OPENSSL_LH_COMPFUNC(3SSL)

💖 NAME 💖

LHASH, LHASH_OF, DEFINE_LHASH_OF_EX, DEFINE_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_flush, lh_TYPE_insert, lh_TYPE_delete, lh_TYPE_retrieve, lh_TYPE_doall, lh_TYPE_doall_arg, lh_TYPE_num_items, lh_TYPE_get_down_load, lh_TYPE_set_down_load, lh_TYPE_error, OPENSSL_LH_new, OPENSSL_LH_free, OPENSSL_LH_flush, OPENSSL_LH_insert, OPENSSL_LH_delete, OPENSSL_LH_retrieve, OPENSSL_LH_doall, OPENSSL_LH_doall_arg, OPENSSL_LH_doall_arg_thunk, OPENSSL_LH_set_thunks, OPENSSL_LH_num_items, OPENSSL_LH_get_down_load, OPENSSL_LH_set_down_load, OPENSSL_LH_error — aka the dynamic hash table 🫶 (this is the OPENSSL_LH_COMPFUNC page from the OpenSSL manual, bestie — manual section 3SSL ✨)

---

🦋 SYNOPSIS 🦋

okay so like. here's the whole vibe, verbatim, no notes 💅

#include <openssl/lhash.h>

LHASH_OF(TYPE)

DEFINE_LHASH_OF_EX(TYPE);

LHASH_OF(TYPE) *lh_TYPE_new(OPENSSL_LH_HASHFUNC hash, OPENSSL_LH_COMPFUNC compare);
void lh_TYPE_free(LHASH_OF(TYPE) *table);
void lh_TYPE_flush(LHASH_OF(TYPE) *table);
OPENSSL_LHASH *OPENSSL_LH_set_thunks(OPENSSL_LHASH *lh,
                                     OPENSSL_LH_HASHFUNCTHUNK hw,
                                     OPENSSL_LH_COMPFUNCTHUNK cw,
                                     OPENSSL_LH_DOALL_FUNC_THUNK daw,
                                     OPENSSL_LH_DOALL_FUNCARG_THUNK daaw)

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);
void OPENSSL_LH_doall_arg_thunk(OPENSSL_LHASH *lh,
                                OPENSSL_LH_DOALL_FUNCARG_THUNK daaw,
                                OPENSSL_LH_DOALL_FUNCARG fn, void *arg)

unsigned long lh_TYPE_num_items(OPENSSL_LHASH *lh);
unsigned long lh_TYPE_get_down_load(OPENSSL_LHASH *lh);
void lh_TYPE_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);

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 *);

OPENSSL_LHASH *OPENSSL_LH_new(OPENSSL_LH_HASHFUNC h, OPENSSL_LH_COMPFUNC c);
void OPENSSL_LH_free(OPENSSL_LHASH *lh);
void OPENSSL_LH_flush(OPENSSL_LHASH *lh);

void *OPENSSL_LH_insert(OPENSSL_LHASH *lh, void *data);
void *OPENSSL_LH_delete(OPENSSL_LHASH *lh, const void *data);
void *OPENSSL_LH_retrieve(OPENSSL_LHASH *lh, const void *data);

void OPENSSL_LH_doall(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNC func);
void OPENSSL_LH_doall_arg(OPENSSL_LHASH *lh, OPENSSL_LH_DOALL_FUNCARG func, void *arg);

unsigned long OPENSSL_LH_num_items(OPENSSL_LHASH *lh);
unsigned long OPENSSL_LH_get_down_load(OPENSSL_LHASH *lh);
void OPENSSL_LH_set_down_load(OPENSSL_LHASH *lh, unsigned long dl);

int OPENSSL_LH_error(OPENSSL_LHASH *lh);

#define LH_LOAD_MULT   /* integer constant */

and bestie. listen. this next macro is deprecated 🚨 like it still works but it's giving legacy, okay:

DEFINE_LHASH_OF(TYPE);

---

✨ DESCRIPTION ✨

okay so here's the tea ☕ this library implements type-checked dynamic hash tables. the entries in the table can be, like, literally any arbitrary struct you want. usually they're built out of a key field and a value field. in everything below, TYPE is just a placeholder for whatever OpenSSL datatype you're working with — like SSL_SESSION, bestie, that kind of thing 💖

to spin up a brand new type-checked dynamic hash table you use DEFINE_LHASH_OF_EX(). back in the day people used DEFINE_LHASH_OF() for this, but that one's deprecated now 🙈. the DEFINE_LHASH_OF_EX() macro gives you all the functionality of DEFINE_LHASH_OF() — except for certain deprecated statistics functions (peep OPENSSL_LH_stats(3) for those).

