443 lines
18 KiB
Groff
443 lines
18 KiB
Groff
.\" $NetBSD: openssl_lhash.3,v 1.8 2003/07/24 14:16:51 itojun Exp $
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.\" ======================================================================
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.\"
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.IX Title "lhash 3"
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.TH lhash 3 "0.9.7b" "2002-07-19" "OpenSSL"
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.UC
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.SH "NAME"
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lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error \- dynamic hash table
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.SH "LIBRARY"
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libcrypto, -lcrypto
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/lhash.h>
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.Ve
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.Vb 2
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\& LHASH *lh_new(LHASH_HASH_FN_TYPE hash, LHASH_COMP_FN_TYPE compare);
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\& void lh_free(LHASH *table);
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.Ve
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.Vb 3
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\& void *lh_insert(LHASH *table, void *data);
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\& void *lh_delete(LHASH *table, void *data);
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\& void *lh_retrieve(LHASH *table, void *data);
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.Ve
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.Vb 3
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\& void lh_doall(LHASH *table, LHASH_DOALL_FN_TYPE func);
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\& void lh_doall_arg(LHASH *table, LHASH_DOALL_ARG_FN_TYPE func,
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\& void *arg);
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.Ve
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.Vb 1
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\& int lh_error(LHASH *table);
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.Ve
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.Vb 4
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\& typedef int (*LHASH_COMP_FN_TYPE)(const void *, const void *);
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\& typedef unsigned long (*LHASH_HASH_FN_TYPE)(const void *);
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\& typedef void (*LHASH_DOALL_FN_TYPE)(const void *);
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\& typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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This library implements dynamic hash tables. The hash table entries
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can be arbitrary structures. Usually they consist of key and value
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fields.
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.PP
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\&\fIlh_new()\fR creates a new \fB\s-1LHASH\s0\fR structure to store arbitrary data
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entries, and provides the 'hash' and 'compare' callbacks to be used in
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organising the table's entries. The \fBhash\fR callback takes a pointer
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to a table entry as its argument and returns an unsigned long hash
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value for its key field. The hash value is normally truncated to a
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power of 2, so make sure that your hash function returns well mixed
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low order bits. The \fBcompare\fR callback takes two arguments (pointers
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to two hash table entries), and returns 0 if their keys are equal,
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non-zero otherwise. If your hash table will contain items of some
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particular type and the \fBhash\fR and \fBcompare\fR callbacks hash/compare
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these types, then the \fB\s-1DECLARE_LHASH_HASH_FN\s0\fR and
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\&\fB\s-1IMPLEMENT_LHASH_COMP_FN\s0\fR macros can be used to create callback
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wrappers of the prototypes required by \fIlh_new()\fR. These provide
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per-variable casts before calling the type-specific callbacks written
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by the application author. These macros, as well as those used for
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the \*(L"doall\*(R" callbacks, are defined as;
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.PP
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.Vb 7
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\& #define DECLARE_LHASH_HASH_FN(f_name,o_type) \e
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\& unsigned long f_name##_LHASH_HASH(const void *);
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\& #define IMPLEMENT_LHASH_HASH_FN(f_name,o_type) \e
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\& unsigned long f_name##_LHASH_HASH(const void *arg) { \e
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\& o_type a = (o_type)arg; \e
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\& return f_name(a); }
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\& #define LHASH_HASH_FN(f_name) f_name##_LHASH_HASH
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.Ve
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.Vb 8
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\& #define DECLARE_LHASH_COMP_FN(f_name,o_type) \e
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\& int f_name##_LHASH_COMP(const void *, const void *);
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\& #define IMPLEMENT_LHASH_COMP_FN(f_name,o_type) \e
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\& int f_name##_LHASH_COMP(const void *arg1, const void *arg2) { \e
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\& o_type a = (o_type)arg1; \e
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\& o_type b = (o_type)arg2; \e
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\& return f_name(a,b); }
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\& #define LHASH_COMP_FN(f_name) f_name##_LHASH_COMP
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.Ve
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.Vb 7
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\& #define DECLARE_LHASH_DOALL_FN(f_name,o_type) \e
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\& void f_name##_LHASH_DOALL(const void *);
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\& #define IMPLEMENT_LHASH_DOALL_FN(f_name,o_type) \e
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\& void f_name##_LHASH_DOALL(const void *arg) { \e
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\& o_type a = (o_type)arg; \e
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\& f_name(a); }
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\& #define LHASH_DOALL_FN(f_name) f_name##_LHASH_DOALL
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.Ve
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.Vb 8
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\& #define DECLARE_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \e
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\& void f_name##_LHASH_DOALL_ARG(const void *, const void *);
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\& #define IMPLEMENT_LHASH_DOALL_ARG_FN(f_name,o_type,a_type) \e
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\& void f_name##_LHASH_DOALL_ARG(const void *arg1, const void *arg2) { \e
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\& o_type a = (o_type)arg1; \e
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\& a_type b = (a_type)arg2; \e
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\& f_name(a,b); }
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\& #define LHASH_DOALL_ARG_FN(f_name) f_name##_LHASH_DOALL_ARG
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.Ve
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An example of a hash table storing (pointers to) structures of type '\s-1STUFF\s0'
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could be defined as follows;
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.PP
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.Vb 14
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\& /* Calculates the hash value of 'tohash' (implemented elsewhere) */
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\& unsigned long STUFF_hash(const STUFF *tohash);
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\& /* Orders 'arg1' and 'arg2' (implemented elsewhere) */
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\& int STUFF_cmp(const STUFF *arg1, const STUFF *arg2);
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\& /* Create the type-safe wrapper functions for use in the LHASH internals */
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\& static IMPLEMENT_LHASH_HASH_FN(STUFF_hash, const STUFF *)
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\& static IMPLEMENT_LHASH_COMP_FN(STUFF_cmp, const STUFF *);
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\& /* ... */
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\& int main(int argc, char *argv[]) {
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\& /* Create the new hash table using the hash/compare wrappers */
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\& LHASH *hashtable = lh_new(LHASH_HASH_FN(STUFF_hash),
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\& LHASH_COMP_FN(STUFF_cmp));
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\& /* ... */
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\& }
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.Ve
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\&\fIlh_free()\fR frees the \fB\s-1LHASH\s0\fR structure \fBtable\fR. Allocated hash table
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entries will not be freed; consider using \fIlh_doall()\fR to deallocate any
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remaining entries in the hash table (see below).
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.PP
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\&\fIlh_insert()\fR inserts the structure pointed to by \fBdata\fR into \fBtable\fR.
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If there already is an entry with the same key, the old value is
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replaced. Note that \fIlh_insert()\fR stores pointers, the data are not
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copied.
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.PP
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\&\fIlh_delete()\fR deletes an entry from \fBtable\fR.
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.PP
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\&\fIlh_retrieve()\fR looks up an entry in \fBtable\fR. Normally, \fBdata\fR is
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a structure with the key \fIfield\fR\|(s) set; the function will return a
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pointer to a fully populated structure.
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.PP
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\&\fIlh_doall()\fR will, for every entry in the hash table, call \fBfunc\fR with
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the data item as its parameter. For \fIlh_doall()\fR and \fIlh_doall_arg()\fR,
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function pointer casting should be avoided in the callbacks (see
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\&\fB\s-1NOTE\s0\fR) \- instead, either declare the callbacks to match the
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prototype required in \fIlh_new()\fR or use the declare/implement macros to
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create type-safe wrappers that cast variables prior to calling your
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type-specific callbacks. An example of this is illustrated here where
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the callback is used to cleanup resources for items in the hash table
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prior to the hashtable itself being deallocated:
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.PP
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.Vb 9
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\& /* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
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\& void STUFF_cleanup(STUFF *a);
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\& /* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
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\& IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF *)
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\& /* ... then later in the code ... */
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\& /* So to run "STUFF_cleanup" against all items in a hash table ... */
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\& lh_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
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\& /* Then the hash table itself can be deallocated */
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\& lh_free(hashtable);
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.Ve
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When doing this, be careful if you delete entries from the hash table
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in your callbacks: the table may decrease in size, moving the item
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that you are currently on down lower in the hash table \- this could
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cause some entries to be skipped during the iteration. The second
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best solution to this problem is to set hash->down_load=0 before
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you start (which will stop the hash table ever decreasing in size).
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The best solution is probably to avoid deleting items from the hash
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table inside a \*(L"doall\*(R" callback!
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.PP
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\&\fIlh_doall_arg()\fR is the same as \fIlh_doall()\fR except that \fBfunc\fR will be
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called with \fBarg\fR as the second argument and \fBfunc\fR should be of
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type \fB\s-1LHASH_DOALL_ARG_FN_TYPE\s0\fR (a callback prototype that is passed
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both the table entry and an extra argument). As with \fIlh_doall()\fR, you
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can instead choose to declare your callback with a prototype matching
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the types you are dealing with and use the declare/implement macros to
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create compatible wrappers that cast variables before calling your
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type-specific callbacks. An example of this is demonstrated here
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(printing all hash table entries to a \s-1BIO\s0 that is provided by the
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caller):
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.PP
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.Vb 7
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\& /* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
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\& void STUFF_print(const STUFF *a, BIO *output_bio);
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\& /* Implement a prototype-compatible wrapper for "STUFF_print" */
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\& static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF_print, const STUFF *, BIO *)
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\& /* ... then later in the code ... */
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\& /* Print out the entire hashtable to a particular BIO */
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\& lh_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), logging_bio);
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.Ve
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\&\fIlh_error()\fR can be used to determine if an error occurred in the last
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operation. \fIlh_error()\fR is a macro.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fIlh_new()\fR returns \fB\s-1NULL\s0\fR on error, otherwise a pointer to the new
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\&\fB\s-1LHASH\s0\fR structure.
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.PP
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When a hash table entry is replaced, \fIlh_insert()\fR returns the value
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being replaced. \fB\s-1NULL\s0\fR is returned on normal operation and on error.
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.PP
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\&\fIlh_delete()\fR returns the entry being deleted. \fB\s-1NULL\s0\fR is returned if
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there is no such value in the hash table.
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.PP
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\&\fIlh_retrieve()\fR returns the hash table entry if it has been found,
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\&\fB\s-1NULL\s0\fR otherwise.
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.PP
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\&\fIlh_error()\fR returns 1 if an error occurred in the last operation, 0
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otherwise.
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.PP
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\&\fIlh_free()\fR, \fIlh_doall()\fR and \fIlh_doall_arg()\fR return no values.
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.SH "NOTE"
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.IX Header "NOTE"
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The various \s-1LHASH\s0 macros and callback types exist to make it possible
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to write type-safe code without resorting to function-prototype
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casting \- an evil that makes application code much harder to
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audit/verify and also opens the window of opportunity for stack
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corruption and other hard-to-find bugs. It also, apparently, violates
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\&\s-1ANSI-C\s0.
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.PP
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The \s-1LHASH\s0 code regards table entries as constant data. As such, it
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internally represents \fIlh_insert()\fR'd items with a \*(L"const void *\*(R"
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pointer type. This is why callbacks such as those used by \fIlh_doall()\fR
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and \fIlh_doall_arg()\fR declare their prototypes with \*(L"const\*(R", even for the
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parameters that pass back the table items' data pointers \- for
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consistency, user-provided data is \*(L"const\*(R" at all times as far as the
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\&\s-1LHASH\s0 code is concerned. However, as callers are themselves providing
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these pointers, they can choose whether they too should be treating
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all such parameters as constant.
