DEFINE_STACK_OF(3ossl)

💅 DEFINE_STACK_OF(3ossl) — OpenSSL

okay so this is the OpenSSL manual for the stack container, filed under DEFINE_STACK_OF(3ossl) in section 3ossl, and bestie it is a lot — but it's honestly so good once it clicks. grab a snack 🍓

🌸 NAME

DEFINE_STACK_OF, DEFINE_STACK_OF_CONST, DEFINE_SPECIAL_STACK_OF, DEFINE_SPECIAL_STACK_OF_CONST, sk_TYPE_num, sk_TYPE_value, sk_TYPE_new, sk_TYPE_new_null, sk_TYPE_reserve, sk_TYPE_free, sk_TYPE_zero, sk_TYPE_delete, sk_TYPE_delete_ptr, sk_TYPE_push, sk_TYPE_unshift, sk_TYPE_pop, sk_TYPE_shift, sk_TYPE_pop_free, sk_TYPE_insert, sk_TYPE_set, sk_TYPE_find, sk_TYPE_find_ex, sk_TYPE_find_all, sk_TYPE_sort, sk_TYPE_is_sorted, sk_TYPE_dup, sk_TYPE_deep_copy, sk_TYPE_set_cmp_func, sk_TYPE_new_reserve, OPENSSL_sk_deep_copy, OPENSSL_sk_delete, OPENSSL_sk_delete_ptr, OPENSSL_sk_dup, OPENSSL_sk_find, OPENSSL_sk_find_ex, OPENSSL_sk_find_all, OPENSSL_sk_free, OPENSSL_sk_insert, OPENSSL_sk_is_sorted, OPENSSL_sk_new, OPENSSL_sk_new_null, OPENSSL_sk_new_reserve, OPENSSL_sk_num, OPENSSL_sk_pop, OPENSSL_sk_pop_free, OPENSSL_sk_push, OPENSSL_sk_reserve, OPENSSL_sk_set, OPENSSL_sk_set_cmp_func, OPENSSL_sk_shift, OPENSSL_sk_sort, OPENSSL_sk_unshift, OPENSSL_sk_value, OPENSSL_sk_zero — stack container 🫶

like… yes, all of those. it's giving "every alias ever" and honestly? iconic.

✨ SYNOPSIS ✨

#include <openssl/safestack.h>

STACK_OF(TYPE)
DEFINE_STACK_OF(TYPE)
DEFINE_STACK_OF_CONST(TYPE)
DEFINE_SPECIAL_STACK_OF(FUNCTYPE, TYPE)
DEFINE_SPECIAL_STACK_OF_CONST(FUNCTYPE, TYPE)

typedef int (*sk_TYPE_compfunc)(const TYPE *const *a, const TYPE *const *b);
typedef TYPE * (*sk_TYPE_copyfunc)(const TYPE *a);
typedef void (*sk_TYPE_freefunc)(TYPE *a);

int sk_TYPE_num(const STACK_OF(TYPE) *sk);
TYPE *sk_TYPE_value(const STACK_OF(TYPE) *sk, int idx);
STACK_OF(TYPE) *sk_TYPE_new(sk_TYPE_compfunc compare);
STACK_OF(TYPE) *sk_TYPE_new_null(void);
int sk_TYPE_reserve(STACK_OF(TYPE) *sk, int n);
void sk_TYPE_free(STACK_OF(TYPE) *sk);
void sk_TYPE_zero(STACK_OF(TYPE) *sk);
TYPE *sk_TYPE_delete(STACK_OF(TYPE) *sk, int i);
TYPE *sk_TYPE_delete_ptr(STACK_OF(TYPE) *sk, TYPE *ptr);
int sk_TYPE_push(STACK_OF(TYPE) *sk, const TYPE *ptr);
int sk_TYPE_unshift(STACK_OF(TYPE) *sk, const TYPE *ptr);
TYPE *sk_TYPE_pop(STACK_OF(TYPE) *sk);
TYPE *sk_TYPE_shift(STACK_OF(TYPE) *sk);
void sk_TYPE_pop_free(STACK_OF(TYPE) *sk, sk_TYPE_freefunc freefunc);
int sk_TYPE_insert(STACK_OF(TYPE) *sk, TYPE *ptr, int idx);
TYPE *sk_TYPE_set(STACK_OF(TYPE) *sk, int idx, const TYPE *ptr);
int sk_TYPE_find(STACK_OF(TYPE) *sk, TYPE *ptr);
int sk_TYPE_find_ex(STACK_OF(TYPE) *sk, TYPE *ptr);
int sk_TYPE_find_all(STACK_OF(TYPE) *sk, TYPE *ptr, int *pnum);
void sk_TYPE_sort(const STACK_OF(TYPE) *sk);
int sk_TYPE_is_sorted(const STACK_OF(TYPE) *sk);
STACK_OF(TYPE) *sk_TYPE_dup(const STACK_OF(TYPE) *sk);
STACK_OF(TYPE) *sk_TYPE_deep_copy(const STACK_OF(TYPE) *sk,
                                  sk_TYPE_copyfunc copyfunc,
                                  sk_TYPE_freefunc freefunc);
sk_TYPE_compfunc (*sk_TYPE_set_cmp_func(STACK_OF(TYPE) *sk,
                                        sk_TYPE_compfunc compare));
STACK_OF(TYPE) *sk_TYPE_new_reserve(sk_TYPE_compfunc compare, int n);

