Elixir NIF- Hello World example on x64 Mac OSX - macos

Question

Hi I'm trying to get the Hello World example for Erlang NIF (Native Implemented Function) shown here
http://www.erlang.org/doc/man/erl_nif.html
to work from Elixir on OSX 64bit.
First I create the C-code:
/* niftest.c */
#include "erl_nif.h"
static ERL_NIF_TERM hello(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
return enif_make_string(env, "Hello world!", ERL_NIF_LATIN1);
}
static ErlNifFunc nif_funcs[] =
{
{"hello", 0, hello}
};
ERL_NIF_INIT(niftest,nif_funcs,NULL,NULL,NULL,NULL)
Then I successfully compile it using gcc for 64 bit architecture as suggested here
Erlang NIF Test -- OS X Lion
gcc -undefined dynamic_lookup -dynamiclib niftest.c -o niftest.so -I /usr/local/Cellar/erlang/R14B02/lib/erlang/usr/include
which produces the necessary file niftest.so that I should be able to interface-into from Erlang/Elixir. My Elixir (niftest.ex) looks like this (inspired by a more complex example reported here):
defmodule Niftest do
#onload :init
def init() do
:erlang.load_nif("./niftest", 0)
:ok
end
def hello() do
"NIF library not loaded"
end
end
Now with niftest.so and niftest.ex in the same directory I fire up elixir using iex and type in Niftest.hello and all I get back is: "NIF library not loaded"
Am I missing a vital step? - please help!

Answer 1

The load of the library is failing silently. You can assert that it succeeds using:
:ok = :erlang.load_nif("./niftest", 0)
This results in an error:
** (MatchError) no match of right hand side value:
{:error, {:bad_lib, 'Library module name \'niftest\' does not match calling module \'\'Elixir.Niftest\'\''}}
niftest.ex:4: Niftest.init/0
That happens because a NIF lib can only be called from its "owning" module. The name of that module is the first argument to the ERL_NIF_INIT macro, so you can fix this by changing that call and recompiling:
ERL_NIF_INIT(Elixir.Niftest,nif_funcs,NULL,NULL,NULL,NULL)
There is also a typo in the load hook. It should be:
#on_load :init