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(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);
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. 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. compare takes
two arguments, and returns 0 if their keys are equal, non-
zero otherwise.
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.
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 parameters. This function can
be quite useful when used as follows:
void cleanup(STUFF *a)
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{ STUFF_free(a); }
lh_doall(hash,cleanup);
lh_free(hash); This can be used to free all the entries.
lh_free() 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 may decrease in
size, moving item that you are currently on down 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 ever being decreased in size.
lh_doall_arg() is the same as lh_doall() except that func
will be called with arg as the second argument.
lh_error() can be used to determine if an error occurred
in the last operation. lh_error() is a macro.
RETURN VALUESlh_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
operation, 0 otherwise.
lh_free(), lh_doall() and lh_doall_arg() return no values.
BUGSlh_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
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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
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 you 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
structures, this routine would not normally be passed to
lh_new(), rather it would be used in the function passed
to lh_new().
SEE ALSOlh_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.
lh_doall_arg, lh_error - dynamic hash table"
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