🦋 DESCRIPTION 🦋

okay so the whole deal here: apps can roll their own stacks by dropping any of the macros below into a header file. those macros define typesafe inline functions that wrap around the utility OPENSSL_sk_ API. and just so we're clear — in everything below, TYPE is a placeholder for any of the OpenSSL datatypes, like X509. that's the main character slot, fill it with whatever 💖

The STACK_OF() macro hands you back the name for a stack of the specified TYPE. it's an opaque pointer to a structure declaration, and you can use it in every header file that references the stack. there are several DEFINE... macros that create static inline functions for all of the functions described on this page. the move is: normally put those in one source file, and wrap the stack manipulation with application-specific functions. periodt.

DEFINE_STACK_OF() creates the set of functions for a stack of TYPE elements. the type is referenced by STACK_OF(TYPE) and every function name starts with sk_TYPE_. DEFINE_STACK_OF_CONST() is literally identical to DEFINE_STACK_OF() except each element is constant. not me pointing out the one const difference 💀

/* DEFINE_STACK_OF(TYPE) */
TYPE *sk_TYPE_value(STACK_OF(TYPE) *sk, int idx);
/* DEFINE_STACK_OF_CONST(TYPE) */
const TYPE *sk_TYPE_value(STACK_OF(TYPE) *sk, int idx);

DEFINE_SPECIAL_STACK_OF() and DEFINE_SPECIAL_STACK_OF_CONST() are the same idea, except FUNCNAME is used in the function names instead:

/* DEFINE_SPECIAL_STACK_OF(TYPE, FUNCNAME) */
TYPE *sk_FUNCNAME_value(STACK_OF(TYPE) *sk, int idx);
/* DEFINE_SPECIAL_STACK_OF(TYPE, FUNCNAME) */
const TYPE *sk_FUNCNAME_value(STACK_OF(TYPE) *sk, int idx);

🎀 the functions, one by one 🎀

sk_TYPE_num() — returns the number of elements in sk, or -1 if sk is NULL. cute, short, tells you if you forgot to init your stack.
sk_TYPE_value() — returns element idx in sk, where idx starts at zero. if idx is out of range then NULL is returned.
sk_TYPE_new() — allocates a new empty stack using comparison function compare. if compare is NULL then no comparison function is used. this one is equivalent to sk_TYPE_new_reserve(compare, 0).
sk_TYPE_new_null() — allocates a new empty stack with no comparison function. equivalent to sk_TYPE_new_reserve(NULL, 0).
sk_TYPE_reserve() — allocates extra memory in the sk structure so that the next n calls to sk_TYPE_insert(), sk_TYPE_push() or sk_TYPE_unshift() will not fail and won't cause memory to be allocated or reallocated. if n is zero, any excess space allocated in the sk structure is freed. on error sk is unchanged. no cap, this is the pre-allocation slay ✨
sk_TYPE_new_reserve() — allocates a new stack. the new stack will have additional memory allocated to hold n elements if n is positive. the next n calls to sk_TYPE_insert(), sk_TYPE_push() or sk_TYPE_unshift() will not fail or cause memory to be allocated or reallocated. if n is zero or less than zero, no memory is allocated. sk_TYPE_new_reserve() also sets the comparison function compare on the newly created stack. if compare is NULL then no comparison function is used.
sk_TYPE_set_cmp_func() — sets the comparison function of sk to compare. the previous comparison function is returned, or NULL if there was no previous one. changing the vibe mid-stack, iykyk.
sk_TYPE_free() — frees up the sk structure. it does not free up any elements of sk. after this call sk is no longer valid. ⚠️ like, gone gone.
sk_TYPE_zero() — sets the number of elements in sk to zero. it does not free sk, so after this call sk is still valid. main character survives 💅
sk_TYPE_pop_free() — frees up all elements of sk and sk itself. the free function freefunc() is called on each element to free it. this is the "actually clean up" one, obsessed.
sk_TYPE_delete() — deletes element i from sk. returns the deleted element, or NULL if i is out of range.
sk_TYPE_delete_ptr() — deletes the element matching ptr from sk. returns the deleted element, or NULL if no element matching ptr was found.
sk_TYPE_insert() — inserts ptr into sk at position idx. any existing elements at or after idx are moved downwards. if idx is out of range the new element is appended to sk. returns either the number of elements in sk after the new element is inserted, or zero if an error occurred (like a memory allocation failure). 🫶
sk_TYPE_push() — appends ptr to sk. it's equivalent to:

