/usr/man/cat.3/lh_doall_arg.3
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>
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;
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#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));
/* ... */
}
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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
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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
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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
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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.
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See also lh_delete(3)
See also lh_doall(3)
See also lh_error(3)
See also lh_free(3)
See also lh_insert(3)
See also lh_new(3)
See also lh_retrieve(3)
See also lhash(3)
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