DEFINE_STACK_OF(3)

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_thunks, OPENSSL_sk_set_cmp_thunks, OPENSSL_sk_set_cmp_func, OPENSSL_sk_shift, OPENSSL_sk_sort, OPENSSL_sk_unshift, OPENSSL_sk_value, OPENSSL_sk_zero - stack container

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(const STACK_OF(TYPE) *sk, TYPE *ptr);
 int sk_TYPE_find_ex(const STACK_OF(TYPE) *sk, TYPE *ptr);
 int sk_TYPE_find_all(const 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
Applications can create and use their own stacks by placing any of the macros described below in a header file. These macros define typesafe inline functions that wrap around the utility OPENSSL_sk_ API. In the description here, TYPE is used as a placeholder for any of the OpenSSL datatypes, such as X509.

The STACK_OF() macro returns the name for a stack of the specified TYPE. This is an opaque pointer to a structure declaration. This can be used 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. This should normally be used in one source file, and the stack manipulation is wrapped with application-specific functions.

DEFINE_STACK_OF() creates set of functions for a stack of TYPE elements. The type is referenced by STACK_OF(TYPE) and each function name begins with sk_TYPE_. DEFINE_STACK_OF_CONST() is identical to DEFINE_STACK_OF() except each element is constant.

 /* 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 similar except FUNCNAME is used in the function names:

 /* 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);

sk_TYPE_num() returns the number of elements in sk or -1 if sk is NULL.

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 function is equivalent to sk_TYPE_new_reserve(compare, 0).

sk_TYPE_new_null() allocates a new empty stack with no comparison function. This function is equivalent to sk_TYPE_new_reserve(NULL, 0).

sk_TYPE_reserve() allocates additional memory in the sk structure such that 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, any excess space allocated in the sk structure is freed. On error sk is unchanged.

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 to 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 comparison function.

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.

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.

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.

sk_TYPE_delete() deletes element i from sk. It returns the deleted element or NULL if i is out of range.

sk_TYPE_delete_ptr() deletes element matching ptr from sk. It 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. sk_TYPE_insert() either returns the number of elements in sk after the new element is inserted or zero if an error (such as memory allocation failure) occurred.

sk_TYPE_push() appends ptr to sk it is equivalent to:

 sk_TYPE_insert(sk, ptr, -1);

sk_TYPE_unshift() inserts ptr at the start of sk it is 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: this will only happen 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, the function performs a linear search for a pointer equal to ptr. In the case where a comparison function has been specified, the function performs a search for a element that the comparison function indicates is a match. If the stack is sorted, a binary search is used, otherwise, a linear search is used. sk_TYPE_find() returns the index of a matching element or -1 if there is no match.

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. The function also does not guarantee that the first matching element in the sorted stack is returned.

sk_TYPE_find_all() operates like sk_TYPE_find() but it also sets the *pnum to number of matching elements in the stack. In case no comparison function has been specified the *pnum will be always set to 1 if matching element was found, 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.

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.

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 performed by the supplied copyfunc() and freeing by freefunc(). The function freefunc() is only called if an error occurs.

NOTES
Care should be taken when accessing stacks in multi-threaded environments. Any operation which increases the size of a stack such as sk_TYPE_insert() or sk_TYPE_push() can "grow" the size of an internal array and cause race conditions if the same stack is accessed in a different thread. Operations such as sk_TYPE_find() and sk_TYPE_sort() can also reorder the stack.

Any comparison function supplied should use a metric suitable for use in a binary search operation. That is it should return zero, a positive or negative value if a is equal to, greater than or less than b respectively.

Care should be taken when checking the return values of the functions sk_TYPE_find() and sk_TYPE_find_ex(). They return an index to the matching element. In particular 0 indicates a matching first element. A failed search is indicated by a -1 return value.

STACK_OF(), DEFINE_STACK_OF(), DEFINE_STACK_OF_CONST(), and DEFINE_SPECIAL_STACK_OF() are implemented as macros.

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, empty stack, or with an invalid index. An error is not raised in these conditions.

The underlying utility OPENSSL_sk_ API should not be used directly. 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_set_thunks(), OPENSSL_sk_set_cmp_thunks(), OPENSSL_sk_shift(), OPENSSL_sk_sort(), OPENSSL_sk_unshift(), OPENSSL_sk_value(), OPENSSL_sk_zero().

OPENSSL_sk_set_thunks() and OPENSSL_sk_set_cmp_thunks(), while public by necessity, are actually internal and should not be used.

RETURN VALUES
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.

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
Before OpenSSL 1.1.0, this was implemented via macros and not inline functions and was not a public API.

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. To avoid any performance implications this change introduces, sk_TYPE_sort() should be called before these find operations.

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.

OPENSSL_sk_set_thunks() was added in OpenSSL 3.6.0.

OPENSSL_sk_set_cmp_thunks() was added in OpenSSL 4.0.0

COPYRIGHT
Copyright 2000-2026 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>.