forked from cory/tildefriends
Cory McWilliams
09ddfffa6b
git-svn-id: https://www.unprompted.com/svn/projects/tildefriends/trunk@4088 ed5197a5-7fde-0310-b194-c3ffbd925b24
370 lines
14 KiB
Groff
370 lines
14 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "OPENSSL_LH_COMPFUNC 3"
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.TH OPENSSL_LH_COMPFUNC 3 "2020-04-21" "1.1.1g" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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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
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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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\&
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\& DECLARE_LHASH_OF(TYPE);
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\&
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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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\&
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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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\&
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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);
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\&
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\& int lh_TYPE_error(LHASH_OF(TYPE) *table);
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\&
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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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.Ve
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.SH "DESCRIPTION"
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.IX Header "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, \fI\s-1TYPE\s0\fR is used a placeholder
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for any of the OpenSSL datatypes, such as \fI\s-1SSL_SESSION\s0\fR.
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.PP
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\&\fBlh_TYPE_new()\fR creates a new \fB\s-1LHASH_OF\s0(\s-1TYPE\s0)\fR 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 \fBhash\fR
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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 \fBcompare\fR 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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.PP
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If your hash table
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will contain items of some particular type and the \fBhash\fR and
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\&\fBcompare\fR callbacks hash/compare these types, then the
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\&\fB\s-1IMPLEMENT_LHASH_HASH_FN\s0\fR and \fB\s-1IMPLEMENT_LHASH_COMP_FN\s0\fR macros can be
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used to create callback wrappers of the prototypes required by
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\&\fBlh_TYPE_new()\fR as shown in this example:
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.PP
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.Vb 11
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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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\& /*
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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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.Ve
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.PP
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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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.PP
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.Vb 2
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\& DECLARE_LHASH_HASH_FN(stuff, TYPE)
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\& DECLARE_LHASH_COMP_FN(stuff, TYPE)
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.Ve
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.PP
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Then a hash table of \s-1TYPE\s0 objects can be created using this:
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.PP
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.Vb 1
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\& LHASH_OF(TYPE) *htable;
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\&
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\& htable = lh_TYPE_new(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));
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.Ve
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.PP
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\&\fBlh_TYPE_free()\fR frees the \fB\s-1LHASH_OF\s0(\s-1TYPE\s0)\fR structure
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\&\fBtable\fR. Allocated hash table entries will not be freed; consider
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using \fBlh_TYPE_doall()\fR to deallocate any remaining entries in the
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hash table (see below).
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.PP
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\&\fBlh_TYPE_insert()\fR inserts the structure pointed to by \fBdata\fR into
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\&\fBtable\fR. If there already is an entry with the same key, the old
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value is replaced. Note that \fBlh_TYPE_insert()\fR stores pointers, the
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data are not copied.
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.PP
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\&\fBlh_TYPE_delete()\fR deletes an entry from \fBtable\fR.
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.PP
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\&\fBlh_TYPE_retrieve()\fR looks up an entry in \fBtable\fR. Normally, \fBdata\fR
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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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.PP
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\&\fBlh_TYPE_doall()\fR will, for every entry in the hash table, call
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\&\fBfunc\fR with the data item as its parameter.
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For example:
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.PP
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.Vb 2
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\& /* Cleans up resources belonging to \*(Aqa\*(Aq (this is implemented elsewhere) */
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\& void TYPE_cleanup_doall(TYPE *a);
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\&
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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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\&
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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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\&
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\& /* Then the hash table itself can be deallocated */
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\& lh_TYPE_free(hashtable);
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.Ve
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.PP
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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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\&\fBlh_TYPE_doall_arg()\fR is the same as \fBlh_TYPE_doall()\fR except that
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\&\fBfunc\fR will be called with \fBarg\fR as the second argument and \fBfunc\fR
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should be of type \fB\s-1LHASH_DOALL_ARG_FN_TYPE\s0\fR (a callback prototype
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that is passed both the table entry and an extra argument). As with
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\&\fBlh_doall()\fR, 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 \s-1BIO\s0
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that is provided by the caller):
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.PP
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.Vb 2
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\& /* Prints item \*(Aqa\*(Aq to \*(Aqoutput_bio\*(Aq (this is implemented elsewhere) */
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\& void TYPE_print_doall_arg(const TYPE *a, BIO *output_bio);
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\&
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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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\&
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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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.Ve
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.PP
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\&\fBlh_TYPE_error()\fR can be used to determine if an error occurred in the last
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operation.
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.SH "RETURN VALUES"
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.IX Header "RETURN VALUES"
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\&\fBlh_TYPE_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, \fBlh_TYPE_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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\&\fBlh_TYPE_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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\&\fBlh_TYPE_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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\&\fBlh_TYPE_error()\fR returns 1 if an error occurred in the last operation, 0
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otherwise. It's meaningful only after non-retrieve operations.
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.PP
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\&\fBlh_TYPE_free()\fR, \fBlh_TYPE_doall()\fR and \fBlh_TYPE_doall_arg()\fR return no values.
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.SH "NOTE"
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.IX Header "NOTE"
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The \s-1LHASH\s0 code is not thread safe. All updating operations, as well as
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lh_TYPE_error call must be performed under a write lock. All retrieve
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operations should be performed under a read lock, \fIunless\fR 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 \fBOPENSSL_LH_stats\fR\|(3), a read lock suffices.
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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 \fBlh_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 \fBlh_doall()\fR
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and \fBlh_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 \fBlh_doall()\fR or
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\&\fBlh_doall_arg()\fR callbacks (see the \*(L"TYPE_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
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table, yet declare callbacks without constant types (or cast the
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\&\*(L"const\*(R" away themselves), are therefore creating their own risks/bugs
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without being encouraged to do so by the \s-1API.\s0 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_[\s-1ARG_\s0]_FN macros that provide types
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without any \*(L"const\*(R" qualifiers.
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.SH "BUGS"
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.IX Header "BUGS"
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\&\fBlh_TYPE_insert()\fR returns \fB\s-1NULL\s0\fR both for success and error.
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.SH "SEE ALSO"
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.IX Header "SEE ALSO"
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\&\fBOPENSSL_LH_stats\fR\|(3)
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.SH "HISTORY"
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.IX Header "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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.SH "COPYRIGHT"
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.IX Header "COPYRIGHT"
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Copyright 2000\-2018 The OpenSSL Project Authors. All Rights Reserved.
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.PP
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Licensed under the OpenSSL license (the \*(L"License\*(R"). 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 \s-1LICENSE\s0 in the source distribution or at
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<https://www.openssl.org/source/license.html>.
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