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    <title>NIFs</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>nif.xml</file>
  </header>
  <p>This section outlines an example of how to solve the example
    problem in <seeguide marker="example">Problem Example</seeguide>
    by using Native Implemented Functions (NIFs).</p>
  <p>NIFs were introduced in Erlang/OTP R13B03 as an experimental
    feature. It is a simpler and more efficient way of calling C-code
    than using port drivers. NIFs are most suitable for synchronous
    functions, such as <c>foo</c> and <c>bar</c> in the example, that
    do some relatively short calculations without side effects and
    return the result.</p>
  <p>A NIF is a function that is implemented in C instead of Erlang.
    NIFs appear as any other functions to the callers. They belong to
    a module and are called like any other Erlang functions. The NIFs
    of a module are compiled and linked into a dynamic loadable,
    shared library (SO in UNIX, DLL in Windows). The NIF library must
    be loaded in runtime by the Erlang code of the module.</p>
  <p>As a NIF library is dynamically linked into the emulator process,
    this is the fastest way of calling C-code from Erlang (alongside
    port drivers). Calling NIFs requires no context switches. But it
    is also the least safe, because a crash in a NIF brings the
    emulator down too.</p>

  <section>
    <title>Erlang Program</title>
    <p>Even if all functions of a module are NIFs, an Erlang
      module is still needed for two reasons:</p>
    <list type="bulleted">
      <item>The NIF library must be explicitly loaded by
      Erlang code in the same module.</item>
      <item>All NIFs of a module must have an Erlang implementation
      as well.</item>
     </list>
   <p>Normally these are minimal stub implementations that throw an
     exception. But they can also be used as fallback implementations
     for functions that do not have native implemenations on some
     architectures.</p>
   <p>NIF libraries are loaded by calling <c>erlang:load_nif/2</c>,
     with the name of the shared library as argument. The second
     argument can be any term that will be passed on to the library
     and used for initialization:</p>

    <codeinclude file="complex6.erl" tag="" type="none"></codeinclude>

    <p>Here, the directive <c>on_load</c> is used to get function
      <c>init</c> to be automatically called when the module is
      loaded. If <c>init</c> returns anything other than <c>ok</c>,
      such when the loading of the NIF library fails in this example,
      the module is unloaded and calls to functions within it,
      fail.</p>
    <p>Loading the NIF library overrides the stub implementations
      and cause calls to <c>foo</c> and <c>bar</c> to be dispatched to
      the NIF implementations instead.</p>
  </section>
  <section>
    <title>NIF Library Code</title>
    <p>The NIFs of the module are compiled and linked into a
      shared library. Each NIF is implemented as a normal C function. The macro
      <c>ERL_NIF_INIT</c> together with an array of structures defines the names,
      arity, and function pointers of all the NIFs in the module. The header
      file <c>erl_nif.h</c> must be included. As the library is a shared
      module, not a program, no main function is to be present.</p>
    <p>The function arguments passed to a NIF appears in an array <c>argv</c>,
      with <c>argc</c> as the length of the array, and thus the arity of the
      function. The Nth argument of the function can be accessed as
      <c>argv[N-1]</c>. NIFs also take an environment argument that
      serves as an opaque handle that is needed to be passed on to
      most API functions. The environment contains information about
      the calling Erlang process:</p>

    <codeinclude file="complex6_nif.c" tag="" type="none"></codeinclude>

    <p>Here,<c>ERL_NIF_INIT</c> has the following arguments:</p>
    <list type="bulleted">
      <item><p>The first argument must be the name of the
      Erlang module as a C-identifier. It will be stringified by the
      macro.</p>
      </item>
      <item>The second argument is the array of <c>ErlNifFunc</c>
      structures containing name, arity, and function pointer of
      each NIF.</item>
      <item>The remaining arguments are pointers to callback functions
      that can be used to initialize the library. They are not used
      in this simple example, hence they are all set to <c>NULL</c>.</item>
    </list>
    <p>Function arguments and return values are represented as values
      of type <c>ERL_NIF_TERM</c>. Here, functions like <c>enif_get_int</c>
      and <c>enif_make_int</c> are used to convert between Erlang term
      and C-type.
      If the function argument <c>argv[0]</c> is not an integer,
      <c>enif_get_int</c> returns false, in which case it returns
      by throwing a <c>badarg</c>-exception with <c>enif_make_badarg</c>.</p>
  </section>

  <section>
    <title>Running the Example</title>
    <p><em>Step 1.</em> Compile the C code:</p>
    <pre>
unix> <input>gcc -o complex6_nif.so -fpic -shared complex.c complex6_nif.c</input>
windows> <input>cl -LD -MD -Fe complex6_nif.dll complex.c complex6_nif.c</input></pre>
    <p><em>Step 2:</em> Start Erlang and compile the Erlang code:</p>
    <pre>
> <input>erl</input>
Erlang R13B04 (erts-5.7.5) [64-bit] [smp:4:4] [rq:4] [async-threads:0] [kernel-poll:false]

Eshell V5.7.5  (abort with ^G)
1> <input>c(complex6).</input>
{ok,complex6}</pre>
    <p><em>Step 3:</em> Run the example:</p>
<pre>
3> <input>complex6:foo(3).</input>
4
4> <input>complex6:bar(5).</input>
10
5> <input>complex6:foo("not an integer").</input>
** exception error: bad argument
     in function  complex6:foo/1
        called as comlpex6:foo("not an integer")
</pre>
</section>
</chapter>
