/xlv3/openssl/0.9.7e-sgipl1/work/0.9.7e-sgipl1/openssl-
0.9.7e/doc/crypto
lhash(3) 18/Jul/2002 (0.9.7e) 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(LHASH_HASH_FN_TYPE hash, LHASH_COMP_FN_TYPE compare);
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, LHASH_DOALL_FN_TYPE func);
void lh_doall_arg(LHASH *table, LHASH_DOALL_ARG_FN_TYPE func,
void *arg);
int lh_error(LHASH *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 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 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_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(f_name,o_type) \
unsigned long f_name##_LHASH_HASH(const void *);
#define IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \
unsigned long f_name##_LHASH_HASH(const void *arg) { \
o_type a = (o_type)arg; \
return f_name(a); }
#define LHASH_HASH_FN(f_name) f_name##_LHASH_HASH
#define DECLARE_LHASH_COMP_FN(f_name,o_type) \
int f_name##_LHASH_COMP(const void *, const void *);
#define IMPLEMENT_LHASH_COMP_FN(f_name,o_type) \
int f_name##_LHASH_COMP(const void *arg1, const void *arg2) { \
o_type a = (o_type)arg1; \
o_type b = (o_type)arg2; \
return f_name(a,b); }
#define LHASH_COMP_FN(f_name) f_name##_LHASH_COMP
#define DECLARE_LHASH_DOALL_FN(f_name,o_type) \
void f_name##_LHASH_DOALL(const void *);
#define IMPLEMENT_LHASH_DOALL_FN(f_name,o_type) \
void f_name##_LHASH_DOALL(const void *arg) { \
o_type a = (o_type)arg; \
f_name(a); }
#define LHASH_DOALL_FN(f_name) f_name##_LHASH_DOALL
#define DECLARE_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \
void f_name##_LHASH_DOALL_ARG(const void *, const void *);
#define IMPLEMENT_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \
void f_name##_LHASH_DOALL_ARG(const void *arg1, const void *arg2) { \
o_type a = (o_type)arg1; \
a_type b = (a_type)arg2; \
f_name(a,b); }
#define LHASH_DOALL_ARG_FN(f_name) f_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_hash, const STUFF *)
static IMPLEMENT_LHASH_COMP_FN(STUFF_cmp, const STUFF *);
/* ... */
int main(int argc, char *argv[]) {
/* Create the new hash table using the hash/compare wrappers */
LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash),
LHASH_COMP_FN(STUFF_cmp));
/* ... */
}
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 (see
below).
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 its parameter. For lh_doall()
and lh_doall_arg(), function pointer casting should be
avoided in the callbacks (see NOTE) - instead, either
declare the callbacks to match the prototype required in
lh_new() or use the declare/implement macros to create
type-safe wrappers that cast variables prior to calling your
type-specific callbacks. An example of this is illustrated
here where the callback is used to cleanup resources for
items in the hash table prior to the hashtable itself being
deallocated:
/* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
void STUFF_cleanup(STUFF *a);
/* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF *)
/* ... then later in the code ... */
/* So to run "STUFF_cleanup" against all items in a hash table ... */
lh_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
/* Then the hash table itself can be deallocated */
lh_free(hashtable);
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_doall_arg() is the same as lh_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(const STUFF *a, BIO *output_bio);
/* Implement a prototype-compatible wrapper for "STUFF_print" */
static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print, const STUFF *, BIO *)
/* ... then later in the code ... */
/* Print out the entire hashtable to a particular BIO */
lh_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), logging_bio);
lh_error() can be used to determine if an error occurred in the last
operation. lh_error() is a macro.
RETURN VALUES
lh_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.
NOTE
The various LHASH macros and callback types exist to make it
possible to write type-safe code without resorting to
function-prototype casting - an evil that makes application
code much harder to audit/verify and also opens the window
of opportunity for stack corruption and other hard-to-find
bugs. It also, apparently, violates ANSI-C.
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 (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_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
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.
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 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.
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