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Allow definition of a load and unload NIF callback #16

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b4bcdd5
Allow definition of a `load` NIF callback
brodeuralexis Sep 11, 2025
d190c14
Rework macro to accept a function instead
brodeuralexis Sep 22, 2025
f19cd5e
Update documentation
brodeuralexis Sep 22, 2025
e3a42a9
Add unload callback too
brodeuralexis Sep 22, 2025
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30 changes: 30 additions & 0 deletions README.md
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Expand Up @@ -604,6 +604,36 @@ Attempting to decode STL containers making use of `std::pmr::polymorphic_allocat
will result in the `std::pmr::get_default_resource()` memory resource being
used.

## Global Callbacks

The Erlang NIF API allows the creation of global callbacks. This section
describes how to leverage these callbacks while using Fine.

Callbacks MUST be used before `FINE_NIF`, since the formers use template
specializations that are instantiated by the latter. If the order is reverse,
there is no guarantee that `FINE_NIF` will actually invoke the callbacks.

### Load

The NIF load callback is called by the ERTS when the NIFs are being loaded by
`:erlang.load_nif/2`. Fine allows customizing the behavior of the load callback
using the `FINE_LOAD` macro:

```c++
static ThreadPool s_pool;

FINE_LOAD(env) {
s_pool = ThreadPool(std::thread::hardware_concurrency());
}

FINE_INIT("Elixir.MyLib.NIF");
```

`FINE_LOAD` takes an identifier that will become the name of the variable
storing the `ErlNifEnv *`. While the NIF load callback returns an `int` to
designate success or failure, `FINE_LOAD` will instead check for a thrown
exception to determine success or failure.

<!-- Docs -->

## Prior work
Expand Down
66 changes: 52 additions & 14 deletions c_include/fine.hpp
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Expand Up @@ -47,7 +47,8 @@ template <typename T, typename SFINAE = void> struct Encoder;

namespace __private__ {
std::vector<ErlNifFunc> &get_erl_nif_funcs();
int load(ErlNifEnv *env, void **priv_data, ERL_NIF_TERM load_info);
void init_atoms(ErlNifEnv *env);
bool init_resources(ErlNifEnv *env);
} // namespace __private__

// Definitions
Expand Down Expand Up @@ -97,8 +98,7 @@ class Atom {
friend struct Decoder<Atom>;
friend struct ::std::hash<Atom>;

friend int __private__::load(ErlNifEnv *env, void **priv_data,
ERL_NIF_TERM load_info);
friend void __private__::init_atoms(ErlNifEnv *env);

// We accumulate all globally defined atom objects and create the
// terms upfront as part of init (called from the NIF load callback).
Expand Down Expand Up @@ -1221,8 +1221,7 @@ class Registration {

friend std::vector<ErlNifFunc> &__private__::get_erl_nif_funcs();

friend int __private__::load(ErlNifEnv *env, void **priv_data,
ERL_NIF_TERM load_info);
friend bool __private__::init_resources(ErlNifEnv *env);

inline static std::vector<std::tuple<ErlNifResourceType **, const char *,
void (*)(ErlNifEnv *, void *)>>
Expand Down Expand Up @@ -1295,23 +1294,41 @@ constexpr unsigned int nif_arity(Ret (*)(Args...)) {
}

namespace __private__ {
void init_atoms(ErlNifEnv *env) { fine::Atom::init_atoms(env); }

bool init_resources(ErlNifEnv *env) {
return fine::Registration::init_resources(env);
}

inline std::vector<ErlNifFunc> &get_erl_nif_funcs() {
return Registration::erl_nif_funcs;
}

inline int load(ErlNifEnv *env, void **, ERL_NIF_TERM) {
Atom::init_atoms(env);

if (!Registration::init_resources(env)) {
return -1;
}
enum class CallbackStatus {
UNIMPLEMENTED,
IMPLEMENTED,
};

return 0;
}
template <CallbackStatus> struct OnLoad {
static void on_load(ErlNifEnv *) {}
};
} // namespace __private__

// Macros

#define FINE_LOAD(env) \
template <> \
struct fine::__private__::OnLoad< \
fine::__private__::CallbackStatus::IMPLEMENTED> { \
static void on_load(ErlNifEnv *); \
}; \
\
void fine::__private__:: \
OnLoad<fine::__private__::CallbackStatus::IMPLEMENTED>::on_load( \
[[maybe_unused]] ErlNifEnv *env)

// Macros

#define FINE_NIF(name, flags) \
static ERL_NIF_TERM name##_nif(ErlNifEnv *env, int argc, \
const ERL_NIF_TERM argv[]) { \
Expand Down Expand Up @@ -1345,7 +1362,28 @@ inline int load(ErlNifEnv *env, void **, ERL_NIF_TERM) {
auto &nif_funcs = fine::__private__::get_erl_nif_funcs(); \
auto num_funcs = static_cast<int>(nif_funcs.size()); \
auto funcs = nif_funcs.data(); \
auto load = fine::__private__::load; \
\
const auto load = [](ErlNifEnv *env, void **, ERL_NIF_TERM) { \
fine::__private__::init_atoms(env); \
\
if (!fine::__private__::init_resources(env)) { \
return -1; \
} \
\
try { \
fine::__private__::OnLoad< \
fine::__private__::CallbackStatus::IMPLEMENTED>::on_load(env); \
} catch (const std::exception &e) { \
enif_fprintf(stderr, "unhandled exception: %s\n", e.what()); \
return -1; \
} catch (...) { \
enif_fprintf(stderr, "unhandled throw\n"); \
return -1; \
} \
\
return 0; \
}; \
\
static ErlNifEntry entry = {ERL_NIF_MAJOR_VERSION, \
ERL_NIF_MINOR_VERSION, \
name, \
Expand Down
5 changes: 5 additions & 0 deletions test/c_src/finest.cpp
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Expand Up @@ -462,6 +462,11 @@ std::uint64_t hash_atom(ErlNifEnv *, fine::Atom atom) noexcept {
}
FINE_NIF(hash_atom, 0);

static bool s_loaded = false;
bool is_loaded(ErlNifEnv *) { return s_loaded; }
FINE_NIF(is_loaded, 0);
} // namespace finest

FINE_LOAD(env) { finest::s_loaded = true; }

FINE_INIT("Elixir.Finest.NIF");
2 changes: 2 additions & 0 deletions test/lib/finest/nif.ex
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Expand Up @@ -80,5 +80,7 @@ defmodule Finest.NIF do
def hash_term(_term), do: err!()
def hash_atom(_term), do: err!()

def is_loaded(), do: err!()

defp err!(), do: :erlang.nif_error(:not_loaded)
end
6 changes: 6 additions & 0 deletions test/test/finest_test.exs
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Expand Up @@ -502,4 +502,10 @@ defmodule FinestTest do
end
end
end

describe "callbacks" do
test "load" do
assert NIF.is_loaded()
end
end
end