete, lh_retrieve,
lh_doall, lh_doall_arg, lh_error - Dynamic hash table
#include <openssl/lhash.h>
LHASH *lh_new(
unsigned long (*hash)(/*void *a*/), int (*compare)(/*void
*a,void *b*/) ); 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, void (*func)(/*void *b*/) ); void
lh_doall_arg(
LHASH *table, void (*func)(/*void *a,void *b*/),
void *arg ); int lh_error(
LHASH *table );
This library implements dynamic hash tables. The hash
table entries can be arbitrary structures. Usually they
consist of key and value fields.
The lh_new() function creates a new LHASH structure. The
hash takes a pointer to the structure and returns an
unsigned long hash value of 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 takes two arguments, and returns 0 if their keys
are equal, non-zero otherwise.
The lh_free() function frees the LHASH structure table.
Allocated hash table entries will not be freed; consider
using the lh_doall() function to deallocate any remaining
entries in the hash table.
The lh_insert() function inserts the structure pointed to
by data into table. If there already is an entry with the
same key, the old value is replaced. The lh_insert() function
stores pointers; the data are not copied.
The lh_delete() function deletes an entry from table.
The lh_retrieve() function looks up an entry in table.
Normally, data is a structure with the key field set; the
function will return a pointer to a fully populated structure.
The lh_doall() function will, for every entry in the hash
table, call func with the data item as parameters. This
function can be quite useful when used as follows: void
cleanup(STUFF *a) { STUFF_free(a); }
lh_doall(hash,cleanup); lh_free(hash). This can be used
to free all the entries. The lh_free() function then
cleans up the 'buckets that point to nothing. When doing
this, be careful if you delete entries from the hash table
in func. The table might decrease in size, moving items
lower in the hash table. This could cause some entries to
be skipped. The best solution to this problem is to set
hash->down_load=0 before you start. This will stop the
hash table from decreasing in size.
The lh_doall_arg() function is the same as lh_doall()
except that func will be called with arg as the second
argument.
The lh_error() macro can be used to determine if an error
occurred in the last operation.
Internals [Toc] [Back]
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 the OPENSSL_realloc()
function. 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 adjusting 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.
The 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_new(), rather it would be used in the function passed
to the lh_new() function.
The lh_insert() function returns NULL both for success and
error.
The lh_new() function returns NULL on error, otherwise a
pointer to the new LHASH structure.
When a hash table entry is replaced, the lh_insert() function
returns the value being replaced. NULL is returned
on normal operation and on error.
The lh_delete() function returns the entry being deleted.
NULL is returned if there is no such value in the hash
table.
The lh_retrieve() function returns the hash table entry if
it has been found, NULL otherwise.
The lh_error() function returns 1 if an error occurred in
the last operation, 0 otherwise.
The lh_free(), lh_doall(), and lh_doall_arg() functions
return no values.
The lhash library is available in all versions of SSLeay
and OpenSSL. The lh_error() function was added in SSLeay
0.9.1b. This reference page is derived from the SSLeay
documentation.
Functions: lh_stats(3)
lhash(3)
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