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Unfortunately, application programs can hardly avoid using the concept because several important OpenSSL APIs rely on it; see the SEE ALSO section for examples. Even though both pages are more complicated than any manual page ought to be, using the concept safely requires a complete understanding of all the details in both this manual page and in OPENSSL_sk_new(3).

The STACK_OF() macro takes a type name as its argument, typically the name of a type that has been defined as an alias for a specific struct type using a typedef declaration. It expands to an incomplete struct type which is intended to represent a “stack” of objects of the given type. That type does not actually exist, so it is not possible to define, for example, an automatic variable ‘STACK_OF(X509) my_certificates’; it is only possible to define pointers to stacks, for example ‘STACK_OF(X509) *my_certificates’. The only way such pointers can ever be used is by wrapper functions casting them to the type _STACK * described in OPENSSL_sk_new(3).

For a considerable number of types, OpenSSL provides one wrapper function for each function described in OPENSSL_sk_new(3). The names of these wrapper functions are usually constructed by inserting the name of the type and an underscore after the ‘sk_’ prefix of the function name. Usually, where the real functions take void * arguments, the wrappers take pointers to the type in questions, and where the real functions take _STACK * arguments, the wrappers take pointers to STACK_OF(type). The same applies to return values. Various exceptions to all this exist, but the above applies to all the types listed below.

Using the above may make sense for the following types because public API functions exist that take stacks of these types as arguments or return them: ACCESS_DESCRIPTION, ASN1_INTEGER, ASN1_OBJECT, ASN1_TYPE, ASN1_UTF8STRING, CONF_VALUE, DIST_POINT, GENERAL_NAME, GENERAL_SUBTREE, PKCS12_SAFEBAG, PKCS7, PKCS7_RECIP_INFO, PKCS7_SIGNER_INFO, POLICY_MAPPING, POLICYINFO, POLICYQUALINFO, X509, X509_ALGOR, X509_ATTRIBUTE, X509_CRL, X509_EXTENSION, X509_INFO, X509_OBJECT, X509_POLICY_NODE, X509_PURPOSE, X509_REVOKED.

Even though the OpenSSL headers declare wrapper functions for many more types and even though the OpenSSL documentation says that users can declare their own stack types, using STACK_OF() with any type not listed here is strongly discouraged. For other types, there may be subtle, undocumented differences in syntax and semantics, and attempting to declare custom stack types is very error prone; using plain C arrays of pointers to the desired type is much simpler and less dangerous.

#include <err.h>
#include <stdio.h>
#include <openssl/x509.h>

int
main(void)
{
	STACK_OF(X509)	*stack;
	X509		*x;

	if ((stack = sk_X509_new_null()) == NULL)
		err(1, NULL);
	if ((x = X509_new()) == NULL)
		err(1, NULL);
	if (sk_X509_push(stack, x) == 0)
		err(1, NULL);
	if (X509_up_ref(x) == 0)
		errx(1, "X509_up_ref failed");
	if (sk_X509_push(stack, x) == 0)
		err(1, NULL);
	printf("%d pointers: %p, %p\n", sk_X509_num(stack),
	    sk_X509_value(stack, 0), sk_X509_value(stack, 1));
	sk_X509_pop_free(stack, X509_free);

	return 0;
}