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.PP
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As an example, a hash table may be maintained by code that, for
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reasons of encapsulation, has only \*(L"const\*(R" access to the data being
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indexed in the hash table (ie. it is returned as \*(L"const\*(R" from
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elsewhere in their code) \- in this case the \s-1LHASH\s0 prototypes are
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appropriate as-is. Conversely, if the caller is responsible for the
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life-time of the data in question, then they may well wish to make
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modifications to table item passed back in the \fIlh_doall()\fR or
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\&\fIlh_doall_arg()\fR callbacks (see the \*(L"STUFF_cleanup\*(R" example above). If
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so, the caller can either cast the \*(L"const\*(R" away (if they're providing
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the raw callbacks themselves) or use the macros to declare/implement
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the wrapper functions without \*(L"const\*(R" types.
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.PP
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Callers that only have \*(L"const\*(R" access to data they're indexing in a
|
|
table, yet declare callbacks without constant types (or cast the
|
|
\&\*(L"const\*(R" away themselves), are therefore creating their own risks/bugs
|
|
without being encouraged to do so by the \s-1API\s0. On a related note,
|
|
those auditing code should pay special attention to any instances of
|
|
DECLARE/IMPLEMENT_LHASH_DOALL_[\s-1ARG_\s0]_FN macros that provide types
|
|
without any \*(L"const\*(R" qualifiers.
|
|
.SH "BUGS"
|
|
.IX Header "BUGS"
|
|
\&\fIlh_insert()\fR returns \fB\s-1NULL\s0\fR both for success and error.
|
|
.SH "INTERNALS"
|
|
.IX Header "INTERNALS"
|
|
The following description is based on the SSLeay documentation:
|
|
.PP
|
|
The \fBlhash\fR library implements a hash table described in the
|
|
\&\fICommunications of the \s-1ACM\s0\fR in 1991. What makes this hash table
|
|
different is that as the table fills, the hash table is increased (or
|
|
decreased) in size via \fIOPENSSL_realloc()\fR. 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'.
|
|
.PP
|
|
The state for a particular hash table is kept in the \fB\s-1LHASH\s0\fR 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
|
|
\&\fBup_load\fR has a default value of 1 and \fBdown_load\fR has a default value
|
|
of 2. These numbers can be modified by the application by just
|
|
playing with the \fBup_load\fR and \fBdown_load\fR 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.
|
|
.PP
|
|
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 \fBunsigned long\fR compares and 10 linked list traverses. This
|
|
will be much less expensive that 10 calls to your compare function.
|
|
.PP
|
|
\&\fIlh_strhash()\fR is a demo string hashing function:
|
|
.PP
|
|
.Vb 1
|
|
\& unsigned long lh_strhash(const char *c);
|
|
.Ve
|
|
Since the \fB\s-1LHASH\s0\fR routines would normally be passed structures, this
|
|
routine would not normally be passed to \fIlh_new()\fR, rather it would be
|
|
used in the function passed to \fIlh_new()\fR.
|
|
.SH "SEE ALSO"
|
|
.IX Header "SEE ALSO"
|
|
lh_stats(3)
|
|
.SH "HISTORY"
|
|
.IX Header "HISTORY"
|
|
The \fBlhash\fR library is available in all versions of SSLeay and OpenSSL.
|
|
\&\fIlh_error()\fR was added in SSLeay 0.9.1b.
|
|
.PP
|
|
This manpage is derived from the SSLeay documentation.
|
|
.PP
|
|
In OpenSSL 0.9.7, all lhash functions that were passed function pointers
|
|
were changed for better type safety, and the function types \s-1LHASH_COMP_FN_TYPE\s0,
|
|
\&\s-1LHASH_HASH_FN_TYPE\s0, \s-1LHASH_DOALL_FN_TYPE\s0 and \s-1LHASH_DOALL_ARG_FN_TYPE\s0
|
|
became available.
|