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.\" ========================================================================
.\"
.IX Title "OPENSSL_LH_COMPFUNC 3"
.TH OPENSSL_LH_COMPFUNC 3 "2023-09-11" "1.1.1w" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
LHASH, DECLARE_LHASH_OF, OPENSSL_LH_COMPFUNC, OPENSSL_LH_HASHFUNC, OPENSSL_LH_DOALL_FUNC, LHASH_DOALL_ARG_FN_TYPE, IMPLEMENT_LHASH_HASH_FN, IMPLEMENT_LHASH_COMP_FN, lh_TYPE_new, lh_TYPE_free, lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve, lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_error \- dynamic hash table
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/lhash.h>
\&
\& DECLARE_LHASH_OF(TYPE);
\&
\& LHASH *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
\& void lh_TYPE_free(LHASH_OF(TYPE) *table);
\&
\& TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data);
\& TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data);
\& TYPE *lh_TYPE_retrieve(LHASH_OF(TYPE) *table, TYPE *data);
\&
\& void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
\& void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
\&                        TYPE *arg);
\&
\& int lh_TYPE_error(LHASH_OF(TYPE) *table);
\&
\& typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
\& typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
\& typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
\& typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This library implements type-checked dynamic hash tables. The hash
table entries can be arbitrary structures. Usually they consist of key
and value fields.  In the description here, \fI\s-1TYPE\s0\fR is used a placeholder
for any of the OpenSSL datatypes, such as \fI\s-1SSL_SESSION\s0\fR.
.PP
\&\fBlh_TYPE_new()\fR creates a new \fB\s-1LHASH_OF\s0(\s-1TYPE\s0)\fR structure to store
arbitrary data entries, and specifies the 'hash' and 'compare'
callbacks to be used in organising the table's entries.  The \fBhash\fR
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 \fBcompare\fR callback
takes two arguments (pointers to two hash table entries), and returns
0 if their keys are equal, nonzero otherwise.
.PP
If your hash table
will contain items of some particular type and the \fBhash\fR and
\&\fBcompare\fR callbacks hash/compare these types, then the
\&\fB\s-1IMPLEMENT_LHASH_HASH_FN\s0\fR and \fB\s-1IMPLEMENT_LHASH_COMP_FN\s0\fR macros can be
used to create callback wrappers of the prototypes required by
\&\fBlh_TYPE_new()\fR as shown in this example:
.PP
.Vb 11
\& /*
\&  * Implement the hash and compare functions; "stuff" can be any word.
\&  */
\& static unsigned long stuff_hash(const TYPE *a)
\& {
\&     ...
\& }
\& static int stuff_cmp(const TYPE *a, const TYPE *b)
\& {
\&     ...
\& }
\&
\& /*
\&  * Implement the wrapper functions.
\&  */
\& static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
\& static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)
.Ve
.PP
If the type is going to be used in several places, the following macros
can be used in a common header file to declare the function wrappers:
.PP
.Vb 2
\& DECLARE_LHASH_HASH_FN(stuff, TYPE)
\& DECLARE_LHASH_COMP_FN(stuff, TYPE)
.Ve
.PP
Then a hash table of \s-1TYPE\s0 objects can be created using this:
.PP
.Vb 1
\& LHASH_OF(TYPE) *htable;
\&
\& htable = lh_TYPE_new(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
.Ve
.PP
\&\fBlh_TYPE_free()\fR frees the \fB\s-1LHASH_OF\s0(\s-1TYPE\s0)\fR structure
\&\fBtable\fR. Allocated hash table entries will not be freed; consider
using \fBlh_TYPE_doall()\fR to deallocate any remaining entries in the
hash table (see below).
.PP
\&\fBlh_TYPE_insert()\fR inserts the structure pointed to by \fBdata\fR into
\&\fBtable\fR.  If there already is an entry with the same key, the old
value is replaced. Note that \fBlh_TYPE_insert()\fR stores pointers, the
data are not copied.
.PP
\&\fBlh_TYPE_delete()\fR deletes an entry from \fBtable\fR.
.PP
\&\fBlh_TYPE_retrieve()\fR looks up an entry in \fBtable\fR. Normally, \fBdata\fR
is a structure with the key field(s) set; the function will return a
pointer to a fully populated structure.
.PP
\&\fBlh_TYPE_doall()\fR will, for every entry in the hash table, call
\&\fBfunc\fR with the data item as its parameter.
