path: root/erts/doc/src/erl_nif.xml

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  <header>
    <copyright>
      <year>2001</year><year>2009</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.
    
      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.
    
    </legalnotice>

    <title>erl_nif</title>
    <prepared>Sverker Eriksson</prepared>
    <responsible>Sverker Eriksson</responsible>
    <docno>1</docno>
    <approved></approved>
    <checked></checked>
    <date>2009-11-17</date>
    <rev>PA1</rev>
    <file>erl_nif.xml</file>
  </header>
  <lib>erl_nif</lib>
  <libsummary>API functions for an Erlang NIF library</libsummary>
  <description>
     <warning><p>The NIF concept is introduced in R13B03 as an
	EXPERIMENTAL feature. The interfaces may be changed in any way
	in coming releases. The API introduced in this release is very
	sparse and contains only the most basic functions to read and
	write Erlang terms.
	</p></warning>

    <p>A NIF library contains native implementation of some functions
    of an erlang module. The native implemented functions (NIFs) are
    called like any other functions without any difference to the
    caller. Each NIF must also have an implementation in Erlang that
    will be invoked if the function is called before the NIF library
    has been successfully loaded. A typical such stub implementation
    is to throw an exception. But it can also be used as a fallback
    implementation if the NIF library is not implemented for some
    architecture.</p> 
    <p>A minimal example of a NIF library can look like this:</p>
      <p/>
      <code type="none">
/* niftest.c */
#include "erl_nif.h"

static ERL_NIF_TERM hello(ErlNifEnv* env)
{
    return enif_make_string(env, "Hello world!");
}

static ErlNifFunc nif_funcs[] =
{
    {"hello", 0, hello}
};

ERL_NIF_INIT(niftest,nif_funcs,NULL,NULL,NULL,NULL)
</code>

    <p>and the erlang module would have to look something like
    this:</p>
      <p/>
      <code type="none">
-module(niftest).

-export([init/0, hello/0]).

init() ->
      erlang:load_nif("./niftest", 0).

hello() ->
      "NIF library not loaded".
</code>
    <p>and compile and test something like this (on Linux):</p>
      <p/>
      <code type="none">
$> gcc -fPIC -shared -o niftest.so niftest.c -I $ERL_ROOT/usr/include/
$> erl

1> c(niftest).
{ok,niftest}
2> niftest:hello().
"NIF library not loaded"
3> niftest:init().
ok
4> niftest:hello().
"Hello world!"
</code>

     <p>A better solution for a real module is to take advantage of
     the new attribute <c>on_load</c> to automatically load the NIF
     library when the module is loaded.</p>
     <p>A loaded NIF library is tied to the Erlang module code version
     that loaded it. If the module is upgraded with a new version, the
     new code will have to load its own NIF library (or maybe choose not
     to). The new code version can however choose to load the exact
     same NIF library as the old code if it wants to. Sharing the same
     dynamic library will mean that static data defined by the library
     will be shared as well. To avoid unintentionally shared static
     data, each Erlang module code can keep its own private data. This
     global private data can be set when the NIF library is loaded and
     then retrieved by calling <seealso marker="erl_nif#enif_get_data">enif_get_data()</seealso>.</p>
    <p>There is currently no way to explicitly unload a NIF
     library. A library will be automatically unloaded when the module
     code that it belongs to is purged by the code server. A NIF
     library will can also be unloaded by replacing it with another
     version of the library by a second call to
     <c>erlang:load_nif/2</c> from the same module code.</p> 
  </description>

  <section>
    <title>INITIALIZATION</title>
    <taglist>
      <tag><marker id="ERL_NIF_INIT"/>ERL_NIF_INIT(MODULE, ErlNifFunc funcs[], load, reload, upgrade, unload)</tag>
      <item><p>This is the magic macro to initialize a NIF library. It
      should be evaluated in global file scope.</p>
      <p><c>MODULE</c> is the name of the Erlang module as an
      identifier without string quotations. It will be stringified by
      the macro.</p>
      <p><c>funcs</c> is a static array of function descriptors for
      all the implemented NIFs in this library.</p>
      <p><c>load</c>, <c>reload</c>, <c>upgrade</c> and <c>unload</c>
      are pointers to functions. One of <c>load</c>, <c>reload</c> or
      <c>upgrade</c> will be called to initialize the library.
      <c>unload</c> is called to release the library. They are all
      described individually below.</p>
      </item>

      <tag><marker id="load"/>int (*load)(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info)</tag>
       <item><p><c>load</c> is called when the NIF library is loaded
        and there is no previously loaded library for this module.</p>
        <p><c>*priv_data</c> can be set to point to some private data
           that the library needs in able to keep a state between NIF 
           calls. <c>enif_get_data()</c> will return this pointer.</p> 
        <p><c>load_info</c> is the second argument to <seealso
           marker="erlang#erlang:load_nif-2">erlang:load_nif/2</seealso>.</p>
        <p>The library will fail to load if <c>load</c> returns
           anything other than 0. <c>load</c> can be NULL in case no
           initialization is needed.</p> 
       </item>

