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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
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 organis-
ing 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 remain-
ing 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(), func-
tion 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 call-
backs. 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 solu-
tion 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 cre-
ate 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 oth-
erwise.
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 consis-
tency, 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. Con-
versely, 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 ta-
ble, 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 audit-
ing code should pay special attention to any instances of
DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types with-
out 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 Communica-
tions 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 redis-
tribute 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 com-
pares 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 rou-
tine 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 point-
ers 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.
0.9.7d 2005-02-24 lhash(3)
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