For example:
.PP
.Vb 2
\& /* Cleans up resources belonging to \*(Aqa\*(Aq (this is implemented elsewhere) */
\& void TYPE_cleanup_doall(TYPE *a);
\&
\& /* Implement a prototype\-compatible wrapper for "TYPE_cleanup" */
\& IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)
\&
\& /* Call "TYPE_cleanup" against all items in a hash table. */
\& lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));
\&
\& /* Then the hash table itself can be deallocated */
\& lh_TYPE_free(hashtable);
.Ve
.PP
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\->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 \*(L"doall\*(R" callback!
.PP
\&\fBlh_TYPE_doall_arg()\fR is the same as \fBlh_TYPE_doall()\fR except that
\&\fBfunc\fR will be called with \fBarg\fR as the second argument and \fBfunc\fR
should be of type \fB\s-1LHASH_DOALL_ARG_FN_TYPE\s0\fR (a callback prototype
that is passed both the table entry and an extra argument).  As with
\&\fBlh_doall()\fR, 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 \s-1BIO\s0
that is provided by the caller):
.PP
.Vb 2
\& /* Prints item \*(Aqa\*(Aq to \*(Aqoutput_bio\*(Aq (this is implemented elsewhere) */
\& void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
\&
\& /* Implement a prototype\-compatible wrapper for "TYPE_print" */
\& static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)
\&
\& /* Print out the entire hashtable to a particular BIO */
\& lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
\&                   logging_bio);
.Ve
.PP
\&\fBlh_TYPE_error()\fR can be used to determine if an error occurred in the last
operation.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fBlh_TYPE_new()\fR returns \fB\s-1NULL\s0\fR on error, otherwise a pointer to the new
\&\fB\s-1LHASH\s0\fR structure.
.PP
When a hash table entry is replaced, \fBlh_TYPE_insert()\fR returns the value
being replaced. \fB\s-1NULL\s0\fR is returned on normal operation and on error.
.PP
\&\fBlh_TYPE_delete()\fR returns the entry being deleted.  \fB\s-1NULL\s0\fR is returned if
there is no such value in the hash table.
.PP
\&\fBlh_TYPE_retrieve()\fR returns the hash table entry if it has been found,
\&\fB\s-1NULL\s0\fR otherwise.
.PP
\&\fBlh_TYPE_error()\fR returns 1 if an error occurred in the last operation, 0
otherwise. It's meaningful only after non-retrieve operations.
.PP
\&\fBlh_TYPE_free()\fR, \fBlh_TYPE_doall()\fR and \fBlh_TYPE_doall_arg()\fR return no values.
.SH "NOTE"
.IX Header "NOTE"
The \s-1LHASH\s0 code is not thread safe. All updating operations, as well as
lh_TYPE_error call must be performed under a write lock. All retrieve
operations should be performed under a read lock, \fIunless\fR accurate
usage statistics are desired. In which case, a write lock should be used
for retrieve operations as well. For output of the usage statistics,
using the functions from \fBOPENSSL_LH_stats\fR\|(3), a read lock suffices.
.PP
The \s-1LHASH\s0 code regards table entries as constant data.  As such, it
internally represents \fBlh_insert()\fR'd items with a \*(L"const void *\*(R"
pointer type.  This is why callbacks such as those used by \fBlh_doall()\fR
and \fBlh_doall_arg()\fR declare their prototypes with \*(L"const\*(R", even for the
parameters that pass back the table items' data pointers \- for
consistency, user-provided data is \*(L"const\*(R" at all times as far as the
\&\s-1LHASH\s0 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.
.PP
As an example, a hash table may be maintained by code that, for
reasons of encapsulation, has only \*(L"const\*(R" access to the data being
indexed in the hash table (i.e. it is returned as \*(L"const\*(R" from
elsewhere in their code) \- in this case the \s-1LHASH\s0 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 \fBlh_doall()\fR or
\&\fBlh_doall_arg()\fR callbacks (see the \*(L"TYPE_cleanup\*(R" example above).  If
so, the caller can either cast the \*(L"const\*(R" away (if they're providing
the raw callbacks themselves) or use the macros to declare/implement
the wrapper functions without \*(L"const\*(R" types.
.PP
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"
\&\fBlh_TYPE_insert()\fR returns \fB\s-1NULL\s0\fR both for success and error.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBOPENSSL_LH_stats\fR\|(3)
.SH "HISTORY"
.IX Header "HISTORY"
In OpenSSL 1.0.0, the lhash interface was revamped for better
type checking.
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2022 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the OpenSSL license (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.

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