OPENSSL_LH_HASHFUNC.3
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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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