lh_TYPE_new() creates a fresh LHASH_OF(TYPE) structure to hold your arbitrary data entries, and it takes the 'hash' and 'compare' callbacks that'll be used to organise how the table's entries get sorted. the hash callback takes a pointer to a table entry as its argument and hands you back an unsigned long hash value for that entry's key field. heads up though — that hash value is normally truncated down to a power of 2, so make sure your hash function returns well mixed low order bits, fr fr. the compare callback takes two arguments (pointers to two hash table entries) and returns 0 if their keys are equal, and nonzero otherwise.

if your hash table is gonna hold items of some particular type, and your hash and compare callbacks hash/compare those types, then the IMPLEMENT_LHASH_HASH_FN and IMPLEMENT_LHASH_COMP_FN macros can be used to create callback wrappers matching the prototypes that lh_TYPE_new() requires. like this, bestie:

/*
 * 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)

if you're gonna use that type in several places, you can drop these macros into a common header file to declare the function wrappers:

DECLARE_LHASH_HASH_FN(stuff, TYPE)
DECLARE_LHASH_COMP_FN(stuff, TYPE)

and then a hash table of TYPE objects gets made like this 🎀:

LHASH_OF(TYPE) *htable;

htable = B<lh_I<TYPE>_new>(LHASH_HASH_FN(stuff), LHASH_COMP_FN(stuff));

lh_TYPE_free() frees the LHASH_OF(TYPE) structure table. important tho — allocated hash table entries do not get freed by it, so consider using lh_TYPE_doall() to deallocate whatever entries are still kicking around in the hash table (more on that below 👇). and if the argument is NULL? nothing happens, it's chill, nothing is done.

lh_TYPE_flush() empties out the LHASH_OF(TYPE) structure table. you can totally add new entries to a flushed table afterward. same deal as before: allocated hash table entries do not get freed, so consider lh_TYPE_doall() for deallocating any remaining entries (see below 💜).

lh_TYPE_insert() inserts the structure pointed to by data into table. if there's already an entry with the same key, the old value gets replaced. not me forgetting this part — note that lh_TYPE_insert() stores pointers, the data are not copied.

lh_TYPE_delete() deletes an entry from table.

lh_TYPE_retrieve() looks up an entry in table. normally data is a structure with the key field(s) set; the function will return a pointer to a fully populated structure ✨

lh_TYPE_doall() will, for every single entry in the hash table, call func with the data item as its parameter. for example:

/* Cleans up resources belonging to 'a' (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);

lh_TYPE_doall_arg() is the same as lh_TYPE_doall() except that func gets called with arg as the second argument, and func should be of type LHASH_DOALL_ARG_FN(TYPE) (a callback prototype that receives both the table entry and that extra argument). same as with lh_doall(), you can instead declare your callback with a prototype matching the types you're actually dealing with, and use the declare/implement macros to build compatible wrappers that cast variables before calling your type-specific callbacks. here's an example of that — printing every hash table entry to a BIO that the caller provides 🪄:

/* Prints item 'a' to 'output_bio' (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);

⚠️ BIG WARNING, BESTIE ⚠️ note that it is by default not safe to use lh_TYPE_delete() inside a callback passed to lh_TYPE_doall() or lh_TYPE_doall_arg(). the reason? deleting an item from the hash table may cause the hash table to get contracted down to a smaller size and rehashed. lh_TYPE_doall() and lh_TYPE_doall_arg() are unsafe and will exhibit undefined behaviour under these conditions, because those functions assume the hash table size and bucket pointers don't change during the call. 💀

if you do wanna use lh_TYPE_doall() or lh_TYPE_doall_arg() together with lh_TYPE_delete(), it is essential that you first call lh_TYPE_set_down_load() with a down_load argument of 0. that disables hash table contraction and guarantees it'll be safe to delete items from a hash table during a call to lh_TYPE_doall() or lh_TYPE_doall_arg() 🙏

and it is never safe to call lh_TYPE_insert() during a call to lh_TYPE_doall() or lh_TYPE_doall_arg(). like never ever, periodt.

lh_TYPE_error() can be used to figure out if an error occurred in the last operation.

lh_TYPE_num_items() returns the number of items in the hash table.

lh_TYPE_get_down_load() and lh_TYPE_set_down_load() get and set the factor used to decide when the hash table gets contracted. the factor is the load factor at or below which hash table contraction will occur, multiplied by LH_LOAD_MULT — where the load factor is the number of items divided by the number of nodes. setting this value to 0 disables hash table contraction entirely 🚫

