Creating an elixir nif in zig

Zig is a language that has excellent interoperability with C, and for this reason, I use it a lot to interact with C libraries. I also use elixir as a daily driver for server applications.

In this post, I'll detail the step to be able to call zig code from elixir.

Zigler

First I'll point to zigler which is a project that allows for easy embedding of zig code in elixir. I am not using it for a few reasons:

• My code needs to be callable from other languages, so I don't want to ember the zig code in .ex files.
• This particular project is moving a lot of raw byte data, and I need more control over how it is called than what zigler provides.
• I need to interact with C libraries in a way that is not practical with zigler.

But it is a very nice way to get some zig code into your elixir and I recommend it.

Doing a NIF from scratch

NIF is a way to write erlang code in C. The first reference you want to read is the tutorial.

The idea is to have C functions that receive elixir arguments and return data as elixir types.

For this, you need two things:

• A dynamic library, .so in nix world and .dll in windows world.
• An elixir module describing the functions. This module can be in erlang, but I'll use elixir.

Let's start with the elixir module as it is the simplest.

As folder paths and such are a bit of a pain to figure out, I'll describe every single steps.

Elixir module

mkdir nif_test
cd nif_test

Now edit nif_test.ex and put this inside:

defmodule NifTest do
  @on_load :load_nifs

  def load_nifs do
    :erlang.load_nif('./build/libnif_test', 0)
  end

  def foo_test(_a) do
    raise "NIF foo_test/1 not implemented"
  end
end

If you fire iex and load the module you will get an error:

$ iex
iex(1)> c "nif_test.ex"

10:48:35.666 [warning] The on_load function for module Elixir.NifTest returned:
{:error,
 {:load_failed,
  'Failed to load NIF library: \'./build/libnif_test.so: cannot open shared object file: No such file or directory\''}}

[NifTest]
iex(2)>

This is normal of course, the dynamic library does not exist yet.

You will notice that the extension .so is added by the loader.

Zig module

Initialize the folder with a build.zig and a main.zig.

We are going to replace them fully, but you can do:

zig int-lib

It will create them in the current folder.

Now update build.zig with the following:

const std = @import("std");
const Pkg = std.build.Pkg;

pub fn build(b: *std.build.Builder) void {
    const mode = b.standardReleaseOptions();

    const nif_step = b.step("nif_lib", "Compiles erlang library");
    const nif_lib = b.addSharedLibrary("nif_test", "./src/main.zig", .unversioned);
    nif_lib.setBuildMode(mode);
    nif_lib.setOutputDir("build");
    nif_lib.addIncludePath("/usr/lib/erlang/usr/include/");

    nif_lib.install();
    nif_lib.linkLibC();
    nif_step.dependOn(&nif_lib.step);
}

This will add the erlang header path to the build environment and also link with the C library.

Finally, in main.zig:

const std = @import("std");
const erl = @cImport({
    @cInclude("erl_nif.h");
});

export fn foo_test(
    env: ?*erl.ErlNifEnv,
    argc: c_int,
    argv: [*c]const erl.ERL_NIF_TERM,
) erl.ERL_NIF_TERM {
    var test_binary: erl.ErlNifBinary = undefined;

    if ((argc != 1) or
        (erl.enif_inspect_binary(env, argv[0], &test_binary) != 1))
    {
        return erl.enif_make_badarg(env);
    }
    var test_slice: []const u8 = &.{};
    test_slice.len = test_binary.size;
    test_slice.ptr = test_binary.data;

    std.debug.print("Test slice: {s}\n\n", .{test_slice});

    return erl.enif_make_int(env, 2);
}

const func_count = 1;

var funcs = [func_count]erl.ErlNifFunc{
    erl.ErlNifFunc{
        .name = "foo_test",
        .arity = 1,
        .fptr = foo_test,
        .flags = 0,
    },
};

var entry = erl.ErlNifEntry{
    .major = erl.ERL_NIF_MAJOR_VERSION,
    .minor = erl.ERL_NIF_MINOR_VERSION,
    .name = "Elixir.NifTest",
    .num_of_funcs = func_count,
    .funcs = &funcs,
    .load = null,
    .reload = null,
    .upgrade = null,
    .unload = null,
    .vm_variant = "beam.vanilla",
    .options = 1,
    .sizeof_ErlNifResourceTypeInit = @sizeOf(erl.ErlNifResourceTypeInit),
    .min_erts = "erts-10.4",
};

export fn nif_init() *erl.ErlNifEntry {
    return &entry;
}

Finally, zig build nif_lib to build the module and run it with:

$ iex
iex(1)> c "nif_test.ex"
[NifTest]
iex(2)> NifTest.foo_test("1234")
Test slice: 1234

                2
iex(3)>

We see the output from zig. Of course, you should not output like that in real code, and instead always return data to erlang/elixir.

