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<h1 class="lined-header"><span>NIFs and port drivers</span></h1>
<p>Erlang.mk can not only build Erlang projects, but also the C code that some projects come with, like NIFs and port drivers.</p>
<p>There are two ways to build the C code: using a custom Makefile, or making Erlang.mk do it directly. The C code will be built as needed when you run <code>make</code>.</p>
<!-- @todo something for easier bootstrapping-->
<h2 id="_c_source_code_location_and_erlang_environment">C source code location and Erlang environment</h2>
<p>The C source code should be located in the <em>$(C_SRC_DIR)</em> directory. It defaults to <em>c_src/</em>. Should you need to modify it, all you need to do is to set the variable in your Makefile before including Erlang.mk:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#009900">C_SRC_DIR =</font> <font color="#009900">$(CURDIR)</font>/my_nif_source</tt></pre>
</div></div>
<p>When this directory exists, Erlang.mk will automatically create a file named <em>$(C_SRC_ENV)</em>. This file defaults to <em>$(C_SRC_DIR)/env.mk</em>. This can also be changed:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#009900">C_SRC_ENV =</font> <font color="#009900">$(C_SRC_DIR)</font>/erlang_env.mk</tt></pre>
</div></div>
<p>It contains a few variable definitions for the environment used for the build:</p>
<dl><dt><code>ERTS_INCLUDE_DIR</code></dt>
<dd><p>Path to the ERTS include files (<em>erl_driver.h</em>, <em>erl_nif.h</em> and more).</p>
</dd>
<dt><code>ERL_INTERFACE_INCLUDE_DIR</code></dt>
<dd><p>Path to the Erl_Interface include files (<em>ei.h</em> and related).</p>
</dd>
<dt><code>ERL_INTERFACE_LIB_DIR</code></dt>
<dd><p>Path to the Erl_Interface static libraries.</p>
</dd>
</dl>
<h2 id="_using_a_custom_makefile">Using a custom Makefile</h2>
<p>Erlang.mk will automatically run <code>make</code> if it detects a Makefile in <em>$(C_SRC_DIR)/Makefile</em>.</p>
<p>The Makefile should have at least two targets: a default target (which can be anything, for example <code>all</code>) which is invoked when building the C code, and a <code>clean</code> target invoked when cleaning it.</p>
<p>You can include the <em>env.mk</em> file to benefit from the Erlang environment detection:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>include env.mk</tt></pre>
</div></div>
<h2 id="_using_erlang_mk_directly">Using Erlang.mk directly</h2>
<p>You don't need to write a Makefile to build C source code, however. Erlang.mk comes with rules to build both shared libraries and executables, using the source files it finds in <em>$(C_SRC_DIR)</em>.</p>
<p>By default, Erlang.mk will create a shared library. To change this and create an executable instead, put this in your Makefile before including Erlang.mk:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#009900">C_SRC_TYPE =</font> executable</tt></pre>
</div></div>
<p>The generated file name varies depending on the type of project you have (shared library or executable) and on the platform you build the project on.</p>
<p>For shared libraries, the generated file name will be <em>$(C_SRC_OUTPUT)$(C_SRC_SHARED_EXTENSION)</em>, with the default being <em>$(CURDIR)/priv/$(PROJECT)</em> followed by the extension: <code>.dll</code> on Windows, <code>.so</code> everywhere else.</p>
<p>For executables, the generated file name is <em>$(C_SRC_OUTPUT)$(C_SRC_EXECUTABLE_EXTENSION)</em>, with the same default except for the extension: <code>.exe</code> on Windows, and otherwise nothing.</p>
<p>Erlang.mk sets appropriate compile and linker flags by default. These flags vary depending on the platform, and can of course be overriden.</p>
<dl><dt><code>CC</code></dt>
<dd><p>The compiler to be used.</p>
</dd>
<dt><code>CFLAGS</code></dt>
<dd><p>C compiler flags.</p>
</dd>
<dt><code>CXXFLAGS</code></dt>
<dd><p>C++ compiler flags.</p>
</dd>
<dt><code>LDFLAGS</code></dt>
<dd><p>Linker flags.</p>
</dd>
<dt><code>LDLIBS</code></dt>
<dd><p>Libraries to link against.</p>
</dd>
</dl>
<p>The source files are automatically gathered from the contents of <em>$(C_SRC_DIR)</em>. Erlang.mk looks for <em>.c</em>, <em>.C</em>, <em>.cc</em> and <em>.cpp</em> source files. You can define the variable <code>SOURCES</code> to manually list the files to compile.</p>
<h2 id="_propagating_compile_and_linker_flags_to_sub_makefiles">Propagating compile and linker flags to sub-Makefiles</h2>
<p>In some cases it might be necessary to propagate the flags you just defined to the sub-Makefiles of your local project. You generally can't just export those as this could impact the building of dependencies.</p>
<p>Makefiles allow you to export variables for specific targets. When doing this, the variables will be exported only when this target runs, and not for other targets. It is therefore possible to export them when building the C code without impacting other build steps.</p>
<p>By adding this to your Makefile all five variables will be made available to sub-Makefiles when building C code:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#990000">app-c_src:</font> <b><font color="#0000FF">export</font></b> CC <font color="#990000">+=</font>
<font color="#990000">app-c_src:</font> <b><font color="#0000FF">export</font></b> CFLAGS <font color="#990000">+=</font>
<font color="#990000">app-c_src:</font> <b><font color="#0000FF">export</font></b> CPPFLAGS <font color="#990000">+=</font>
<font color="#990000">app-c_src:</font> <b><font color="#0000FF">export</font></b> LDFLAGS <font color="#990000">+=</font>
<font color="#990000">app-c_src:</font> <b><font color="#0000FF">export</font></b> LDLIBS <font color="#990000">+=</font></tt></pre>
</div></div>
<p>Appending an empty string to the existing value is necessary because Makefiles expect an assignment for target-specific exports. Alternatively you can set a new value:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#990000">app-c_src:</font> <b><font color="#0000FF">export</font></b> CFLAGS <font color="#990000">=</font> -O<font color="#993399">3</font></tt></pre>
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