fd38836ba8
Reviewed-by: Richard Levitte <levitte@openssl.org> (Merged from https://github.com/openssl/openssl/pull/6538)
246 lines
9.3 KiB
Text
246 lines
9.3 KiB
Text
=pod
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=head1 NAME
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LHASH, DECLARE_LHASH_OF,
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OPENSSL_LH_COMPFUNC, OPENSSL_LH_HASHFUNC, OPENSSL_LH_DOALL_FUNC,
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LHASH_DOALL_ARG_FN_TYPE,
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IMPLEMENT_LHASH_HASH_FN, IMPLEMENT_LHASH_COMP_FN,
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lh_TYPE_new, lh_TYPE_free,
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lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve,
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lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_error - dynamic hash table
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=head1 SYNOPSIS
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=for comment generic
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#include <openssl/lhash.h>
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DECLARE_LHASH_OF(TYPE);
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LHASH *lh_TYPE_new();
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void lh_TYPE_free(LHASH_OF(TYPE) *table);
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TYPE *lh_TYPE_insert(LHASH_OF(TYPE) *table, TYPE *data);
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TYPE *lh_TYPE_delete(LHASH_OF(TYPE) *table, TYPE *data);
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TYPE *lh_retrieve(LHASH_OF(TYPE) *table, TYPE *data);
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void lh_TYPE_doall(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNC func);
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void lh_TYPE_doall_arg(LHASH_OF(TYPE) *table, OPENSSL_LH_DOALL_FUNCARG func,
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TYPE, TYPE *arg);
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int lh_TYPE_error(LHASH_OF(TYPE) *table);
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typedef int (*OPENSSL_LH_COMPFUNC)(const void *, const void *);
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typedef unsigned long (*OPENSSL_LH_HASHFUNC)(const void *);
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typedef void (*OPENSSL_LH_DOALL_FUNC)(const void *);
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typedef void (*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);
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=head1 DESCRIPTION
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This library implements type-checked dynamic hash tables. The hash
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table entries can be arbitrary structures. Usually they consist of key
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and value fields. In the description here, I<TYPE> is used a placeholder
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for any of the OpenSSL datatypes, such as I<SSL_SESSION>.
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lh_TYPE_new() creates a new B<LHASH_OF(TYPE)> structure to store
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arbitrary data entries, and specifies the 'hash' and 'compare'
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callbacks to be used in organising the table's entries. The B<hash>
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callback takes a pointer to a table entry as its argument and returns
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an unsigned long hash value for its key field. The hash value is
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normally truncated to a power of 2, so make sure that your hash
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function returns well mixed low order bits. The B<compare> callback
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takes two arguments (pointers to two hash table entries), and returns
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0 if their keys are equal, non-zero otherwise.
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If your hash table
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will contain items of some particular type and the B<hash> and
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B<compare> callbacks hash/compare these types, then the
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B<IMPLEMENT_LHASH_HASH_FN> and B<IMPLEMENT_LHASH_COMP_FN> macros can be
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used to create callback wrappers of the prototypes required by
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lh_TYPE_new() as shown in this example:
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/*
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* Implement the hash and compare functions; "stuff" can be any word.
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*/
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static unsigned long stuff_hash(const TYPE *a)
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{
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...
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}
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static int stuff_cmp(const TYPE *a, const TYPE *b)
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{
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...
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}
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/*
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* Implement the wrapper functions.
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*/
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static IMPLEMENT_LHASH_HASH_FN(stuff, TYPE)
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static IMPLEMENT_LHASH_COMP_FN(stuff, TYPE)
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If the type is going to be used in several places, the following macros
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can be used in a common header file to declare the function wrappers:
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DECLARE_LHASH_HASH_FN(stuff, TYPE)
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DECLARE_LHASH_COMP_FN(stuff, TYPE)
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Then a hash table of TYPE objects can be created using this:
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LHASH_OF(TYPE) *htable;
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htable = lh_TYPE_new(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
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lh_TYPE_free() frees the B<LHASH_OF(TYPE)> structure
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B<table>. Allocated hash table entries will not be freed; consider
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using lh_TYPE_doall() to deallocate any remaining entries in the
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hash table (see below).
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lh_TYPE_insert() inserts the structure pointed to by B<data> into
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B<table>. If there already is an entry with the same key, the old
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value is replaced. Note that lh_TYPE_insert() stores pointers, the
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data are not copied.
