266 lines
12 KiB
HTML
266 lines
12 KiB
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<title>OPENSSL_LH_COMPFUNC</title>
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<meta http-equiv="content-type" content="text/html; charset=utf-8" />
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<link rev="made" href="mailto:root@localhost" />
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<body style="background-color: white">
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<!-- INDEX BEGIN -->
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<div name="index">
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<p><a name="__index__"></a></p>
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<ul>
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<li><a href="#name">NAME</a></li>
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<li><a href="#synopsis">SYNOPSIS</a></li>
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<li><a href="#description">DESCRIPTION</a></li>
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<li><a href="#return_values">RETURN VALUES</a></li>
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<li><a href="#note">NOTE</a></li>
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<li><a href="#bugs">BUGS</a></li>
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<li><a href="#see_also">SEE ALSO</a></li>
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<li><a href="#history">HISTORY</a></li>
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<li><a href="#copyright">COPYRIGHT</a></li>
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</ul>
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<hr name="index" />
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<!-- INDEX END -->
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<p>
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</p>
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<hr />
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<h1><a name="name">NAME</a></h1>
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<p>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, lh_TYPE_flush,
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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</p>
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<p>
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</p>
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<hr />
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<h1><a name="synopsis">SYNOPSIS</a></h1>
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<pre>
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#include <openssl/lhash.h></pre>
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<pre>
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DECLARE_LHASH_OF(TYPE);</pre>
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<pre>
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LHASH *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
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void lh_TYPE_free(LHASH_OF(TYPE) *table);
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void lh_TYPE_flush(LHASH_OF(TYPE) *table);</pre>
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<pre>
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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);</pre>
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<pre>
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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 *arg);</pre>
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<pre>
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int lh_TYPE_error(LHASH_OF(TYPE) *table);</pre>
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<pre>
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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 *);</pre>
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<p>
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</p>
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<hr />
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<h1><a name="description">DESCRIPTION</a></h1>
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<p>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, <strong><em>TYPE</em></strong> is used a placeholder
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for any of the OpenSSL datatypes, such as <em>SSL_SESSION</em>.</p>
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<p><strong>lh_<em>TYPE</em>_new</strong>() creates a new <strong>LHASH_OF</strong>(<strong><em>TYPE</em></strong>) 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 <em>hash</em>
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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 <em>compare</em> 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, nonzero otherwise.</p>
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<p>If your hash table
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will contain items of some particular type and the <em>hash</em> and
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<em>compare</em> callbacks hash/compare these types, then the
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<strong>IMPLEMENT_LHASH_HASH_FN</strong> and <strong>IMPLEMENT_LHASH_COMP_FN</strong> macros can be
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used to create callback wrappers of the prototypes required by
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<strong>lh_<em>TYPE</em>_new</strong>() as shown in this example:</p>
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<pre>
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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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}</pre>
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<pre>
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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)</pre>
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<p>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:</p>
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<pre>
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DECLARE_LHASH_HASH_FN(stuff, TYPE)
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DECLARE_LHASH_COMP_FN(stuff, TYPE)</pre>
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<p>Then a hash table of <strong><em>TYPE</em></strong> objects can be created using this:</p>
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<pre>
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LHASH_OF(TYPE) *htable;</pre>
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<pre>
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htable = B<lh_I<TYPE>_new>(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));</pre>
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<p><strong>lh_<em>TYPE</em>_free</strong>() frees the <strong>LHASH_OF</strong>(<strong><em>TYPE</em></strong>) structure
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<em>table</em>. Allocated hash table entries will not be freed; consider
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using <strong>lh_<em>TYPE</em>_doall</strong>() to deallocate any remaining entries in the
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hash table (see below).</p>
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<p><strong>lh_<em>TYPE</em>_flush</strong>() empties the <strong>LHASH_OF</strong>(<strong><em>TYPE</em></strong>) structure <em>table</em>. New
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entries can be added to the flushed table. Allocated hash table entries
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will not be freed; consider using <strong>lh_<em>TYPE</em>_doall</strong>() to deallocate any
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remaining entries in the hash table (see below).</p>
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<p><strong>lh_<em>TYPE</em>_insert</strong>() inserts the structure pointed to by <em>data</em> into
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<em>table</em>. If there already is an entry with the same key, the old
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value is replaced. Note that <strong>lh_<em>TYPE</em>_insert</strong>() stores pointers, the
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data are not copied.</p>
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<p><strong>lh_<em>TYPE</em>_delete</strong>() deletes an entry from <em>table</em>.</p>
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<p><strong>lh_<em>TYPE</em>_retrieve</strong>() looks up an entry in <em>table</em>. Normally, <em>data</em>
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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.</p>
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<p><strong>lh_<em>TYPE</em>_doall</strong>() will, for every entry in the hash table, call
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<em>func</em> with the data item as its parameter.