sk_TYPE_insert(sk, ptr, -1);

sk_TYPE_unshift() — inserts ptr at the start of sk. it's equivalent to:

sk_TYPE_insert(sk, ptr, 0);

sk_TYPE_pop() — returns and removes the last element from sk.
sk_TYPE_shift() — returns and removes the first element from sk.
sk_TYPE_set() — sets element idx of sk to ptr, replacing the current element. the new element value is returned, or NULL if an error occurred — and just so you know, that only happens if sk is NULL or idx is out of range.
sk_TYPE_find() — searches sk for the element ptr. in the case where no comparison function has been specified, it does a linear search for a pointer equal to ptr, and returns the index of the first matching element, or -1 if there's no match. in the case where a comparison function has been specified, sk is sorted and sk_TYPE_find() returns the index of a matching element, or -1 if there's no match. heads up tho: in that case the comparison function will usually compare the values pointed to rather than the pointers themselves, and the order of elements in sk can change. 🙈
sk_TYPE_find_ex() — operates like sk_TYPE_find() except when a comparison function has been specified and no matching element is found. instead of returning -1, sk_TYPE_find_ex() returns the index of the element either before or after the location where ptr would be if it were present in sk. it also does not guarantee that the first matching element in the sorted stack is returned. spicy.
sk_TYPE_find_all() — operates like sk_TYPE_find() but it also sets *pnum to the number of matching elements in the stack. in the case where no comparison function has been specified, *pnum will always be set to 1 if a matching element was found, and 0 otherwise.
sk_TYPE_sort() — sorts sk using the supplied comparison function.
sk_TYPE_is_sorted() — returns 1 if sk is sorted and 0 otherwise. a vibe check, literally.
sk_TYPE_dup() — returns a shallow copy of sk, or an empty stack if the passed stack is NULL. note the pointers in the copy are identical to the original — same objects, new list 🌈
sk_TYPE_deep_copy() — returns a new stack where each element has been copied, or an empty stack if the passed stack is NULL. copying is done by the supplied copyfunc() and freeing by freefunc(). and the fine print: freefunc() is only called if an error occurs. 👑

okay that's the whole description covered, and the page keeps going — the next part picks up from here, so don't go anywhere 🧚💜

🌸 NOTES 🌸

ok so real talk bestie, there are some things you need to know before you go feral with stacks 💅

Thread safety check, babes. Be super careful when you're touching stacks in multi-threaded environments. Any operation that makes a stack bigger — think sk_TYPE_insert() or sk_TYPE_push() — can "grow" the size of that internal array, and that's exactly how you get race conditions if some other thread is poking the same stack at the same time. And it's not just the growing ones: sk_TYPE_find() and sk_TYPE_sort() can reorder the stack too. So, like, coordinate with your threads, mkay? 😭

Your comparison function? It has to use a metric that actually works for a binary search operation. Translation: it should return zero, a positive value, or a negative value depending on whether a is equal to, greater than, or less than b respectively. That's the vibe check your cmp function has to pass 🫶

the find() return value trap, i cannot stress this enough ⭐

Be careful when you're checking the return values of sk_TYPE_find() and sk_TYPE_find_ex(). They return an index to the matching element — and here's the thing, 0 means it matched the first element. Zero is a legit success, bestie, do not treat it like false! A failed search is signalled by a -1 return value. iykyk 💀

STACK_OF(), DEFINE_STACK_OF(), DEFINE_STACK_OF_CONST(), and DEFINE_SPECIAL_STACK_OF() are all implemented as macros. macros. yes. that's just how it is ✨