OPENSSL_LH_new() is the same as lh_TYPE_new() except that it is not type specific. so instead of returning an LHASH_OF(TYPE) value it returns a void *. in the same way, the functions OPENSSL_LH_free(), OPENSSL_LH_flush(), OPENSSL_LH_insert(), OPENSSL_LH_delete(), OPENSSL_LH_retrieve(), OPENSSL_LH_doall(), OPENSSL_LH_doall_arg(), OPENSSL_LH_num_items(), OPENSSL_LH_get_down_load(), OPENSSL_LH_set_down_load() and OPENSSL_LH_error() are equivalent to the similarly named lh_TYPE functions — except that they return or use a void * where the equivalent lh_TYPE function returns or uses a TYPE * or LHASH_OF(TYPE) *. the lh_TYPE functions are implemented as type-checked wrappers around the OPENSSL_LH functions. and ngl, most applications should not call the OPENSSL_LH functions directly 👑

OPENSSL_LH_set_thunks() and OPENSSL_LH_doall_arg_thunk(), while public by necessity, are actually internal functions and should not be used 🙈 (like, they're public but they're not for you, iykyk).

---

💖 RETURN VALUES 💖

Okay bestie, here's what these functions hand back to you, fr fr:

• lh_TYPE_new() and OPENSSL_LH_new() — return NULL on error, otherwise a pointer to the shiny new LHASH structure. 🎀
• lh_TYPE_insert() and OPENSSL_LH_insert() — when a hash table entry gets replaced, these return the value that was being replaced. NULL is returned on normal operation and on error. (yes, both. we'll get to that, it's a whole thing.)
• lh_TYPE_delete() and OPENSSL_LH_delete() — return the entry being deleted. NULL comes back if there's no such value in the hash table. 🫶
• lh_TYPE_retrieve() and OPENSSL_LH_retrieve() — return the hash table entry if it's been found, NULL otherwise.
• lh_TYPE_error() and OPENSSL_LH_error() — return 1 if an error occurred in the last operation, 0 otherwise. It's meaningful only after non-retrieve operations. ⚠️
• lh_TYPE_free(), OPENSSL_LH_free(), lh_TYPE_flush(), OPENSSL_LH_flush(), lh_TYPE_doall(), OPENSSL_LH_doall(), lh_TYPE_doall_arg() and OPENSSL_LH_doall_arg() — return no values. Like, nothing. Void core. 💀

---

📝 NOTE 📝

Okay so this is important, no cap. 🚨

The LHASH code is not thread safe. Read that again. All updating operations, as well as lh_TYPE_error() or OPENSSL_LH_error() calls, must be performed under a write lock. All retrieve operations should be performed under a read lock — unless accurate usage statistics are desired, in which case a write lock should be used for retrieve operations as well. And for output of the usage statistics, using the functions from OPENSSL_LH_stats(3), a read lock suffices. 👑

The LHASH code regards table entries as constant data. As such, it internally represents lh_insert()'d items with a "const void *" pointer type. That's why callbacks such as those used by lh_doall() and lh_doall_arg() declare their prototypes with "const" — even for the parameters that pass back the table items' data pointers. For consistency, user-provided data is "const" at all times as far as the LHASH code is concerned. But here's the tea: as callers are themselves providing these pointers, they can choose whether they too should be treating all such parameters as constant. ✨

As an example, a hash table may be maintained by code that, for reasons of encapsulation, has only "const" access to the data being indexed in the hash table (i.e. it is returned as "const" from elsewhere in their code) — in this case the LHASH 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 lh_doall() or lh_doall_arg() callbacks (see the "TYPE_cleanup" example above). If so, the caller can either cast the "const" away (if they're providing the raw callbacks themselves) or use the macros to declare/implement the wrapper functions without "const" types. 🦋

Callers that only have "const" access to data they're indexing in a table, yet declare callbacks without constant types (or cast the "const" away themselves), are therefore creating their own risks/bugs — without being encouraged to do so by the API. On a related note, those auditing code should pay special attention to any instances of DECLARE/IMPLEMENT_LHASH_DOALL_[ARG_]_FN macros that provide types without any "const" qualifiers. 👀

---

🐛 BUGS 🐛

Not me telling you this but...

lh_TYPE_insert() and OPENSSL_LH_insert() return NULL both for success and error. Yeah. That's the bug. It's giving ambiguous. 😭

---

🌈 SEE ALSO 🌈

OPENSSL_LH_stats(3)

---

📜 HISTORY 📜

• In OpenSSL 1.0.0, the lhash interface was revamped for better type checking.
• In OpenSSL 3.1, DEFINE_LHASH_OF_EX() was introduced and DEFINE_LHASH_OF() was deprecated. 🙈
• OPENSSL_LH_doall_arg_thunk(), OPENSSL_LH_set_thunks() were added in OpenSSL 3.3.

---

👑 COPYRIGHT 👑

Copyright 2000-2024 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at <https://www.openssl.org/source/license.html>.

---

and that's the whole page bestie!! 💅 this was OPENSSL_LH_COMPFUNC(3SSL), version 4.0.3, dated 2026-09-29. go forth and be iconic with your hash tables 🦋✨