How to call a fun from a NIF?

robashton · January 10, 2022, 12:57pm

When we wrote Erlang.NET early last year, we “needed” a way for VB.NET (or C#) to synchronously call functions in Erlang in quite a few places, so this is the means we came up with

github.com

robashton/erlang.net/blob/master/c_src/utils.cpp#L12

    
      
          
          
#include <stdio.h>
          #include <stdint.h>
          #include <stdlib.h>
          #include <string.h>
          #include <assert.h>
          #include <unistd.h>
          
          
#include "types.h"
          
          
ERL_NIF_TERM call_erlang_fn(ErlNifEnv* env, ERL_NIF_TERM mfa) {
            nif_globals* globals = (nif_globals*)enif_priv_data(env);
          
          
  // We'll be nice and do this as a resource so we can pass it to erlang and back to us
            callback_resource* callback = (callback_resource*)enif_alloc_resource(globals->callback_resource, sizeof(callback_resource));
            memset(callback, 0, sizeof(callback_resource));
            
            callback->condition =  enif_cond_create((char*)"call_fn_wait");
            ErlNifMutex* wait_mutex = enif_mutex_create((char*)"call_fn_mutex");
            callback->env = env;

The resource is sent up to Erlang using an enif_send, we then do a spin on the result - which then written into the resource by the Erlang and the function can proceed and the result can be returned.

This is not the best way, it’d be best to design the code as stated above (keep an state accumulator and repeatedly invoke the C with it), but if that’s not an option…