Definitions like #define SOME_VALUE 25 are not exported

The world of C preprocessor directives is dark and impossible to formalise. Potentially in the future we can add a mode to export the simplest of definitions as global constants, but it's very limited and inflexible.

Currently we recommend adding C forwarder functions for those cases, e.g. in the example of OpenSSL:

#include "openssl/evp.h"

// preprocessor constants
int get_EVP_MAC_KEY() { return EVP_PKEY_HMAC; }

int get_EVP_MAX_MD_SIZE() { return EVP_MAX_MD_SIZE; }

// macro definition
void get_OpenSSL_add_all_digests() { OpenSSL_add_all_digests(); }

Variadic functions are not rendered (only before version 0.0.16)

Note: in bindgen >= 0.0.17 and Scala Native 0.4.12+ you can now have variadic functions

Scala Native does support constructing a va_list, but a function taking such list must exist in the first place.

In the case of, for example, cURL, the public interface is defined in terms of ... and there's nothing in C that allows you to call a variadic function if all you have is a value of va_list.

The layout of said va_list also varies by platform.