=pod

=head1 NAME

sk_X509_num, sk_X509_value, sk_X509_new, sk_X509_new_null, sk_X509_free,
sk_X509_zero, sk_X509_delete, sk_X509_delete_ptr, sk_X509_push,
sk_X509_unshift, sk_X509_pop, sk_X509_shift, sk_X509_pop_free, sk_X509_insert,
sk_X509_set, sk_X509_find, sk_X509_find_ex, sk_X509_sort, sk_X509_is_sorted,
sk_X509_dup, sk_X509_deep_copy, sk_X509_set_cmp_func - X509 stack

=head1 SYNOPSIS

 #include <openssl/x509.h>

 int sk_X509_num(const STACK_OF(X509) *sk);
 X509 *sk_X509_value(const STACK_OF(X509) *sk, int idx);
 STACK_OF(X509) *sk_X509_new(int (*cmpf)(const X509 * const *a,
                                         const X509 * const *b));
 STACK_OF(X509) *sk_X509_new_null(void);
 int (*sk_X509_set_cmp_func (STACK_OF(X509) *sk,
                             int (*cmpf) (const X509 * const *a,
                                          const X509 * const *b)))
                             (const X509 * const *, const X509 * const *);
 void sk_X509_free(const STACK_OF(X509) *sk);
 void sk_X509_zero(const STACK_OF(X509) *sk);
 void sk_X509_pop_free(STACK_OF(X509) *sk, void (*func) (X509 *a));
 X509 *sk_X509_delete(STACK_OF(X509) *sk, int i);
 X509 *sk_X509_delete_ptr(STACK_OF(X509) *sk, X509 *ptr);
 int sk_X509_insert(STACK_OF(X509) *sk, X509 *ptr, int idx);
 int sk_X509_push(STACK_OF(X509) *sk, X509 *ptr);
 int sk_X509_unshift(STACK_OF(X509) *sk, X509 *ptr);
 X509 *sk_X509_pop(STACK_OF(X509) *sk);
 X509 *sk_X509_shift(STACK_OF(X509) *sk);
 X509 *sk_X509_set(STACK_OF(X509) *sk, int idx, X509 *ptr);
 int sk_X509_find(STACK_OF(X509) *sk, X509 *ptr);
 int sk_X509_find_ex(STACK_OF(X509) *sk, X509 *ptr);
 void sk_X509_sort(const STACK_OF(X509) *sk);
 int sk_X509_is_sorted(const STACK_OF(X509) *sk);
 STACK_OF(X509) *sk_X509_dup(STACK_OF(X509) *sk);
 STACK_OF(X509) *sk_X509_deep_copy(STACK_OF(X509) *sk,
                                   X509 * (*copyfn) (const X509 *),
                                   void (*freefn) (X509 *));

=head1 DESCRIPTION

sk_X509_num() returns the number of elements in B<sk> or -1 if B<sk> is
B<NULL>.

sk_X509_value() returns element B<idx> in B<sk>. Where B<idx> runs from 0
to sk_X509_num(sk) - 1 inclusive. If B<idx> is out of range then B<NULL>
is returned.

sk_X509_new() allocates a new empty stack using comparison function B<cmpf>.
If B<cmpf> is B<0> then no comparison function is used.

sk_X509_new_null() allocates a new empty stack with no comparison function.

sk_X509_set_cmp_func() sets the comparison function of B<sk> to B<cmpf>.
The previous comparison function is returned or B<0> if there was
no previous comparison function.

sk_X509_free() frees up the B<sk> structure. It does B<not> free up any
elements of B<sk>. After this call B<sk> is no longer valid.

sk_X509_zero() sets the number of elements in B<sk> to zero. It does not free
B<sk> so after this call B<sk> is still valid.

sk_X509_pop_free() frees up all elements of B<sk> and B<sk> itself. The
free function func() is called on each element to free it.

sk_X509_delete() deletes element B<i> from B<sk>. It returns the deleted
element or B<NULL> if B<i> is out of range.

sk_X509_delete_ptr() deletes element matching B<ptr> from B<sk>. It returns
the deleted element or B<NULL> if no element matching B<ptr> was found.

sk_X509_insert() inserts B<ptr> into B<sk> at position B<idx>. Any existing
elements at or after B<idx> are moved downwards. If B<idx> is out of range
the new element is appended to B<sk>. sk_X509_insert() either returns the
number of elements in B<sk> after the new element is inserted or zero if
an error occured: which will happen if there is a memory allocation failure.

sk_X509_push() appends B<ptr> to B<sk> it is equivalent to:

 sk_X509_insert(sk, ptr, -1);

sk_X509_unshift() inserts B<ptr> at the start of B<sk> it is equivalent to:

 sk_X509_insert(sk, ptr, 0);

sk_X509_pop() returns and removes the last element from B<sk>.

sk_X509_shift() returns and removes the first element from B<sk>.

sk_X509_set() sets element B<idx> of B<sk> to B<ptr> replacing the current
element. The new element value is returned or B<NULL> if an error occured:
this will only happen if B<sk> is B<NULL> or B<idx> is out of range.

sk_X509_find() and int sk_X509_find_ex() search B<sk> using the supplied
comparison function for an element matching B<ptr>. sk_X509_find() returns
the index of the first matching element or B<-1> if there is no match.
sk_X509_find_ex() returns a matching element or the nearest element that
does not match B<ptr>. Note: if a comparison function is set then  B<sk> is
sorted before the search which may change its order. If no comparison
function is set then a linear search is made for a pointer matching B<ptr>
and the stack is not reordered.

sk_X509_sort() sorts B<sk> using the supplied comparison function.

sk_X509_is_sorted() returns B<1> if B<sk> is sorted and B<0> otherwise.

sk_X509_dup() returns a copy of B<sk>. Note the pointers in the copy
are identical to the original.

sk_X509_deep_copy() returns a new stack where each element has been copied.
Copying is performed by the supplied copyfn() and freeing by freefn(). The
function freefn() is only called if an error occurs.

=head1 NOTES

This manual page documents the functions which operate on a stack of
B<X509> pointers. A stack can contain pointers to any structure with B<X509>
replaced by the appropriate structure name.

Care should be taken when accessing stacks in multi-threaded environments.
Any operation which increases the size of a stack such as sk_X509_insert() or
sk_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_find() and sk_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 B<a> is equal to, greater than
or less than B<b> respectively.

Care should be taken when checking the return values of the functions
sk_X509_find() and sk_X509_find_ex(). They return an index to the
matching element. In particular B<0> indicates a matching first element.
A failed search is indicated by a B<-1> return value.

=head1 APPLICATION DEFINED STACKS

Applications can create and use their own stacks by placing any of the macros
described below in a header file.

DEFINE_STACK_OF(NAME) creates set of functions for a stack of B<NAME>. This
will mean that type B<NAME> is stored in each stack, the type is referenced by
STACK_OF(NAME) and each function name begins with sk_NAME_. For example:

 NAME *sk_NAME_value(STACK_OF(NAME) *sk, int idx);

DEFINE_STACK_OF_CONST(NAME) is identical to DEFINE_STACK_OF(NAME) except
each element is constant for example:

 const NAME *sk_name_value(STACK_OF(NAME) *sk, int idx);

DEFINE_SPECIAL_STACK_OF(FNAME, STNAME) defines a stack of B<STNAME> but
each function uses B<FNAME>. For example:

 STNAME *sk_FNAME_value(STACK_OF(STNAME) *sk, int idx);

=head1 RETURN VALUES

sk_X509_num() returns the number of elements in the stack or B<-1> if the
passed stack is B<NULL>.

sk_X509_value() returns a pointer to a stack element or B<NULL> if the
index is out of range.

sk_X509_new() and sk_X509_new_null() return an empty stack or B<NULL> if
an error occurs.

sk_X509_set_cmp_func() returns the old comparison function or B<NULL> if
there was no old comparison function.

sk_X509_free(), sk_X509_zero(), sk_X509_pop_free() and sk_X509_sort() do
not return values.

sk_X509_pop(), sk_X509_shift(), sk_X509_delete() and sk_X509_delete_ptr()
return a pointer to the deleted element or B<NULL> on error.

sk_X509_insert(), sk_X509_push() and sk_X509_unshift() return the total
number of elements in the stack and 0 if an error occurred.

sk_X509_set() returns a pointer to the replacement element or B<NULL> on
error.

sk_X509_find() and sk_X509_find_ex() return an index to the found element
or B<-1> on error.

sk_X509_is_sorted() returns B<1> if the stack is sorted and B<0> if it is
not.

sk_X509_dup() and sk_X509_deep_copy() return a pointer to the copy of the
stack.

=head1 HISTORY

Use of inline functions and application defined stacks first appeared in
OpenSSL 1.1.0. Previous versions of OpenSSL implemented stacks as macros.