Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
OPENSSL_LH_COMPFUNC(3)		    OpenSSL		OPENSSL_LH_COMPFUNC(3)

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_insert, lh_TYPE_delete, lh_TYPE_retrieve,
       lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_error - dynamic hash table

SYNOPSIS
	#include <openssl/lhash.h>

	DECLARE_LHASH_OF(TYPE);

	LHASH *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
	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, 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 *);

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, TYPE	is used	a placeholder
       for any of the OpenSSL datatypes, such as SSL_SESSION.

       lh_TYPE_new() creates a new LHASH_OF(TYPE) 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
       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
       IMPLEMENT_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros can be used
       to create callback wrappers of the prototypes required by lh_TYPE_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 TYPE objects can be	created	using this:

	LHASH_OF(TYPE) *htable;

	htable = lh_TYPE_new(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));

       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 example:

	/* Cleans up resources belonging to 'a'	(this is implemented elsewhere)	*/
	void TYPE_cleanup_doall(TYPE *a);

	/* Implement a prototype-compatible wrapper for	"TYPE_cleanup" */
	IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)

	/* Call	"TYPE_cleanup" against all items in a hash table. */
	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_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 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_TYPE_error() can be used to determine	if an error occurred in	the
       last operation.

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. It's meaningful only after non-retrieve operations.

       lh_TYPE_free(), lh_TYPE_doall() and lh_TYPE_doall_arg() return no
       values.

NOTE
       The LHASH code is not thread safe. All updating operations, as well as
       lh_TYPE_error call 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 (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
       "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_TYPE_insert()	returns	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 2000-2018 The OpenSSL Project Authors.	All Rights Reserved.

       Licensed	under the OpenSSL license (the "License").  You	may not	use
       this file except	in compliance with the License.	 You can obtain	a copy
       in the file LICENSE in the source distribution or at
       <https://www.openssl.org/source/license.html>.

1.1.1d				  2019-09-10		OPENSSL_LH_COMPFUNC(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | NOTE | BUGS | SEE ALSO | HISTORY | COPYRIGHT

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=LHASH&sektion=3&manpath=FreeBSD+12.1-RELEASE+and+Ports>

home | help