Erlang/Elixir: 外部通信之-NIF

系列:
Erlang/Elixir: 外部通信之-NIF
Erlang/Elixir: 外部通信之-端口驱动
Erlang/Elixir: 外部通信之-C节点

本文阐述了如何使用rebar3来构建一个简单的NIF(内建函数)

使用Makefile模板

让我们从创建一个新的名为test_nif的lib, 使用test_nif项目中的cmake模板开始

$ rebar3 new lib test_nif
===> Writing test_nif/src/test_nif.erl
===> Writing test_nif/src/test_nif.app.src
===> Writing test_nif/rebar.config
===> Writing test_nif/.gitignore
===> Writing test_nif/LICENSE
===> Writing test_nif/README.md
$ cd test_nif
$ rebar3 new cmake
===> Writing c_src/Makefile

设置钩子

{erl_opts, [debug_info]}.
{deps, []}.
{pre_hooks,[
    {"(linux|darwin|solaris)", compile, "make -C c_src"},
    {"(freebsd)", compile, "gmake -C c_src"}
]}.
{post_hooks,[
    {"(linux|darwin|solaris)", clean, "make -C c_src clean"},
    {"(freebsd)", clean, "gmake -C c_src clean"}
]}.

Makefile 是由rebar3 new cmake命令创建的.

下面是一个NIF, 包含一个repeat函数, 接受一个pid和一个erlang term作为参数, 向该pid发送消息:

c_src/test_nif.c

#include "erl_nif.h"

ERL_NIF_TERM
mk_atom(ErlNifEnv* env, const char* atom)
{
    ERL_NIF_TERM ret;

    if(!enif_make_existing_atom(env, atom, &ret, ERL_NIF_LATIN1))
    {
        return enif_make_atom(env, atom);
    }

    return ret;
}

ERL_NIF_TERM
mk_error(ErlNifEnv* env, const char* mesg)
{
    return enif_make_tuple2(env, mk_atom(env, "error"), mk_atom(env, mesg));
}

static ERL_NIF_TERM
repeat(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
{
    ErlNifEnv* msg_env;
    ErlNifPid pid;
    ERL_NIF_TERM copy;

    if(argc != 2)
    {
        return enif_make_badarg(env);
    }

    if(!enif_is_pid(env, argv[0]))
    {
        return mk_error(env, "not_a_pid");
    }

    if(!enif_get_local_pid(env, argv[0], &pid))
    {
        return mk_error(env, "not_a_local_pid");
    }

    msg_env = enif_alloc_env();
    if(msg_env == NULL)
    {
        return mk_error(env, "environ_alloc_error");
    }

    copy = enif_make_copy(msg_env, argv[1]);

    if(!enif_send(env, &pid, msg_env, copy))
    {
        enif_free(msg_env);
        return mk_error(env, "error_sending_term");
    }

    enif_free_env(msg_env);
    return mk_atom(env, "ok");
}

static ErlNifFunc nif_funcs[] = {
    {"repeat", 2, repeat}
};

ERL_NIF_INIT(test_nif, nif_funcs, NULL, NULL, NULL, NULL);

测试代码 src/test_nif.erl

-module(test_nif).
-export([repeat/2]).
-on_load(init/0).

-define(APPNAME, test_nif).
-define(LIBNAME, test_nif).

repeat(_, _) ->
    not_loaded(?LINE).

init() ->
    SoName = case code:priv_dir(?APPNAME) of
        {error, bad_name} ->
            case filelib:is_dir(filename:join(["..", priv])) of
                true ->
                    filename:join(["..", priv, ?LIBNAME]);
                _ ->
                    filename:join([priv, ?LIBNAME])
            end;
        Dir ->
            filename:join(Dir, ?LIBNAME)
    end,
    erlang:load_nif(SoName, 0).

not_loaded(Line) ->
    exit({not_loaded, [{module, ?MODULE}, {line, Line}]}).

运行 rebar3 shell

$ rebar3 shell
===> Verifying dependencies...
===> Compiling test_nif
Erlang/OTP 17 [erts-6.3] [source] [64-bit] [smp:4:4] [async-threads:0] [kernel-poll:false]

Eshell V6.3  (abort with ^G)
1> test_nif:repeat(self(), hello).
ok
2> receive X -> X end.
hello