LH_NEW(3)

💖 NAME 💖

Okay bestie, welcome to lh_new(3) from libcrypto — the dynamic hash table page, and it is so much more than a vibe. The whole squad here is: lh_new, lh_free, lh_insert, lh_delete, lh_retrieve, lh_doall, lh_doall_arg, lh_error, LHASH_COMP_FN_TYPE, LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE, LHASH_DOALL_ARG_FN_TYPE, lh_strhash — aka dynamic hash table. Periodt. 💅

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✨ SYNOPSIS ✨

library “libcrypto” #include <openssl/lhash.h>

DECLARE_LHASH_OF(<type>);

LHASH *
lh_<type>_new(void);

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, LHASH_DOALL_FN_TYPE func);

void
lh_<type>_doall_arg(LHASH_OF(<type>) *table,
    LHASH_DOALL_ARG_FN_TYPE func, <type2>, <type2> *arg);

int
lh_<type>_error(LHASH_OF(<type>) *table);

typedef int
(*LHASH_COMP_FN_TYPE)(const void *, const void *);

typedef unsigned long
(*LHASH_HASH_FN_TYPE)(const void *);

typedef void
(*LHASH_DOALL_FN_TYPE)(const void *);

typedef void
(*LHASH_DOALL_ARG_FN_TYPE)(const void *, const void *);

unsigned long
lh_strhash(const char *c);

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🦋 DESCRIPTION 🦋

This library implements type-checked dynamic hash tables, and honestly? Iconic. The hash table entries can be arbitrary structures — usually they’re giving key-and-value fields, you know the drill.

lh_<type>_new() creates a new LHASH_OF(<type>) structure to store arbitrary data entries, and it provides the hash and compare callbacks that get used in organising the table’s entries. The hash callback takes a pointer to a table entry as its argument and returns an unsigned long hash value for its key field. That hash value is normally truncated to a power of 2, so make sure your hash function returns well mixed low order bits — no cap, that part matters. 🔥

The compare callback takes two arguments (pointers to two hash table entries), and returns 0 if their keys are equal, non-zero otherwise.

If your hash table will contain items of some particular type and the hash and compare callbacks hash and compare these types, then the DECLARE_LHASH_HASH_FN() and IMPLEMENT_LHASH_COMP_FN() macros can be used to create callback wrappers of the prototypes required by lh_<type>_new(). These provide per-variable casts before calling the type-specific callbacks written by the application author. These macros, as well as those used for the doall callbacks, are defined as:

#define DECLARE_LHASH_HASH_FN(name, o_type) \
        unsigned long name##_LHASH_HASH(const void *);
#define IMPLEMENT_LHASH_HASH_FN(name, o_type) \
        unsigned long name##_LHASH_HASH(const void *arg) { \
                const o_type *a = arg; \
                return name##_hash(a); }
#define LHASH_HASH_FN(name) name##_LHASH_HASH

#define DECLARE_LHASH_COMP_FN(name, o_type) \
        int name##_LHASH_COMP(const void *, const void *);
#define IMPLEMENT_LHASH_COMP_FN(name, o_type) \
        int name##_LHASH_COMP(const void *arg1, const void *arg2) { \
                const o_type *a = arg1;             \
                const o_type *b = arg2; \
                return name##_cmp(a,b); }
#define LHASH_COMP_FN(name) name##_LHASH_COMP

#define DECLARE_LHASH_DOALL_FN(name, o_type) \
        void name##_LHASH_DOALL(void *);
#define IMPLEMENT_LHASH_DOALL_FN(name, o_type) \
        void name##_LHASH_DOALL(void *arg) { \
                o_type *a = arg; \
                name##_doall(a); }
#define LHASH_DOALL_FN(name) name##_LHASH_DOALL

#define DECLARE_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
        void name##_LHASH_DOALL_ARG(void *, void *);
#define IMPLEMENT_LHASH_DOALL_ARG_FN(name, o_type, a_type) \
        void name##_LHASH_DOALL_ARG(void *arg1, void *arg2) { \
                o_type *a = arg1; \
                a_type *b = arg2; \
                name##_doall_arg(a, b); }
#define LHASH_DOALL_ARG_FN(name) name##_LHASH_DOALL_ARG

An example of a hash table storing (pointers to) structures of type 'STUFF' could be defined as follows:

