I've been looking into the C API definitions in dave.h and noticed a significant limitation regarding callback handling.

Currently, the callback typedefs (like DAVEMLSFailureCallback and DAVEPairwiseFingerprintCallback) are defined as plain function pointers without a user context parameter:

And the registration functions do not accept a context pointer to pass through:

The Problem

While the underlying C++ implementation uses std::function (which supports capturing state/this), the C API strips this context away.

This makes it effectively impossible to map a callback invocation back to a specific session instance or language-level object without resorting to unsafe global state or thread-local storage.

For consumers using C++ or things like JNI, this isn't strictly a blocker because we can wrap the C++ API directly. However, for any consumer attempting to use the C API via FFI mechanisms (like Java's JNA/Project Panama, C# P/Invoke, Python ctypes, or Rust bindgen), this is a major pain point.

To make this work currently with FFI, one would have to resort to extreme hacks, like JIT-compiling a unique trampoline function for every session instance just to bake the instance pointer into the instruction stream.

Suggested Fix

Add a void* user_data argument to every callback type definition and any function consuming the callback.

The signatures should ideally look something like this:

typedef void (*DAVEMLSFailureCallback)(void* user_data, const char* source, const char* reason);

DAVE_EXPORT DAVESessionHandle daveSessionCreate(void* context,
                                                const char* authSessionId,
                                                DAVEMLSFailureCallback callback,
                                                void* user_data); // <--- Passed back to callback