Sortix nightly manual
This manual documents Sortix nightly, a development build that has not been officially released. You can instead view this document in the latest official manual.
LH_NEW(3) | Library Functions Manual | LH_NEW(3) |
NAME
lh_new
,
lh_free
,
lh_insert
,
lh_delete
,
lh_retrieve
,
lh_doall
,
lh_doall_arg
,
lh_error
,
LHASH_COMP_FN_TYPE
,
LHASH_HASH_FN_TYPE
,
LHASH_DOALL_FN_TYPE
,
LHASH_DOALL_ARG_FN_TYPE
,
lh_strhash
—
dynamic hash table
SYNOPSIS
#include
<openssl/lhash.h>
DECLARE_LHASH_OF
(<type>);
LHASH *
lh_<type>_new
(void);
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_<type>_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 *);
unsigned long
lh_strhash
(const
char *c);
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 and
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
/* Calculate the hash value of 'tohash' (implemented elsewhere) */ unsigned long STUFF_hash(const STUFF *tohash); /* Order 'arg1' and 'arg2' (implemented elsewhere) */ int stuff_cmp(const STUFF *arg1, const STUFF *arg2); /* Create 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
NOTES) — 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:
/* Clean 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);
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_<type>_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):
/* Print 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, or NULL
otherwise.
lh_<type>_error
() returns 1 if an error
occurred in the last operation, or 0 otherwise.
NOTES
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 representslh_<type>_insert
()'ed
items with a const void * pointer type. This
is why callbacks such as those used by
lh_<type>_doall
() and
lh_<type>_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_<type>_doall
() or
lh_<type>_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 are 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.
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 reallocarray(3). 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. 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
crypto(3), lh_stats(3)HISTORY
lh_new
(),
lh_free
(),
lh_insert
(),
lh_delete
(),
lh_retrieve
(),
lh_doall
(), and
lh_strhash
() appeared in SSLeay 0.4 or
earlier. lh_doall_arg
() first appeared in
SSLeay 0.5.1. These functions have been available since
OpenBSD 2.4.
lh_<type>_error
() was added in SSLeay
0.9.1b.
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.
BUGS
lh_<type>_insert
() returns
NULL
both for success and error.March 31, 2022 | Debian |