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>
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); }
#define LHASH_HASH_FN(name) name##_LHASH_HASH
#define DECLARE_LHASH_COMP_FN(name, o_type) \
int name##_LHASH_COMP(const void *, const void *);
#define IMPLEMENT_LHASH_COMP_FN(name, o_type) \
int name##_LHASH_COMP(const void *arg1, const void *arg2) { \
const o_type *a = arg1; \
const o_type *b = arg2; \
return name##_cmp(a,b); }
#define LHASH_COMP_FN(name) name##_LHASH_COMP
#define DECLARE_LHASH_DOALL_FN(name, o_type) \
void name##_LHASH_DOALL(void *);
#define IMPLEMENT_LHASH_DOALL_FN(name, o_type) \
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 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_doall(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_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
/* Then the hash table itself can be deallocated */
lh_STUFF_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_<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.
NOTE¶
The various LHASH macros and callback types exist to make it possible to write
type-checked 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_<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 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_<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.