      <tag><marker id="reload"/>int (*reload)(ErlNifEnv* env, void** priv_data, ERL_NIF_TERM load_info)</tag>
       <item><p><c>reload</c> is called when the NIF library is loaded
        and there is already a previously loaded library for this
        module code.</p>
        <p>Works the same as <c>load</c>. The only difference is that
        <c>*priv_data</c> already contains the value set by the
        previous call to <c>load</c> or <c>reload</c>.</p>
        <p>The library will fail to load if <c>reload</c> returns
           anything other than 0 or if <c>reload</c> is NULL.</p> 
      </item>

      <tag><marker id="upgrade"/>int (*upgrade)(ErlNifEnv* env, void** priv_data, void** old_priv_data, ERL_NIF_TERM load_info)</tag>
       <item><p><c>upgrade</c> is called when the NIF library is loaded
        and there is no previously loaded library for this module
        code, BUT there is old code of this module with a
        loaded NIF library.</p>
        <p>Works the same as <c>load</c>. The only difference is that
        <c>*old_priv_data</c> already contains the value set by the
         last call to <c>load</c> or <c>reload</c> for the old module
         code. It is allowed to write to both *priv_data and
         *old_priv_data.</p> 
        <p>The library will fail to load if <c>upgrade</c> returns
           anything other than 0 or if <c>upgrade</c> is NULL.</p>
      </item>

      <tag><marker id="unload"/>void (*unload)(ErlNifEnv* env, void* priv_data)</tag>
       <item><p><c>unload</c> is called when the module code that
       the NIF library belongs to is purged as old. New code
       of the same module may or may not exist.</p>
      </item>


    </taglist>
  </section>

  <section>
    <title>DATA TYPES</title>

    <taglist>
     <tag><marker id="ErlDrvEnv"/>ErlDrvEnv</tag>
      <item>
        <p><c>ErlNifEnv</c> contains information about the context in
          which a NIF call is made. This pointer should not be
          dereferenced in any way, but only passed on to API
          functions. An <c>ErlNifEnv</c> pointer is only valid until
          the function, where is what supplied as argument,
          returns. There is thus useless and dangerous to store <c>ErlNifEnv</c>
          pointers in between NIF calls.</p>
      </item>
    <tag><marker id="ErlNifFunc"/>ErlNifFunc</tag>
     <item>
      <p/>
      <code type="none">
typedef struct {
    const char* name;
    unsigned arity;
    ERL_NIF_TERM (*fptr)(ErlNifEnv* env, ...);
} ErlNifFunc;
</code>
        <p>Describes a NIF by its name, arity and implementation.
        <c>fptr</c> is a pointer to the function that implements the
        NIF. The number of arguments must match the arity. A NIF of
        arity 2 will thus look like:</p> 
      <p/>
      <code type="none">
ERL_NIF_TERM my_nif(ErlNifEnv* env, ERL_NIF_TERM arg1, ERL_NIF_TERM arg2)
{
    /* ... */
}
</code>
        <p>The maximum allowed arity for a NIF is 3 in current implementation.</p>
      </item>
    <tag><marker id="ErlNifBinary"/>ErlNifBinary</tag>
     <item>
      <p/>
      <code type="none">
typedef struct {
    unsigned size;
    unsigned char* data;
} ErlNifBinary;
</code>
        <p><c>ErlNifBinary</c> contains transient information about an
          inspected binary term. <c>data</c> is a pointer to a buffer
          of <c>size</c> bytes with the raw content of the binary.</p>
      </item>
      <tag><marker id="ERL_NIF_TERM"/>ERL_NIF_TERM</tag>
       <item>
        <p>Variables of type <c>ERL_NIF_TERM</c> can refere to any
        Erlang term. This is an opaque type and values of it can only
        by used either as arguments to API functions or as return
        values from NIFs. A variable of type <c>ERL_NIF_TERM</c> is
        only valid until the NIF call, where it was obtained,
        returns.</p>
      </item>
    </taglist>
  </section>