Detailed explaination

As we have a running minimal example, I will explain the code in details.

nif_test.ex

defmodule NifTest do
  # This is called when the module is loaded
  # It is important to call load_nifs from within the module, as
  # NIF can only be loaded within the module it will populate
  @on_load :load_nifs

  def load_nifs do
    # In a real nif, you would adjust this path
    # Second argument is what you C code will receive, we are ignoring it
    # so just pass 0 which is NULL on the C side
    :erlang.load_nif('./build/libnif_test', 0)
  end

  # Our first function, it needs to have the same name and arity as C version
  # Implementation is not important, but it is good manner to raise
  # if tthe NIF is not loaded.
  # The NIF version will come override this one.
  def foo_test(_a) do
    raise "NIF foo_test/1 not implemented"
  end
end

build.zig

const std = @import("std");
const Pkg = std.build.Pkg;

pub fn build(b: *std.build.Builder) void {

    // Add a build step, this will now be callable with `zig build nif_lib`
    // Steps are optional, you can ommit it, and `zig build` will invoque it
    const nif_step = b.step("nif_lib", "Compiles erlang library");
    // Create a shared library, the output name will have `lib` and `.so` added to it
    // It is not versioned as we will usually use elixir package versioning
    // instead, and we do not plan to distribute the library by itself
    const nif_lib = b.addSharedLibrary("nif_test", "./src/main.zig", .unversioned);

    const mode = b.standardReleaseOptions();
    // Use release mode
    nif_lib.setBuildMode(mode);

    // Override build dir, this is a convention of mine as it plays well with
    // IDE, but the default `zig-out` is fine
    nif_lib.setOutputDir("build");

    // We need to tell zig where to find the erlang headers
    nif_lib.addIncludePath("/usr/lib/erlang/usr/include/");

    // As erlang nif library is in C, we need this
    nif_lib.linkLibC();

    // Install the lib inside the
    nif_lib.install();
    nif_step.dependOn(&nif_lib.step);
}

main.zig

// Load zig standard library, here we need it for debug print to stderr
const std = @import("std");

// Load ERL library header
const erl = @cImport({
    @cInclude("erl_nif.h");
});

// Declare a NIF function
// Every function in your module will have the same zig signature
export fn foo_test(
    // This is the erlang environment, we need this for all NIF operations
    env: ?*erl.ErlNifEnv,
    // The number of argument your function was called with
    argc: c_int,
    // An array of arguments
    argv: [*c]const erl.ERL_NIF_TERM,
) erl.ERL_NIF_TERM { // Returned value is also a erlang term
    // Our test function is going to accept a single argument, a binary
    // We need to allocate the stack for it
    var test_binary: erl.ErlNifBinary = undefined;

    // We check that we have one argument and that we can transform this
    // argument into a binary, a binary is just a serie of bytes
    if ((argc != 1) or
        // we try to convert the argument at position 0 into a binary
        (erl.enif_inspect_binary(env, argv[0], &test_binary) != 1))
    {
        // If it fails, we return a bad argument error
        return erl.enif_make_badarg(env);
    }
    // If we are here, it's a success, we got our binary
    // Note that the binary data contained into `test_binary` will be
    // deallocated when this function returns
    // If you want to keep it longer, you need top copy it
    //
    // What we do here is that we build a slice and point it to the binary
    // this does NOT copy any data and this slice would be invalid after this
    // function returns
    //
    // Create an empty slice (pointer + len)
    var test_slice: []const u8 = &.{};
    // Set len
    test_slice.len = test_binary.size;
    // Set ptr
    test_slice.ptr = test_binary.data;

    // Now our slice is printable as a C string by zig formatter
    // This prints to stderr by default
    std.debug.print("Test slice: {s}\n\n", .{test_slice});

    // We return an int, 2, to demonstrate how to return data
    return erl.enif_make_int(env, 2);
}


// How many functions will our module have?
const func_count = 1;

// Create an array of function
var funcs = [func_count]erl.ErlNifFunc{
    // Our test function
    erl.ErlNifFunc{
        // The erlang/elixir name, this must match the name in the module
        .name = "foo_test",
        // The arity, note that you can declare multiple functions with same
        // name and different arity
        .arity = 1,
        // The function implementation, this must match the name in the zig code
        // above.
        // You can use the same zig function for multiple arity, thus the
        // argc argument to the zig function
        .fptr = foo_test,
        // Flags can be set to mark the function as dirty
        // Dirty functions can take more CPU time
        // Regular function must be fast, <1 ms
        // You can set this to erl.ERL_NIF_DIRTY_JOB_CPU_BOUND or
        // erl.ERL_NIF_DIRTY_JOB_IO_BOUND
        .flags = 0,
    },
};


// All the following code is required because it is implemented as macro in the
// C version With C, you just call the magic macro ERL_NIF_INIT
// but we cannot do that in zig, so here is a full manual implementation

// This is the NIF entry for out module
var entry = erl.ErlNifEntry{
    // We just the version of the header
    .major = erl.ERL_NIF_MAJOR_VERSION,
    .minor = erl.ERL_NIF_MINOR_VERSION,
    // If this is an elixir module that will be available as `NifTest`
    // on the elixir side, we need to add `Elixir` in front of it
    .name = "Elixir.NifTest",
    // Hor many functions
    .num_of_funcs = func_count,
    // Pointer to function definition array
    .funcs = &funcs,
    // Callbacks for the lifecycle of our NIF, they can be nil
    .load = null,
    .reload = null,
    .upgrade = null,
    .unload = null,
    // Which VM variant
    .vm_variant = "beam.vanilla",
    // This can be erl.ERL_NIF_DIRTY_NIF_OPTION
    .options = 0,
    // This must be defined as is
    .sizeof_ErlNifResourceTypeInit = @sizeOf(erl.ErlNifResourceTypeInit),
    // Minimum erts version
    .min_erts = "erts-10.4",
};

// This is the function that will be called by the erlang runtime when loading
// the module
export fn nif_init() *erl.ErlNifEntry {
    return &entry;
}

The repository with sample code is available on my github.