openssl - what is STACK_OF?

Question

static STACK_OF(CMS_RevocationInfoChoice)
**cms_get0_revocation_choices(CMS_ContentInfo *cms)
{
    switch (OBJ_obj2nid(cms->contentType)) {

    case NID_pkcs7_signed:
        return &cms->d.signedData->crls;

source

Another question says it's

#define STACK_OF(type) struct stack_st_##type

But when I search the openssl code with regex #define\s*STACK_OF I do not find any entries.

What is STACK_OF?

Answer 1

"Stacks" are the way OpenSSL handles a set/array of objects. They are macro-accessed structures that provide the ability to operate on those objects.

The STACK_OF() macro is defined as:

# define STACK_OF(type) struct stack_st_##type

For the X509 type, the structure is defined as:

struct x509_st {
    X509_CINF cert_info;
    X509_ALGOR sig_alg;
      .
      .
      .
    char *propq;
} /* X509 */ ;

Per the OpenSSL Wiki:

STACK API

The stack library provides a generic way to handle collections of objects in OpenSSL. A comparison function can be registered to sort the collection.

Interface is split in two headers, <openssl/stack.h> and <openssl/safestack.h>. The former declares the C functions that will execute the insert, delete, pop, push, and other operations on the stack, while the latter declares a bunch of macros to enforce some type-checking by the compiler; these macros are mostly auto-generated by mkstack.pl.

It is highly discouraged to use the C functions declared in <openssl/stack.h>. Rather, use the macros defined in <openssl/safestack.h> for OpenSSL built-in stacks, and declare your own type-checking wrappers for your custom stacks.

Basic Use

A stack type is defined with the DECLARE_STACK_OF() macro and its instances are declared with the STACK_OF() macro.

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