NAME
lhnew, lhfree, lhinsert, lhdelete, lhretrieve, lhdoall,lhdoallarg, lherror - dynamic hash table
SYNOPSIS
#include
LHASH *lhnew(LHASHHASHFNTYPE hash, LHASHCOMPFNTYPE compare); void lhfree(LHASH *table); void *lhinsert(LHASH *table, void *data); void *lhdelete(LHASH *table, void *data); void *lhretrieve(LHASH *table, void *data); void lhdoall(LHASH *table, LHASHDOALLFNTYPE func); void lhdoallarg(LHASH *table, LHASHDOALLARGFNTYPE func, void *arg); int lherror(LHASH *table); typedef int (*LHASHCOMPFNTYPE)(const void *, const void *); typedef unsigned long (*LHASHHASHFNTYPE)(const void *); typedef void (*LHASHDOALLFNTYPE)(const void *); typedef void (*LHASHDOALLARGFNTYPE)(const void *, const void *);DESCRIPTION
This library implements dynamic hash tables. The hash table entries can be arbitrary structures. Usually they consist of key and value fields. lhnew() creates a new LLHHAASSHH structure to store arbitrary data entries, and provides the 'hash' and 'compare' callbacks to be used in organising the table's entries. The hhaasshh 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 ccoommppaarree callback takes two arguments (pointers to two hash tableentries), and returns 0 if their keys are equal, non-zero otherwise.
If your hash table will contain items of some particular type and the hhaasshh and ccoommppaarree callbacks hash/compare these types, then the DDEECCLLAARREELLHHAASSHHHHAASSHHFFNN and IIMMPPLLEEMMEENNTTLLHHAASSHHCCOOMMPPFFNN macros can be used to create callback wrappers of the prototypes required by lhnew(). Theseprovide 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 DECLARELHASHHASHFN(fname,otype) \
unsigned long fname##LHASHHASH(const void *);
#define IMPLEMENTLHASHHASHFN(fname,otype) \
unsigned long fname##LHASHHASH(const void *arg) { \
otype a = (otype)arg; \ return fname(a); }#define LHASHHASHFN(fname) fname##LHASHHASH
#define DECLARELHASHCOMPFN(fname,otype) \
int fname##LHASHCOMP(const void *, const void *);
#define IMPLEMENTLHASHCOMPFN(fname,otype) \
int fname##LHASHCOMP(const void *arg1, const void *arg2) { \
otype a = (otype)arg1; \ otype b = (otype)arg2; \ return fname(a,b); }#define LHASHCOMPFN(fname) fname##LHASHCOMP
#define DECLARELHASHDOALLFN(fname,otype) \
void fname##LHASHDOALL(const void *);
#define IMPLEMENTLHASHDOALLFN(fname,otype) \
void fname##LHASHDOALL(const void *arg) { \
otype a = (otype)arg; \ fname(a); }#define LHASHDOALLFN(fname) fname##LHASHDOALL
#define DECLARELHASHDOALLARGFN(fname,otype,atype) \
void fname##LHASHDOALLARG(const void *, const void *);
#define IMPLEMENTLHASHDOALLARGFN(fname,otype,atype) \
void fname##LHASHDOALLARG(const void *arg1, const void *arg2) { \
otype a = (otype)arg1; \ atype b = (atype)arg2; \ fname(a,b); }#define LHASHDOALLARGFN(fname) fname##LHASHDOALLARG
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 STUFFhash(const STUFF *tohash); /* Orders 'arg1' and 'arg2' (implemented elsewhere) */ int STUFFcmp(const STUFF *arg1, const STUFF *arg2);/* Create the type-safe wrapper functions for use in the LHASH internals */
static IMPLEMENTLHASHHASHFN(STUFFhash, const STUFF *) static IMPLEMENTLHASHCOMPFN(STUFFcmp, const STUFF *); /* ... */ int main(int argc, char *argv[]) { /* Create the new hash table using the hash/compare wrappers */ LHASH *hashtable = lhnew(LHASHHASHFN(STUFFhash), LHASHCOMPFN(STUFFcmp)); /* ... */ } lhfree() frees the LLHHAASSHH structure ttaabbllee. Allocated hash table entries will not be freed; consider using lhdoall() to deallocate any remaining entries in the hash table (see below). lhinsert() inserts the structure pointed to by ddaattaa into ttaabbllee. If there already is an entry with the same key, the old value is replaced. Note that lhinsert() stores pointers, the data are not copied. lhdelete() deletes an entry from ttaabbllee. lhretrieve() looks up an entry in ttaabbllee. Normally, ddaattaa is a structure with the key field(s) set; the function will return a pointer to a fully populated structure. lhdoall() will, for every entry in the hash table, call ffuunncc with the data item as its parameter. For lhdoall() and lhdoallarg(), function pointer casting should be avoided in the callbacks (see NNOOTTEE)- instead, either declare the callbacks to match the prototype required
in lhnew() 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 STUFFcleanup(STUFF *a);/* Implement a prototype-compatible wrapper for "STUFFcleanup" */
IMPLEMENTLHASHDOALLFN(STUFFcleanup, STUFF *) /* ... then later in the code ... */ /* So to run "STUFFcleanup" against all items in a hash table ... */ lhdoall(hashtable, LHASHDOALLFN(STUFFcleanup)); /* Then the hash table itself can be deallocated */ lhfree(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 thatyou are currently on down lower in the hash table - this could cause
some entries to be skipped during the iteration. The second bestsolution to this problem is to set hash->download=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! lhdoallarg() is the same as lhdoall() except that ffuunncc will be called with aarrgg as the second argument and ffuunncc should be of type LLHHAASSHHDDOOAALLLLAARRGGFFNNTTYYPPEE (a callback prototype that is passed both the table entry and an extra argument). As with lhdoall(), 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 yourtype-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 'outputbio' (this is implemented elsewhere) */ void STUFFprint(const STUFF *a, BIO *outputbio);/* Implement a prototype-compatible wrapper for "STUFFprint" */
static IMPLEMENTLHASHDOALLARGFN(STUFFprint, const STUFF *, BIO *) /* ... then later in the code ... */ /* Print out the entire hashtable to a particular BIO */ lhdoallarg(hashtable, LHASHDOALLARGFN(STUFFprint), loggingbio); lherror() can be used to determine if an error occurred in the last operation. lherror() is a macro.RETURN VALUES
lhnew() returns NNUULLLL on error, otherwise a pointer to the new LLHHAASSHH structure. When a hash table entry is replaced, lhinsert() returns the value being replaced. NNUULLLL is returned on normal operation and on error. lhdelete() returns the entry being deleted. NNUULLLL is returned if there is no such value in the hash table. lhretrieve() returns the hash table entry if it has been found, NNUULLLL otherwise. lherror() returns 1 if an error occurred in the last operation, 0 otherwise. lhfree(), lhdoall() and lhdoallarg() return no values. NNOOTTEE The various LHASH macros and callback types exist to make it possibleto 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 window of opportunity for stack corruption and otherhard-to-find bugs. It also, apparently, violates ANSI-C.
The LHASH code regards table entries as constant data. As such, it internally represents lhinsert()'d items with a "const void *" pointer type. This is why callbacks such as those used by lhdoall() and lhdoallarg() declare their prototypes with "const", even for theparameters 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 theircode) - 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 lhdoall() or lhdoallarg() callbacks (see the "STUFFcleanup" 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/IMPLEMENTLHASHDOALL[ARG]FN macros that provide types without any "const" qualifiers.BUGS
lhinsert() returns NNUULLLL both for success and error. IINNTTEERRNNAALLSS The following description is based on the SSLeay documentation: The llhhaasshh 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 OPENSSLrealloc(). 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 LLHHAASSHH 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. Thedefault values are as follows. If (hash->upload < load) => expand.
if (hash->download > load) => contract. The uuppllooaadd has a default
value of 1 and ddoowwnnllooaadd has a default value of 2. These numbers can be modified by the application by just playing with the uuppllooaadd and ddoowwnnllooaadd variables. The 'load' is kept in a form which is multipliedby 256. So hash->upload=8*256; will cause a load of 8 to be set.
If you are interested in performance the field to watch is numcompcalls. The hash library keeps track of the 'hash' value for each item so when a lookup is done, the 'hashes' are compared, if thereis a match, then a full compare is done, and hash->numcompcalls is
incremented. If numcompcalls is not equal to numdelete plus numretrieve 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 uunnssiiggnneedd lloonngg compares and 10 linked list traverses. This will be much less expensive that 10 calls to your compare function. lhstrhash() is a demo string hashing function: unsigned long lhstrhash(const char *c); Since the LLHHAASSHH routines would normally be passed structures, this routine would not normally be passed to lhnew(), rather it would be used in the function passed to lhnew().SEE ALSO
lhstats(3) HISTORY The llhhaasshh library is available in all versions of SSLeay and OpenSSL. lherror() 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 LHASHCOMPFNTYPE, LHASHHASHFNTYPE, LHASHDOALLFNTYPE and LHASHDOALLARGFNTYPE became available.0.9.7l 2002-07-18 lhash(3)