OPENSSL_SK_NEW(3) | Library Functions Manual | OPENSSL_SK_NEW(3)

# NAME

`sk_new_null`, `sk_new`, `sk_set_cmp_func`, `sk_dup`, `sk_free`, `sk_pop_free`, `sk_num`, `sk_value`, `sk_find`, `sk_sort`, `sk_is_sorted`, `sk_push`, `sk_unshift`, `sk_insert`, `sk_set`, `sk_pop`, `sk_shift`, `sk_delete`, `sk_delete_ptr`, `sk_zero` — variable-sized arrays of void pointers, called OpenSSL stacks

# SYNOPSIS

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

_STACK *
`sk_new_null`(void);

_STACK *
`sk_new`(int (*compfunc)(const void *, const void *));

old_function_pointer
`sk_set_cmp_func`(_STACK *stack, int (*compfunc)(const void *, const void *));

_STACK *
`sk_dup`(_STACK *stack);

void
`sk_free`(_STACK *stack);

void
`sk_pop_free`(_STACK *stack, void (*freefunc)(void *));

int
`sk_num`(const _STACK *stack);

void *
`sk_value`(const _STACK *stack, int index);

int
`sk_find`(_STACK *stack, void *wanted);

void
`sk_sort`(_STACK *stack);

int
`sk_is_sorted`(const _STACK *stack);

int
`sk_push`(_STACK *stack, void *new_item);

int
`sk_unshift`(_STACK *stack, void *new_item);

int
`sk_insert`(_STACK *stack, void *new_item, int index);

void *
`sk_set`(_STACK *stack, int index, void *new_item);

void *
`sk_pop`(_STACK *stack);

void *
`sk_shift`(_STACK *stack);

void *
`sk_delete`(_STACK *stack, int index);

void *
`sk_delete_ptr`(_STACK *stack, void *wanted);

void
`sk_zero`(_STACK *stack);

# DESCRIPTION

OpenSSL introduced an idiosyncratic concept of variable sized arrays of pointers and somewhat misleadingly called such an array a “stack”. Intrinsically, and as documented in this manual page, OpenSSL stacks are not type safe but only handle void * function arguments and return values.

OpenSSL also provides a fragile, unusually complicated system of macro-generated wrappers that offers superficial type safety at the expense of extensive obfuscation, implemented using large amounts of autogenerated code involving exceedingly ugly, nested cpp(1) macros; see the STACK_OF(3) manual page for details.

The fundamental data type is the _STACK structure. It stores a variable number of void pointers and remembers the number of pointers currently stored. It can optionally hold a pointer to a comparison function. As long as no comparison function is installed, the order of pointers is meaningful; as soon as a comparison function is installed, it becomes ill-defined.

`sk_new_null`() allocates and initializes a new, empty stack. `sk_new`() is identical except that it also installs compfunc as the comparison function for the new stack object. `sk_set_cmp_func`() installs compfunc for the existing stack. The compfunc is allowed to be `NULL`, but the stack is not.

`sk_dup`() creates a shallow copy of the given stack, which must not be a `NULL` pointer. It neither copies the objects pointed to from the stack nor increases their reference counts, but merely copies the pointers. Extreme care must be taken in order to avoid freeing the memory twice, for example by calling `sk_free`() on one copy and only calling `sk_pop_free`() on the other.

`sk_free`() frees the given stack. It does not free any of the pointers stored on the stack. Unless these pointers are merely copies of pointers owned by other objects, they must be freed before calling `sk_free`(), in order to avoid leaking memory. If stack is a `NULL` pointer, no action occurs.

`sk_pop_free`() is severely misnamed. It does not at all do what one would expect from a function called “pop”. Instead, it does the same as `sk_free`(), except that it also calls the function freefunc on each of the pointers contained in the stack. If the calls to freefunc are intended to free the memory in use by the objects on the stack, ensure that no other pointers to the same objects remain elsewhere.

`sk_find`() searches the stack for the wanted pointer. If the stack contains more than one copy of the wanted pointer, only the first match is found. If a comparison function is installed for the stack, the stack is first sorted with `sk_sort`(), and instead of comparing pointers, two pointers are considered to match if the comparison function returns 0.

`sk_sort`() sorts the stack using qsort(3) and the installed comparison function. If stack is a `NULL` pointer or already considered sorted, no action occurs. This function can only be called if a comparison function is installed.

`sk_is_sorted`() reports whether the stack is considered sorted. Calling `sk_new_null`() or `sk_new`(), successfully calling `sk_push`(), `sk_unshift`(), `sk_insert`(), or `sk_set`(), or changing the comparison function sets the state to unsorted. If a comparison function is installed, calling `sk_sort`(), or `sk_find`() sets the state to sorted.