  <funcs>
    <func><name><ret>void*</ret><nametext>enif_get_data(ErlNifEnv* env)</nametext></name>
      <fsummary>Get the private data of a NIF library</fsummary>
      <desc><p>Returns the pointer to the private data that was set by <c>load</c>, <c>reload</c> or <c>upgrade</c>.</p></desc>
    </func>
    <func><name><ret>void*</ret><nametext>enif_alloc(ErlNifEnv* env, size_t size)</nametext></name>
      <fsummary>Allocate dynamic memory.</fsummary>
      <desc><p>Allocate memory of <c>size</c> bytes.</p></desc>
    </func>
    <func><name><ret>void</ret><nametext>enif_free(ErlNifEnv* env, void* ptr)</nametext></name>
      <fsummary>Free dynamic memory</fsummary>
      <desc><p>Free memory allocated by <c>enif_alloc</c>.</p></desc>
    </func>
    <func><name><ret>int</ret><nametext>enif_is_binary(ErlNifEnv* env, ERL_NIF_TERM term)</nametext></name>
      <fsummary>Determine if a term is a binary</fsummary>
      <desc><p>Return true if <c>term</c> is a binary</p></desc>
    </func>
    <func><name><ret>int</ret><nametext>enif_inspect_binary(ErlNifEnv* env, ERL_NIF_TERM bin_term, ErlNifBinary* bin)</nametext></name>
      <fsummary>Inspect the content of a binary</fsummary>
      <desc><p>Initialize the structure pointed to by <c>bin</c> with
      transient information about the binary term
      <c>bin_term</c>. Return false if <c>bin_term</c> is not a binary.</p></desc> 
    </func>
    <func><name><ret>int</ret><nametext>enif_alloc_binary(ErlNifEnv* env, unsigned size, ErlNifBinary* bin)</nametext></name>
      <fsummary>Create a new binary.</fsummary>
      <desc><p>Allocate a new binary of size of <c>size</c>
      bytes. Initialize the structure pointed to by <c>bin</c> to
      refer to the allocated binary.</p></desc>
    </func>
    <func><name><ret>void</ret><nametext>enif_release_binary(ErlNifEnv* env, ErlNifBinary* bin)</nametext></name>
      <fsummary>Release a binary.</fsummary>
      <desc><p>Release a binary obtained from <c>enif_alloc_binary</c> or <c>enif_inspect_binary</c>.</p></desc>
    </func>
    <func><name><ret>int</ret><nametext>enif_get_int(ErlNifEnv* env, ERL_NIF_TERM term, int* ip)</nametext></name>
      <fsummary>Read an integer term.</fsummary>
      <desc><p>Set <c>*ip</c> to the integer value of
      <c>term</c> or return false if <c>term</c> is not an integer or is
      outside the bounds of type <c>int</c></p></desc>
    </func>
    <func><name><ret>int</ret><nametext>enif_get_ulong(ErlNifEnv* env, ERL_NIF_TERM term, unsigned long* ip)</nametext></name>
      <fsummary>Read an unsigned long integer</fsummary>
      <desc><p>Set <c>*ip</c> to the unsigned long integer value of
      <c>term</c> or return false if <c>term</c> is not an unsigned
      integer or is outside the bounds of type <c>unsigned long</c></p></desc>
    </func>
    <func><name><ret>int</ret><nametext>enif_get_list_cell(ErlNifEnv* env, ERL_NIF_TERM list, ERL_NIF_TERM* head, ERL_NIF_TERM* tail)</nametext></name>
      <fsummary>Get head and tail from a list</fsummary>
      <desc><p>Set <c>*head</c> and <c>*tail</c> from
      <c>list</c> or return false if <c>list</c> is not a non-empty
      list.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_binary(ErlNifEnv* env, ErlNifBinary* bin)</nametext></name>
      <fsummary>Make a binary term.</fsummary>
      <desc><p>Make a binary term from <c>bin</c>. Will also release
      the binary.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_badarg(ErlNifEnv* env)</nametext></name>
      <fsummary>Make a badarg exception.</fsummary>
      <desc><p>Make a badarg exception to be returned from a NIF.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_int(ErlNifEnv* env, int i)</nametext></name>
      <fsummary>Create an integer term</fsummary>
      <desc><p>Create an integer term.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_ulong(ErlNifEnv* env, unsigned long i)</nametext></name>
      <fsummary>Create an integer term from an unsigned long int</fsummary>
      <desc><p>Create an integer term from an <c>unsigned long int</c>.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_atom(ErlNifEnv* env, const char* name)</nametext></name>
      <fsummary>Create an atom term</fsummary>
      <desc><p>Create an atom term from the C-string <c>name</c>. Atom
      terms may be saved and used between NIF calls.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_tuple(ErlNifEnv* env, unsigned cnt, ...)</nametext></name>
      <fsummary>Create a tuple term.</fsummary>
      <desc><p>Create a tuple term of arity <c>cnt</c>. Expects
      <c>cnt</c> number of arguments (after <c>cnt</c>) of type ERL_NIF_TERM as the
      elements of the tuple.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_list(ErlNifEnv* env, unsigned cnt, ...)</nametext></name>
      <fsummary>Create a list term.</fsummary>
      <desc><p>Create an ordinary list term of length <c>cnt</c>. Expects
      <c>cnt</c> number of arguments (after <c>cnt</c>) of type ERL_NIF_TERM as the
      elements of the list. An empty list is returned if <c>cnt</c> is 0.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_list_cell(ErlNifEnv* env, ERL_NIF_TERM head, ERL_NIF_TERM tail)</nametext></name>
      <fsummary>Create a list cell.</fsummary>
      <desc><p>Create a list cell <c>[head | tail]</c>.</p></desc>
    </func>
    <func><name><ret>ERL_NIF_TERM</ret><nametext>enif_make_string(ErlNifEnv* env, const char* string)</nametext></name>
      <fsummary>Create a string.</fsummary>
      <desc><p>Creates a list containing the characters of the
      C-string <c>string</c>.</p></desc>
    </func>
  </funcs>
  <section>
    <title>SEE ALSO</title>
    <p><seealso marker="erlang#erlang:load_nif-2">load_nif(3)</seealso></p>
  </section>
</cref>