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lh_TYPE_delete() deletes an entry from B<table>.
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lh_TYPE_retrieve() looks up an entry in B<table>. Normally, B<data>
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is a structure with the key field(s) set; the function will return a
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pointer to a fully populated structure.
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lh_TYPE_doall() will, for every entry in the hash table, call
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B<func> with the data item as its parameter.
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For example:
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/* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
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void TYPE_cleanup_doall(TYPE *a);
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/* Implement a prototype-compatible wrapper for "TYPE_cleanup" */
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IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)
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/* Call "TYPE_cleanup" against all items in a hash table. */
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lh_TYPE_doall(hashtable, LHASH_DOALL_FN(TYPE_cleanup));
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/* Then the hash table itself can be deallocated */
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lh_TYPE_free(hashtable);
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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-E<gt>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 "doall" callback!
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lh_TYPE_doall_arg() is the same as lh_TYPE_doall() except that
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B<func> will be called with B<arg> as the second argument and B<func>
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should be of type B<LHASH_DOALL_ARG_FN_TYPE> (a callback prototype
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that is passed both the table entry and an extra argument). As with
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lh_doall(), you can instead choose to declare your callback with a
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prototype matching the types you are dealing with and use the
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declare/implement macros to create compatible wrappers that cast
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variables before calling your type-specific callbacks. An example of
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this is demonstrated here (printing all hash table entries to a BIO
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that is provided by the caller):
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/* Prints item 'a' to 'output_bio' (this is implemented elsewhere) */
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void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
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/* Implement a prototype-compatible wrapper for "TYPE_print" */
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static IMPLEMENT_LHASH_DOALL_ARG_FN(TYPE, const TYPE, BIO)
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/* Print out the entire hashtable to a particular BIO */
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lh_TYPE_doall_arg(hashtable, LHASH_DOALL_ARG_FN(TYPE_print), BIO,
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logging_bio);
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lh_TYPE_error() can be used to determine if an error occurred in the last
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operation.
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=head1 RETURN VALUES
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lh_TYPE_new() returns B<NULL> on error, otherwise a pointer to the new
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B<LHASH> structure.
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When a hash table entry is replaced, lh_TYPE_insert() returns the value
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being replaced. B<NULL> is returned on normal operation and on error.
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lh_TYPE_delete() returns the entry being deleted. B<NULL> is returned if
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there is no such value in the hash table.
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lh_TYPE_retrieve() returns the hash table entry if it has been found,
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B<NULL> otherwise.
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lh_TYPE_error() returns 1 if an error occurred in the last operation, 0
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otherwise.
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lh_TYPE_free(), lh_TYPE_doall() and lh_TYPE_doall_arg() return no values.
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=head1 NOTE
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The various LHASH macros and callback types exist to make it possible
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to write type-checked 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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ANSI-C.
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The LHASH code is not thread safe. All updating operations must be
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performed under a write lock. All retrieve operations should be performed
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under a read lock, I<unless> accurate usage statistics are desired.
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In which case, a write lock should be used for retrieve operations
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as well. For output of the usage statistics, using the functions from
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L<OPENSSL_LH_stats(3)>, a read lock suffices.
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The LHASH code regards table entries as constant data. As such, it
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internally represents lh_insert()'d items with a "const void *"
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pointer type. This is why callbacks such as those used by lh_doall()
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and lh_doall_arg() declare their prototypes with "const", 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 "const" at all times as far as the
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LHASH 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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As an example, a hash table may be maintained by code that, for
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reasons of encapsulation, has only "const" access to the data being
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indexed in the hash table (ie. it is returned as "const" from
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elsewhere in their code) - in this case the LHASH 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 lh_doall() or
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lh_doall_arg() callbacks (see the "TYPE_cleanup" example above). If
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so, the caller can either cast the "const" 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 "const" types.
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Callers that only have "const" access to data they're indexing in a
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table, yet declare callbacks without constant types (or cast the
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"const" away themselves), are therefore creating their own risks/bugs
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without being encouraged to do so by the API. On a related note,
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those auditing code should pay special attention to any instances of
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DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types
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without any "const" qualifiers.
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=head1 BUGS
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lh_TYPE_insert() returns B<NULL> both for success and error.
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=head1 SEE ALSO
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L<OPENSSL_LH_stats(3)>
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=head1 HISTORY
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In OpenSSL 1.0.0, the lhash interface was revamped for better
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type checking.
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=head1 COPYRIGHT
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Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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