Update the LHASH man page.

* Correct some prototypes and macros with respect to "const"ness.

* Add the extra macros and examples due to the lh_doall[_arg] modifications
  made recently. The existing example is also reworked for consistency.

* Rewrite, tweak, and supplement bits of the existing comments that seemed
  (IMHO) to be a little convoluted and misleading.

* Add a NOTE section that explains the use of macros and avoiding function
  casts (ie. generate a wrapper as with the macros, or prototype any
  callback functions exactly to not require casting). Also, explain the
  "const" approach taken in LHASH for the purposes of API comprehensibility
  and also application code auditing.

doc/crypto/lhash.pod

@@ -2,8 +2,7 @@
 
 =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
 
@@ -22,10 +21,10 @@ lh_doall_arg, lh_error - dynamic hash table
 
  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
 
@@ -33,53 +32,78 @@ 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 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
@@ -93,25 +117,53 @@ 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 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.
 
@@ -134,6 +186,45 @@ otherwise.
 
 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.
@@ -174,7 +265,7 @@ 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 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: