Why can't I use `BIGNUM` in this C struct declaration?

Question

I was experimenting with some ~2005 C code (using OpenSSL 0.9.8, I think) and I tried make -ing it with OpenSSL 3.0.2 on Ubuntu 22.04.

Minimum example:

#include <openssl/bn.h>

struct fraction
{
    BIGNUM numerator;
    BIGNUM denominator;
}

Expected: everything builds, just as intended.

Actual: complier complains about incomplete type declaration for both fields.

Why does this happen? Is this not a valid declaration? Or is it something else?

Answer 1

Later versions of OpenSSL made several types opaque, including BIGNUM , which means you can't instantiate them directly in user code. The BIGNUM type in particular became an opaque type starting with version 1.1.0.

You would instead need to use pointers:

struct fraction
{
    BIGNUM *numerator;
    BIGNUM *denominator;
};

And use the appropriate functions to create and destroy them, as well as use the relevant accessor functions to access the members.