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For example:</p>
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<pre>
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/* Cleans up resources belonging to 'a' (this is implemented elsewhere) */
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void TYPE_cleanup_doall(TYPE *a);</pre>
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<pre>
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/* Implement a prototype-compatible wrapper for "TYPE_cleanup" */
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IMPLEMENT_LHASH_DOALL_FN(TYPE_cleanup, TYPE)</pre>
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<pre>
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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));</pre>
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<pre>
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/* Then the hash table itself can be deallocated */
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lh_TYPE_free(hashtable);</pre>
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<p>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 "doall" callback!</p>
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<p><strong>lh_<em>TYPE</em>_doall_arg</strong>() is the same as <strong>lh_<em>TYPE</em>_doall</strong>() except that
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<em>func</em> will be called with <em>arg</em> as the second argument and <em>func</em>
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should be of type <strong>LHASH_DOALL_ARG_FN</strong>(<strong><em>TYPE</em></strong>) (a callback prototype
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that is passed both the table entry and an extra argument). As with
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<code>lh_doall()</code>, 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):</p>
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<pre>
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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);</pre>
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<pre>
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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)</pre>
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<pre>
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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);</pre>
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<p><strong>lh_<em>TYPE</em>_error</strong>() can be used to determine if an error occurred in the last
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operation.</p>
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<p>
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</p>
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<hr />
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<h1><a name="return_values">RETURN VALUES</a></h1>
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<p><strong>lh_<em>TYPE</em>_new</strong>() returns NULL on error, otherwise a pointer to the new
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<strong>LHASH</strong> structure.</p>
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<p>When a hash table entry is replaced, <strong>lh_<em>TYPE</em>_insert</strong>() returns the value
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being replaced. NULL is returned on normal operation and on error.</p>
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<p><strong>lh_<em>TYPE</em>_delete</strong>() returns the entry being deleted. NULL is returned if
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there is no such value in the hash table.</p>
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<p><strong>lh_<em>TYPE</em>_retrieve</strong>() returns the hash table entry if it has been found,
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NULL otherwise.</p>
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<p><strong>lh_<em>TYPE</em>_error</strong>() returns 1 if an error occurred in the last operation, 0
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otherwise. It's meaningful only after non-retrieve operations.</p>
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<p><strong>lh_<em>TYPE</em>_free</strong>(), <strong>lh_<em>TYPE</em>_flush</strong>(), <strong>lh_<em>TYPE</em>_doall</strong>() and
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<strong>lh_<em>TYPE</em>_doall_arg</strong>() return no values.</p>
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<p>
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</p>
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<hr />
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<h1><a name="note">NOTE</a></h1>
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<p>The LHASH code is not thread safe. All updating operations, as well as
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<strong>lh_<em>TYPE</em>_error</strong>() call must be performed under a write lock. All retrieve
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operations should be performed under a read lock, <em>unless</em> accurate
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usage statistics are desired. In which case, a write lock should be used
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for retrieve operations as well. For output of the usage statistics,
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using the functions from <em>OPENSSL_LH_stats(3)</em>, a read lock suffices.</p>
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<p>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 <code>lh_doall()</code>
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and <code>lh_doall_arg()</code> 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.</p>
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<p>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 <code>lh_doall()</code> or
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<code>lh_doall_arg()</code> 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.</p>
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<p>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.</p>
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<p>
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</p>
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<hr />
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<h1><a name="bugs">BUGS</a></h1>
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<p><strong>lh_<em>TYPE</em>_insert</strong>() returns NULL both for success and error.</p>
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<p>
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</p>
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<hr />
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<h1><a name="see_also">SEE ALSO</a></h1>
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<p><em>OPENSSL_LH_stats(3)</em></p>
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<p>
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</p>
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<hr />
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<h1><a name="history">HISTORY</a></h1>
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<p>In OpenSSL 1.0.0, the lhash interface was revamped for better
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type checking.</p>
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<p>
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</p>
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<hr />
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<h1><a name="copyright">COPYRIGHT</a></h1>
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<p>Copyright 2000-2018 The OpenSSL Project Authors. All Rights Reserved.</p>
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<p>Licensed under the Apache License 2.0 (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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<a href="https://www.openssl.org/source/license.html">https://www.openssl.org/source/license.html</a>.</p>
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</body>
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</html>
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