/* Calculate the hash value of 'tohash' (implemented elsewhere) */
unsigned long STUFF_hash(const STUFF *tohash);
/* Order 'arg1' and 'arg2' (implemented elsewhere) */
int stuff_cmp(const STUFF *arg1, const STUFF *arg2);
/* Create type-safe wrapper functions for use in the LHASH internals */
static IMPLEMENT_LHASH_HASH_FN(stuff, STUFF);
static IMPLEMENT_LHASH_COMP_FN(stuff, STUFF);
/* ... */
int main(int argc, char *argv[]) {
        /* Create the new hash table using the hash/compare wrappers */
        LHASH_OF(STUFF) *hashtable =
            lh_STUFF_new(LHASH_HASH_FN(STUFF_hash),
                LHASH_COMP_FN(STUFF_cmp));
        /* ... */
}

lh_<type>_free() frees the LHASH_OF(<type>) structure table. Allocated hash table entries will not be freed — consider using lh_<type>_doall() to deallocate any remaining entries in the hash table (see below). Don’t forget, bestie, or you’ll be leaking like it’s a sad playlist. 😭

lh_<type>_insert() inserts the structure pointed to by data into table. If there already is an entry with the same key, the old value is replaced. Note that lh_<type>_insert() stores pointers — the data are not copied. It’s giving very “we’re all sharing the same brain cell” energy. 🫶

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 entry in the hash table, call func with the data item as its parameter. For lh_<type>_doall() and lh_<type>_doall_arg(), function pointer casting should be avoided in the callbacks (see “NOTES”) — instead use the declare/implement macros to create type-checked wrappers that cast variables prior to calling your type-specific callbacks. An example of this is illustrated here where the callback is used to cleanup resources for items in the hash table prior to the hashtable itself being deallocated:

/* Clean up resources belonging to 'a' (this is implemented elsewhere) */
void STUFF_cleanup_doall(STUFF *a);
/* Implement a prototype-compatible wrapper for "STUFF_cleanup" */
IMPLEMENT_LHASH_DOALL_FN(STUFF_cleanup, STUFF)
        /* ... then later in the code ... */
/* So to run "STUFF_cleanup" against all items in a hash table ... */
lh_STUFF_doall(hashtable, LHASH_DOALL_FN(STUFF_cleanup));
/* Then the hash table itself can be deallocated */
lh_STUFF_free(hashtable);

A callback may delete entries from the hash table, however, it is not safe to insert new entries. ⚠️

lh_<type>_doall_arg() is the same as lh_<type>_doall() except that func will be called with arg as the second argument and func should be of type LHASH_DOALL_ARG_FN_TYPE (a callback prototype that is passed both the table entry and an extra argument). As with lh_<type>_doall(), 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 BIO that is provided by the caller):

/* Print item 'a' to 'output_bio' (this is implemented elsewhere) */
void STUFF_print_doall_arg(const STUFF *a, BIO *output_bio);
/* Implement a prototype-compatible wrapper for "STUFF_print" */
static IMPLEMENT_LHASH_DOALL_ARG_FN(STUFF, const STUFF, BIO)
        /* ... then later in the code ... */
/* Print out the entire hashtable to a particular BIO */
lh_STUFF_doall_arg(hashtable, LHASH_DOALL_ARG_FN(STUFF_print), BIO,
        logging_bio);

lh_<type>_error() can be used to determine if an error occurred in the last operation.

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🌸 RETURN VALUES 🌸

• lh_<type>_new() returns NULL on error, otherwise a pointer to the new LHASH structure. Slay when it works, weep when it’s NULL. 💀
• When a hash table entry is replaced, lh_<type>_insert() returns the value being replaced. NULL is returned on normal operation and on error.
• lh_<type>_delete() returns the entry being deleted. NULL is returned if there is no such value in the hash table.
• lh_<type>_retrieve() returns the hash table entry if it has been found, or NULL otherwise.
• lh_<type>_error() returns 1 if an error occurred in the last operation, or 0 otherwise.

