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OPENSSL_LH_COMPFUNC(3ossl) OpenSSL OPENSSL_LH_COMPFUNC(3ossl)
NAME
LHASH, DECLARE_LHASH_OF, OPENSSL_LH_COMPFUNC, OPENSSL_LH_HASHFUNC,
OPENSSL_LH_DOALL_FUNC, LHASH_DOALL_ARG_FN_TYPE,
IMPLEMENT_LHASH_HASH_FN, IMPLEMENT_LHASH_COMP_FN, lh_TYPE_new,
lh_TYPE_free, lh_TYPE_flush, lh_TYPE_insert, lh_TYPE_delete,
lh_TYPE_retrieve, lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_error,
OPENSSL_LH_new, OPENSSL_LH_free, OPENSSL_LH_flush, OPENSSL_LH_insert,
OPENSSL_LH_delete, OPENSSL_LH_retrieve, OPENSSL_LH_doall,
OPENSSL_LH_doall_arg, OPENSSL_LH_error - dynamic hash table
SYNOPSIS
#include <openssl/lhash.h>
DECLARE_LHASH_OF(TYPE);
LHASH_OF(TYPE) *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
void lh_TYPE_free(LHASH_OF(TYPE) *table);
void lh_TYPE_flush(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_TYPE_retrieve(LHASH_OF(TYPE) *table, TYPE *data);
void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
TYPE *arg);
int lh_TYPE_error(LHASH_OF(TYPE) *table);
typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
OPENSSL_LHASH *OPENSSL_LH_new(OPENSSL_LH_HASHFUNC h, OPENSSL_LH_COMPFUNC c);
void OPENSSL_LH_free(OPENSSL_LHASH *lh);
void OPENSSL_LH_flush(OPENSSL_LHASH *lh);
void *OPENSSL_LH_insert(OPENSSL_LHASH *lh, void *data);
void *OPENSSL_LH_delete(OPENSSL_LHASH *lh, const void *data);
void *OPENSSL_LH_retrieve(OPENSSL_LHASH *lh, const void *data);
void OPENSSL_LH_doall(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNC func);
void OPENSSL_LH_doall_arg(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNCARG func, void *arg);
int OPENSSL_LH_error(OPENSSL_LHASH *lh);
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. In the description here, TTYYPPEE is used a placeholder
for any of the OpenSSL datatypes, such as SSL_SESSION.
lh_TTYYPPEE_new() creates a new LHASH_OF(TTYYPPEE) structure to store arbitrary
data entries, and specifies the 'hash' and 'compare' callbacks to be
used in organising the table's entries. The hash callback takes a
If your hash table will contain items of some particular type and the
hash and compare callbacks hash/compare these types, then the
IMPLEMENT_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros can be used
to create callback wrappers of the prototypes required by lh_TTYYPPEE_new()
as shown in this example:
/*
* Implement the hash and compare functions; "stuff" can be any word.
*/
static unsigned long stuff_hash(const TYPE *a)
{
...
}
static int stuff_cmp(const TYPE *a, const TYPE *b)
{
...
}
/*
* Implement the wrapper functions.
*/
static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)
If the type is going to be used in several places, the following macros
can be used in a common header file to declare the function wrappers:
DECLARE_LHASH_HASH_FN(stuff, TYPE)
DECLARE_LHASH_COMP_FN(stuff, TYPE)
Then a hash table of TTYYPPEE objects can be created using this:
LHASH_OF(TYPE) *htable;
htable = B<lh_I<TYPE>_new>(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
lh_TTYYPPEE_free() frees the LHASH_OF(TTYYPPEE) structure table. Allocated hash
table entries will not be freed; consider using lh_TTYYPPEE_doall() to
deallocate any remaining entries in the hash table (see below).
lh_TTYYPPEE_flush() empties the LHASH_OF(TTYYPPEE) structure table. New entries
can be added to the flushed table. Allocated hash table entries will
not be freed; consider using lh_TTYYPPEE_doall() to deallocate any
remaining entries in the hash table (see below).
lh_TTYYPPEE_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_TTYYPPEE_insert() stores pointers, the data are not
copied.
lh_TTYYPPEE_delete() deletes an entry from table.
lh_TTYYPPEE_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_TTYYPPEE_doall() will, for every entry in the hash table, call func with
the data item as its parameter. For example:
lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));
/* Then the hash table itself can be deallocated */
lh_TYPE_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_TTYYPPEE_doall_arg() is the same as lh_TTYYPPEE_doall() except that func
will be called with arg as the second argument and func should be of
type LHASH_DOALL_ARG_FN(TTYYPPEE) (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 TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
/* Implement a prototype-compatible wrapper for "TYPE_print" */
static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)
/* Print out the entire hashtable to a particular BIO */
lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
logging_bio);
lh_TTYYPPEE_error() can be used to determine if an error occurred in the
last operation.
OPENSSL_LH_new() is the same as the lh_TTYYPPEE_new() except that it is not
type specific. So instead of returning an LHASH_OF(TTYYPPEE) value it
returns a void *. In the same way the functions OPENSSL_LH_free(),
OPENSSL_LH_flush(), OPENSSL_LH_insert(), OPENSSL_LH_delete(),
OPENSSL_LH_retrieve(), OPENSSL_LH_doall(), OPENSSL_LH_doall_arg(), and
OPENSSL_LH_error() are equivalent to the similarly named lh_TTYYPPEE
functions except that they return or use a void * where the equivalent
lh_TTYYPPEE function returns or uses a TTYYPPEE * or LHASH_OF(TTYYPPEE) *. lh_TTYYPPEE
functions are implemented as type checked wrappers around the
OPENSSL_LH functions. Most applications should not call the OPENSSL_LH
functions directly.
RETURN VALUES
lh_TTYYPPEE_new() and OPENSSL_LH_new() return NULL on error, otherwise a
pointer to the new LHASH structure.
When a hash table entry is replaced, lh_TTYYPPEE_insert() or
OPENSSL_LH_insert() return the value being replaced. NULL is returned
on normal operation and on error.
lh_TTYYPPEE_delete() and OPENSSL_LH_delete() return the entry being
retrieve operations.
lh_TTYYPPEE_free(), OPENSSL_LH_free(), lh_TTYYPPEE_flush(), OPENSSL_LH_flush(),
lh_TTYYPPEE_doall() OPENSSL_LH_doall(), lh_TTYYPPEE_doall_arg() and
OPENSSL_LH_doall_arg() return no values.
NOTE
The LHASH code is not thread safe. All updating operations, as well as
lh_TTYYPPEE_error() or OPENSSL_LH_error() calls must be performed under a
write lock. All retrieve operations should be performed under a read
lock, unless accurate usage statistics are desired. In which case, a
write lock should be used for retrieve operations as well. For output
of the usage statistics, using the functions from OPENSSL_LH_stats(3),
a read lock suffices.
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 (i.e. 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
"TYPE_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_TTYYPPEE_insert() and OPENSSL_LH_insert() return NULL both for success
and error.
SEE ALSO
OPENSSL_LH_stats(3)
HISTORY
In OpenSSL 1.0.0, the lhash interface was revamped for better type
checking.
COPYRIGHT
Copyright 2000-2022 The OpenSSL Project Authors. All Rights Reserved.