`sk_push`() pushes new_item onto the end of the stack, increasing the number of pointers by 1. If stack is a `NULL` pointer, no action occurs.

`sk_unshift`() inserts new_item at the beginning of the stack, such that it gets the index 0. The number of pointers increases by 1. If stack is a `NULL` pointer, no action occurs.

`sk_insert`() inserts the new_item into the stack such that it gets the given index. If index is less than 0 or greater than or equal to `sk_num`(stack), the effect is the same as for `sk_push`(). If stack is a `NULL` pointer, no action occurs.

`sk_set`() replaces the pointer with the given index on the stack with the new_item. The old pointer is not freed, which may leak memory if no copy of it exists elsewhere. If stack is a `NULL` pointer or if index is less than 0 or greater than or equal to `sk_num`(stack), no action occurs.

`sk_pop`() and `sk_shift`() remove the pointer with the highest or lowest index from the stack, respectively, reducing the number of pointers by 1. If stack is a `NULL` pointer or if it is empty, no action occurs.

`sk_delete`() removes the pointer with the given index from the stack, reducing the number of pointers by 1. If stack is a `NULL` pointer or the index is less than 0 or greater than or equal to `sk_num`(stack), no action occurs.

`sk_delete_ptr`() removes the wanted pointer from the stack, reducing the number of pointers by 1 if it is found. It never uses a comparison function but only compares pointers themselves. The stack pointer must not be `NULL`.

`sk_zero`() removes all pointers from the stack. It does not free any of the pointers. Unless these pointers are merely copies of pointers owned by other objects, they must be freed before calling `sk_zero`(), in order to avoid leaking memory. If stack is a `NULL` pointer, no action occurs.

# RETURN VALUES

`sk_new_null`(), `sk_new`(), and `sk_dup`() return a pointer to the newly allocated stack object or `NULL` if insufficient memory is available.

`sk_set_cmp_func`() returns a pointer to the comparison function that was previously installed for the stack or `NULL` if none was installed.

`sk_num`() returns the number of pointers currently stored on the stack, or -1 if stack is a `NULL` pointer.

`sk_value`() returns the pointer with the given index from the stack, or `NULL` if stack is a `NULL` pointer or if the index is less than 0 or greater than or equal to `sk_num`(stack).

`sk_find`() returns the lowest index considered to match or -1 if stack is a `NULL` pointer or if no match is found.

`sk_is_sorted`() returns 1 if the stack is considered sorted or if it is a `NULL` pointer, or 0 otherwise.

`sk_push`(), `sk_unshift`(), and `sk_insert`() return the new number of pointers on the stack or 0 if stack is a `NULL` pointer or if memory allocation fails.

`sk_set`() returns new_item or `NULL` if stack is a `NULL` pointer or if the index is less than 0 or greater than or equal to `sk_num`(stack).

`sk_pop`() and `sk_shift`() return the deleted pointer or `NULL` if stack is a `NULL` pointer or if it is empty.

`sk_delete`() returns the deleted pointer or `NULL` if stack is a `NULL` pointer or if the index is less than 0 or greater than or equal to `sk_num`(stack).

`sk_delete_ptr`() returns wanted or `NULL` if it is not found.

# SEE ALSO

STACK_OF(3)

# HISTORY

`sk_new_null`(), `sk_new`(), `sk_free`(), `sk_pop_free`(), `sk_num`(), `sk_value`(), `sk_find`(), `sk_push`(), `sk_unshift`(), `sk_insert`(), `sk_pop`(), `sk_shift`(), `sk_delete`(), and `sk_delete_ptr`() first appeared in SSLeay 0.5.1. `sk_set_cmp_func`(), `sk_dup`(), and `sk_zero`() first appeared in SSLeay 0.8.0. These functions have been available since OpenBSD 2.4.

`sk_set`() first appeared in OpenSSL 0.9.3. `sk_sort`() first appeared in OpenSSL 0.9.4. Both functions have been available since OpenBSD 2.6.

`sk_is_sorted`() first appeared in OpenSSL 0.9.7e and has been available since OpenBSD 3.8.

# BUGS

Even if a comparison function is installed, empty stacks and stacks containing a single pointer are sometimes considered sorted and sometimes considered unsorted.

If a comparison function is installed, the concept of “first match” in `sk_find`() is ill-defined because qsort(3) is not a stable sorting function. It is probably best to only assume that they return an arbitrary match.

June 8, 2025 | Debian