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🎀 NOTES 🎀

The various LHASH macros and callback types exist to make it possible to write type-checked code without resorting to function-prototype casting — an evil that makes application code much harder to audit/verify and also opens the window of opportunity for stack corruption and other hard-to-find bugs. It also, apparently, violates ANSI-C. Not me defending the chaos, just reporting it. 🙈

The LHASH code regards table entries as constant data. As such, it internally represents lh_<type>_insert()’ed items with a const void * pointer type. This is why callbacks such as those used by lh_<type>_doall() and lh_<type>_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. However, 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_<type>_doall() or lh_<type>_doall_arg() callbacks (see the "STUFF_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 are 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. Auditing era — bring snacks. 🍓

🧠 INTERNALS 🧠

okay bestie, this next bit is straight-up lifted from the SSLeay docs, so it's giving ✨historical artifact✨ but it's also lowkey fascinating, stay with me 💅

The lhash library implements a hash table that was described in the Communications of the ACM in 1991. and what makes this hash table different is this: as the table fills up, the hash table gets increased (or decreased) in size via reallocarray(3).

But here's the iconic part 🎀 — when a 'resize' happens, instead of redistributing all the hashes over twice as many 'buckets', only one bucket gets split. so when an 'expand' goes down, there's only a minimal cost to redistribute some values. subsequent inserts will cause more single 'bucket' redistributions, sure, but there will never be a sudden large cost from redistributing all the 'buckets'. no cap, that's the whole design flex 💪

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The state for a particular hash table lives in the LHASH structure. the decision to increase or decrease the hash table size is made depending on the 'load' of the hash table, and the load is literally just the number of items in the hash table divided by the size of the hash table. the default values are as follows:

• If (hash->up_load < load) => expand
• If (hash->down_load > load) => contract

The up_load has a default value of 1 and down_load has a default value of 2. these numbers can be modified by the application by just playing with the up_load and down_load variables — go off, customize your vibe check 🦋

The 'load' is kept in a form which is multiplied by 256. so hash->up_load=8*256 will cause a load of 8 to be set. math girlies rise 😭

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If you're interested in performance (and who isn't, main character energy ✨), the field to watch is num_comp_calls. here's why: the hash library keeps track of the 'hash' value for each item, so when a lookup is done, the 'hashes' are compared, and if there's a match, then a full compare is done, and hash->num_comp_calls gets incremented.

So if num_comp_calls is not equal to num_delete plus num_retrieve, that means your hash function is generating hashes that are the same for different values 👀🚩 It is probably worth changing your hash function if this is the case, because even if your hash table has 10 items in a 'bucket', it can be searched with 10 unsigned long compares and 10 linked list traverses. and that's much less expensive than 10 calls to your compare function. frugal queen behavior 💅

lh_strhash() is a demo string hashing function. since the LHASH routines would normally be passed structures, this routine would not normally be passed to lh_<type>_new(), rather it would be used in the function passed to lh_<type>_new(). iykyk 🫶

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👀 SEE ALSO 👀

• crypto(3)

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🌈 HISTORY 🌈

okay so here's the lore, and it's honestly so much:

• lh_new(), lh_free(), lh_insert(), lh_delete(), lh_retrieve(), lh_doall(), and lh_strhash() appeared in SSLeay 0.4 or earlier. ancient. prehistoric even 💀
• lh_doall_arg() first appeared in SSLeay 0.5.1. a little baby sibling 🍼
• These functions have been available since OpenBSD 2.4.
• lh_<type>_error() was added in SSLeay 0.9.1b.

Then the glow-ups happened ✨:

• In OpenSSL 0.9.7, all lhash functions that were passed function pointers were changed for better type safety, and the function types LHASH_COMP_FN_TYPE, LHASH_HASH_FN_TYPE, LHASH_DOALL_FN_TYPE, and LHASH_DOALL_ARG_FN_TYPE became available. slay, type safety era 🎀
• In OpenSSL 1.0.0, the lhash interface was revamped for even better type checking. they said "we can do even better" and they DID 👑

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💀 BUGS 💀

lh_<type>_insert() returns NULL both for success and error.

that's it. that's the bug. you literally cannot tell the difference between "it worked ✨" and "it flopped 😭" from the return value alone. not me crying over an ambiguous NULL 🙈

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and that's a wrap on LH_NEW(3), bestie 🫶 this page came from the Debian crew and is dated June 8, 2025 — go forth, hash responsibly, and remember: if your num_comp_calls is sus, fix your hash function 💖✨