OpenSSL lhash(3openssl) NNNNAAAAMMMMEEEE
lh_new, lh_free, lh_insert, lh_delete, lh_retrieve,
lh_doall, lh_doall_arg, lh_error - dynamic hash table
SSSSYYYYNNNNOOOOPPPPSSSSIIIISSSS#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 *);
DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN 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 LLLLHHHHAAAASSSSHHHH structure to store arbitrary
data entries, and provides the 'hash' and 'compare' callbacks to be used in organising the table's entries. The hhhhaaaasssshhhh 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 ccccoooommmmppppaaaarrrreeee callback takes two arguments (pointers to two hash table entries), and returns 0 if theirkeys are equal, non-zero otherwise. If your hash table will
contain items of some particular type and the hhhhaaaasssshhhh and ccccoooommmmppppaaaarrrreeee callbacks hash/compare these types, then theDDDDEEEECCCCLLLLAAAARRRREEEE_LLLLHHHHAAAASSSSHHHH_HHHHAAAASSSSHHHH_FFFFNNNN and IIIIMMMMPPPPLLLLEEEEMMMMEEEENNNNTTTT_LLLLHHHHAAAASSSSHHHH_CCCCOOOOMMMMPPPP_FFFFNNNN macros can
be used to create callback wrappers of the prototypesrequired 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;18/Jul/2002 Last change: 0.9.8o 1 OpenSSL lhash(3openssl)
#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));
/* ... */ }18/Jul/2002 Last change: 0.9.8o 2 OpenSSL lhash(3openssl)
lh_free() frees the LLLLHHHHAAAASSSSHHHH structure ttttaaaabbbblllleeee. Allocated hash
table entries will not be freed; consider using lh_doall()
to deallocate any remaining entries in the hash table (see below).lh_insert() inserts the structure pointed to by ddddaaaattttaaaa into
ttttaaaabbbblllleeee. If there already is an entry with the same key, theold value is replaced. Note that lh_insert() stores
pointers, the data are not copied.lh_delete() deletes an entry from ttttaaaabbbblllleeee.
lh_retrieve() looks up an entry in ttttaaaabbbblllleeee. Normally, ddddaaaattttaaaa 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
ffffuuuunnnncccc with the data item as its parameter. For lh_doall()
and lh_doall_arg(), function pointer casting should be
avoided in the callbacks (see NNNNOOOOTTTTEEEE) - instead, either
declare the callbacks to match the prototype required inlh_new() or use the declare/implement macros to create
type-safe 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(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 lowerin the hash table - this could cause some entries to be
skipped during the iteration. The second best solution tothis 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 ffffuuuunnnncccc
will be called with aaaarrrrgggg as the second argument and ffffuuuunnnnccccshould be of type LLLLHHHHAAAASSSSHHHH_DDDDOOOOAAAALLLLLLLL_AAAARRRRGGGG_FFFFNNNN_TTTTYYYYPPPPEEEE (a callback
prototype that is passed both the table entry and an extra18/Jul/2002 Last change: 0.9.8o 3 OpenSSL lhash(3openssl)
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 beforecalling 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.
RRRREEEETTTTUUUURRRRNNNN VVVVAAAALLLLUUUUEEEESSSSlh_new() returns NNNNUUUULLLLLLLL on error, otherwise a pointer to the
new LLLLHHHHAAAASSSSHHHH structure.When a hash table entry is replaced, lh_insert() returns the
value being replaced. NNNNUUUULLLLLLLL is returned on normal operation and on error.lh_delete() returns the entry being deleted. NNNNUUUULLLLLLLL is
returned if there is no such value in the hash table.lh_retrieve() returns the hash table entry if it has been
found, NNNNUUUULLLLLLLL otherwise.lh_error() returns 1 if an error occurred in the last
operation, 0 otherwise.lh_free(), lh_doall() and lh_doall_arg() return no values.
NNNNOOOOTTTTEEEE The various LHASH macros and callback types exist to make itpossible 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 windowof 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. Assuch, it internally represents lh_insert()'d items with a
"const void *" pointer type. This is why callbacks such asthose used by lh_doall() and lh_doall_arg() declare their
prototypes with "const", even for the parameters that pass18/Jul/2002 Last change: 0.9.8o 4 OpenSSL lhash(3openssl)
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 totable 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 ofDECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide
types without any "const" qualifiers. BBBBUUUUGGGGSSSSlh_insert() returns NNNNUUUULLLLLLLL both for success and error.
IIIINNNNTTTTEEEERRRRNNNNAAAALLLLSSSS The following description is based on the SSLeay documentation: The llllhhhhaaaasssshhhh 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 tableis 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 LLLLHHHHAAAASSSSHHHH 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 table18/Jul/2002 Last change: 0.9.8o 5 OpenSSL lhash(3openssl) 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 uuuupppp_llllooooaaaadddd has a
default value of 1 and ddddoooowwwwnnnn_llllooooaaaadddd has a default value of 2.
These numbers can be modified by the application by justplaying with the uuuupppp_llllooooaaaadddd and ddddoooowwwwnnnn_llllooooaaaadddd variables. The
'load' is kept in a form which is multiplied by 256. Sohash->up_load=8*256; will cause a load of 8 to be set.
If you are interested in performance the field to watch isnum_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 isdone, 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 uuuunnnnssssiiiiggggnnnneeeedddd lllloooonnnngggg 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 LLLLHHHHAAAASSSSHHHH routines would normally be passed structures, this routine would not normally be passed tolh_new(), rather it would be used in the function passed to
lh_new().
SSSSEEEEEEEE AAAALLLLSSSSOOOOlh_stats(3)
HHHHIIIISSSSTTTTOOOORRRRYYYY The llllhhhhaaaasssshhhh library is available in all versions of SSLeay andOpenSSL. 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, andthe function types LHASH_COMP_FN_TYPE, LHASH_HASH_FN_TYPE,
LHASH_DOALL_FN_TYPE and LHASH_DOALL_ARG_FN_TYPE became
available.18/Jul/2002 Last change: 0.9.8o 6 OpenSSL lhash(3openssl)
18/Jul/2002 Last change: 0.9.8o 7