Also, super important and kind of a relief: it is not an error to call sk_TYPE_num(), sk_TYPE_value(), sk_TYPE_free(), sk_TYPE_zero(), sk_TYPE_pop_free(), sk_TYPE_delete(), sk_TYPE_delete_ptr(), sk_TYPE_pop(), sk_TYPE_shift(), sk_TYPE_find(), sk_TYPE_find_ex(), and sk_TYPE_find_all() on a NULL stack, an empty stack, or with an invalid index. An error is not raised in these conditions. no drama, no explosion 💖

🚩 And now the big one: the underlying utility OPENSSL_sk_ API should not be used directly. Like, at all. It's the plumbing, not the vibes. For the record, it defines these functions: OPENSSL_sk_deep_copy(), OPENSSL_sk_delete(), OPENSSL_sk_delete_ptr(), OPENSSL_sk_dup(), OPENSSL_sk_find(), OPENSSL_sk_find_ex(), OPENSSL_sk_find_all(), OPENSSL_sk_free(), OPENSSL_sk_insert(), OPENSSL_sk_is_sorted(), OPENSSL_sk_new(), OPENSSL_sk_new_null(), OPENSSL_sk_new_reserve(), OPENSSL_sk_num(), OPENSSL_sk_pop(), OPENSSL_sk_pop_free(), OPENSSL_sk_push(), OPENSSL_sk_reserve(), OPENSSL_sk_set(), OPENSSL_sk_set_cmp_func(), OPENSSL_sk_shift(), OPENSSL_sk_sort(), OPENSSL_sk_unshift(), OPENSSL_sk_value(), OPENSSL_sk_zero(). Okay? Okay. 💅

🎀 RETURN VALUES 🎀

okay so here's the receipts on what each one hands back to you 👑

sk_TYPE_num() — returns the number of elements in the stack, or -1 if the passed stack is NULL.
sk_TYPE_value() — returns a pointer to a stack element, or NULL if the index is out of range.
sk_TYPE_new(), sk_TYPE_new_null() and sk_TYPE_new_reserve() — return an empty stack, or NULL if an error occurs.
sk_TYPE_reserve() — returns 1 on successful allocation of the required memory, or 0 on error.
sk_TYPE_set_cmp_func() — returns the old comparison function, or NULL if there was no old comparison function.
sk_TYPE_free(), sk_TYPE_zero(), sk_TYPE_pop_free() and sk_TYPE_sort() — do not return values. they give nothing. they simply are 🦋
sk_TYPE_pop(), sk_TYPE_shift(), sk_TYPE_delete() and sk_TYPE_delete_ptr() — return a pointer to the deleted element, or NULL on error.
sk_TYPE_insert(), sk_TYPE_push() and sk_TYPE_unshift() — return the total number of elements in the stack, and 0 if an error occurred.
sk_TYPE_set() — returns a pointer to the replacement element, or NULL on error.
sk_TYPE_find() and sk_TYPE_find_ex() — return an index to the found element, or -1 on error.
sk_TYPE_is_sorted() — returns 1 if the stack is sorted and 0 if it is not.
sk_TYPE_dup() and sk_TYPE_deep_copy() — return a pointer to the copy of the stack, or NULL on error.

🦋 HISTORY 🦋

storytime, because this API has lore 📖

Before OpenSSL 1.1.0, this was implemented via macros and not inline functions, and was not a public API. ancient times, truly 🙈
sk_TYPE_reserve() and sk_TYPE_new_reserve() were added in OpenSSL 1.1.1.
From OpenSSL 3.2.0, the sk_TYPE_find(), sk_TYPE_find_ex() and sk_TYPE_find_all() calls are read-only and do not sort the stack. And to avoid any performance implications this change introduces, sk_TYPE_sort() should be called before these find operations. so like, sort first, then find. put that on a sticky note ⭐
Before OpenSSL 3.3.0, sk_TYPE_push() returned -1 if sk was NULL. It was changed to return 0 in this condition, as for other errors. glow-up behavior honestly ✨

👑 COPYRIGHT 👑

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, you ate that 🫶 this one's from the OpenSSL 3.5.0 era, dated 2026-09-25, and it lives under the name DEFINE_STACK_OF(3ossl) — go forth and stack responsibly, mwah 💋🌈

📦 where this page comes from

opensuse-leap 16.0: openssl-3-doc 3.5.0-160000.11.1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms