Update the LHASH man page.

=head1 NAME

lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall,

lh_doall_arg, lh_error - dynamic hash table

lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error - dynamic hash table

=head1 SYNOPSIS

 int lh_error(LHASH *table);

 typedef int (*LHASH_COMP_FN_TYPE)(void *, void *);

 typedef unsigned long (*LHASH_HASH_FN_TYPE)(void *);

 typedef void (*LHASH_DOALL_FN_TYPE)(void *);

 typedef void (*LHASH_DOALL_ARG_FN_TYPE)(void *, void *);

 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 *);

=head1 DESCRIPTION

can be arbitrary structures. Usually they consist of key and value

fields.

lh_new() creates a new B<LHASH> structure. B<hash> takes a pointer to

the structure and returns an unsigned long hash value of 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. B<compare> takes two arguments, and returns 0 if their keys are

equal, non-zero otherwise. If your hash table will contain items of

some uniform type, and similarly the B<hash> and B<compare> callbacks

hash or compare the same type, then the B<DECLARE_LHASH_HASH_FN> and

lh_new() creates a new B<LHASH> structure to store arbitrary data

entries, and provides the 'hash' and 'compare' callbacks to be used in

organising the table's entries.  The B<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 B<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 B<hash> and B<compare> callbacks hash/compare

these types, then the B<DECLARE_LHASH_HASH_FN> and

B<IMPLEMENT_LHASH_COMP_FN> macros can be used to create callback

wrappers of the prototypes required in lh_new(). These provide

wrappers of the prototypes required by lh_new().  These provide

per-variable casts before calling the type-specific callbacks written

by the application author. These macros are defined as;

by the application author.  These macros, as well as those used for

the "doall" callbacks, are defined as;

 #define DECLARE_LHASH_HASH_FN(f_name,o_type) \

         unsigned long f_name##_LHASH_HASH(void *);

         unsigned long f_name##_LHASH_HASH(const void *);

 #define IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \

         unsigned long f_name##_LHASH_HASH(void *arg) { \

         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(void *, void *);

         int f_name##_LHASH_COMP(const void *, const void *);

 #define IMPLEMENT_LHASH_COMP_FN(f_name,o_type) \

         int f_name##_LHASH_COMP(void *arg1, void *arg2) { \

         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

An example of a hash table storing (pointers to) a structure type 'foo'

 #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;

 unsigned long   foo_hash(foo *tohash);

 int             foo_compare(foo *arg1, foo *arg2);

 static IMPLEMENT_LHASH_HASH_FN(foo_hash, foo *)

 static IMPLEMENT_LHASH_COMP_FN(foo_compare, foo *);

 /* 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[]) {

         LHASH *hashtable = lh_new(LHASH_HASH_FN(foo_hash),

                                   LHASH_COMP_FN(foo_compare));

         /* Create the new hash table using the hash/compare wrappers */

         LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash),

                                   LHASH_COMP_FN(STUFF_cmp));

	 /* ... */

 }

lh_free() frees the B<LHASH> structure B<table>. Allocated hash table

entries will not be freed; consider using lh_doall() to deallocate any

remaining entries in the hash table.

remaining entries in the hash table (see below).

lh_insert() inserts the structure pointed to by B<data> into B<table>.

If there already is an entry with the same key, the old value is

pointer to a fully populated structure.

lh_doall() will, for every entry in the hash table, call B<func> with

the data item as parameters.

This function can be quite useful when used as follows:

 void cleanup(STUFF *a)

  { STUFF_free(a); }

 lh_doall(hash,(LHASH_DOALL_FN_TYPE)cleanup);

 lh_free(hash);

This can be used to free all the entries. lh_free() then cleans up the

'buckets' that point to nothing. When doing this, be careful if you

delete entries from the hash table in B<func>: the table may decrease

in size, moving item that you are currently on down lower in the hash

table.  This could cause some entries to be skipped.  The best

solution to this problem is to set hash-E<gt>down_load=0 before you

start.  This will stop the hash table ever being decreased in size.

lh_doall_arg() is the same as lh_doall() except that B<func> will

be called with B<arg> as the second argument and B<func> should be

of type B<LHASH_DOALL_ARG_FN_TYPE> (a callback prototype that is

passed an extra argument).

the data item as its parameter.  For lh_doall() and lh_doall_arg(),

function pointer casting should be avoided in the callbacks (see

B<NOTE>) - instead, either declare the callbacks to match the

prototype required in lh_new() or use the decare/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 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-E<gt>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 B<func> will be

called with B<arg> as the second argument and B<func> should be of

type B<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 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.

lh_free(), lh_doall() and lh_doall_arg() return no values.

=head1 NOTE

The various LHASH macros and callback types exist to make it possible

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 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 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 "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 of

DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types

without any "const" qualifiers.

=head1 BUGS

lh_insert() returns B<NULL> both for success and error.

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 B<unsigned long> compares and 10 linked list traverses.  This

will be much less expensive that 10 calls to you compare function.

will be much less expensive that 10 calls to your compare function.

lh_strhash() is a demo string hashing function: