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-<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
-<html>
-<head>
-<title>FFI Library</title>
-<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
-<meta name="Author" content="Mike Pall">
-<meta name="Copyright" content="Copyright (C) 2005-2013, Mike Pall">
-<meta name="Language" content="en">
-<link rel="stylesheet" type="text/css" href="bluequad.css" media="screen">
-<link rel="stylesheet" type="text/css" href="bluequad-print.css" media="print">
-</head>
-<body>
-<div id="site">
-<a href="http://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
-</div>
-<div id="head">
-<h1>FFI Library</h1>
-</div>
-<div id="nav">
-<ul><li>
-<a href="luajit.html">LuaJIT</a>
-<ul><li>
-<a href="http://luajit.org/download.html">Download <span class="ext">&raquo;</span></a>
-</li><li>
-<a href="install.html">Installation</a>
-</li><li>
-<a href="running.html">Running</a>
-</li></ul>
-</li><li>
-<a href="extensions.html">Extensions</a>
-<ul><li>
-<a class="current" href="ext_ffi.html">FFI Library</a>
-<ul><li>
-<a href="ext_ffi_tutorial.html">FFI Tutorial</a>
-</li><li>
-<a href="ext_ffi_api.html">ffi.* API</a>
-</li><li>
-<a href="ext_ffi_semantics.html">FFI Semantics</a>
-</li></ul>
-</li><li>
-<a href="ext_jit.html">jit.* Library</a>
-</li><li>
-<a href="ext_c_api.html">Lua/C API</a>
-</li></ul>
-</li><li>
-<a href="status.html">Status</a>
-<ul><li>
-<a href="changes.html">Changes</a>
-</li></ul>
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-<a href="faq.html">FAQ</a>
-</li><li>
-<a href="http://luajit.org/performance.html">Performance <span class="ext">&raquo;</span></a>
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-<a href="http://wiki.luajit.org/">Wiki <span class="ext">&raquo;</span></a>
-</li><li>
-<a href="http://luajit.org/list.html">Mailing List <span class="ext">&raquo;</span></a>
-</li></ul>
-</div>
-<div id="main">
-<p>
-
-The FFI library allows <b>calling external C&nbsp;functions</b> and
-<b>using C&nbsp;data structures</b> from pure Lua code.
-
-</p>
-<p>
-
-The FFI library largely obviates the need to write tedious manual
-Lua/C bindings in C. No need to learn a separate binding language
-&mdash; <b>it parses plain C&nbsp;declarations!</b> These can be
-cut-n-pasted from C&nbsp;header files or reference manuals. It's up to
-the task of binding large libraries without the need for dealing with
-fragile binding generators.
-
-</p>
-<p>
-The FFI library is tightly integrated into LuaJIT (it's not available
-as a separate module). The code generated by the JIT-compiler for
-accesses to C&nbsp;data structures from Lua code is on par with the
-code a C&nbsp;compiler would generate. Calls to C&nbsp;functions can
-be inlined in JIT-compiled code, unlike calls to functions bound via
-the classic Lua/C API.
-</p>
-<p>
-This page gives a short introduction to the usage of the FFI library.
-<em>Please use the FFI sub-topics in the navigation bar to learn more.</em>
-</p>
-
-<h2 id="call">Motivating Example: Calling External C Functions</h2>
-<p>
-It's really easy to call an external C&nbsp;library function:
-</p>
-<pre class="code mark">
-<span class="codemark">&#9312;
-&#9313;
-
-
-&#9314;</span>local ffi = require("ffi")
-ffi.cdef[[
-<span style="color:#00a000;">int printf(const char *fmt, ...);</span>
-]]
-ffi.C.printf("Hello %s!", "world")
-</pre>
-<p>
-So, let's pick that apart:
-</p>
-<p>
-<span class="mark">&#9312;</span> Load the FFI library.
-</p>
-<p>
-<span class="mark">&#9313;</span> Add a C&nbsp;declaration
-for the function. The part inside the double-brackets (in green) is
-just standard C&nbsp;syntax.
-</p>
-<p>
-<span class="mark">&#9314;</span> Call the named
-C&nbsp;function &mdash; Yes, it's that simple!
-</p>
-<p style="font-size: 8pt;">
-Actually, what goes on behind the scenes is far from simple: <span
-style="color:#4040c0;">&#9314;</span> makes use of the standard
-C&nbsp;library namespace <tt>ffi.C</tt>. Indexing this namespace with
-a symbol name (<tt>"printf"</tt>) automatically binds it to the
-standard C&nbsp;library. The result is a special kind of object which,
-when called, runs the <tt>printf</tt> function. The arguments passed
-to this function are automatically converted from Lua objects to the
-corresponding C&nbsp;types.
-</p>
-<p>
-Ok, so maybe the use of <tt>printf()</tt> wasn't such a spectacular
-example. You could have done that with <tt>io.write()</tt> and
-<tt>string.format()</tt>, too. But you get the idea ...
-</p>
-<p>
-So here's something to pop up a message box on Windows:
-</p>
-<pre class="code">
-local ffi = require("ffi")
-ffi.cdef[[
-<span style="color:#00a000;">int MessageBoxA(void *w, const char *txt, const char *cap, int type);</span>
-]]
-ffi.C.MessageBoxA(nil, "Hello world!", "Test", 0)
-</pre>
-<p>
-Bing! Again, that was far too easy, no?
-</p>
-<p style="font-size: 8pt;">
-Compare this with the effort required to bind that function using the
-classic Lua/C API: create an extra C&nbsp;file, add a C&nbsp;function
-that retrieves and checks the argument types passed from Lua and calls
-the actual C&nbsp;function, add a list of module functions and their
-names, add a <tt>luaopen_*</tt> function and register all module
-functions, compile and link it into a shared library (DLL), move it to
-the proper path, add Lua code that loads the module aaaand ... finally
-call the binding function. Phew!
-</p>
-
-<h2 id="cdata">Motivating Example: Using C Data Structures</h2>
-<p>
-The FFI library allows you to create and access C&nbsp;data
-structures. Of course the main use for this is for interfacing with
-C&nbsp;functions. But they can be used stand-alone, too.
-</p>
-<p>
-Lua is built upon high-level data types. They are flexible, extensible
-and dynamic. That's why we all love Lua so much. Alas, this can be
-inefficient for certain tasks, where you'd really want a low-level
-data type. E.g. a large array of a fixed structure needs to be
-implemented with a big table holding lots of tiny tables. This imposes
-both a substantial memory overhead as well as a performance overhead.
-</p>
-<p>
-Here's a sketch of a library that operates on color images plus a
-simple benchmark. First, the plain Lua version:
-</p>
-<pre class="code">
-local floor = math.floor
-
-local function image_ramp_green(n)
- local img = {}
- local f = 255/(n-1)
- for i=1,n do
- img[i] = { red = 0, green = floor((i-1)*f), blue = 0, alpha = 255 }
- end
- return img
-end
-
-local function image_to_grey(img, n)
- for i=1,n do
- local y = floor(0.3*img[i].red + 0.59*img[i].green + 0.11*img[i].blue)
- img[i].red = y; img[i].green = y; img[i].blue = y
- end
-end
-
-local N = 400*400
-local img = image_ramp_green(N)
-for i=1,1000 do
- image_to_grey(img, N)
-end
-</pre>
-<p>
-This creates a table with 160.000 pixels, each of which is a table
-holding four number values in the range of 0-255. First an image with
-a green ramp is created (1D for simplicity), then the image is
-converted to greyscale 1000 times. Yes, that's silly, but I was in
-need of a simple example ...
-</p>
-<p>
-And here's the FFI version. The modified parts have been marked in
-bold:
-</p>
-<pre class="code mark">
-<span class="codemark">&#9312;
-
-
-
-
-
-&#9313;
-
-&#9314;
-&#9315;
-
-
-
-
-
-
-&#9314;
-&#9316;</span><b>local ffi = require("ffi")
-ffi.cdef[[
-</b><span style="color:#00a000;">typedef struct { uint8_t red, green, blue, alpha; } rgba_pixel;</span><b>
-]]</b>
-
-local function image_ramp_green(n)
- <b>local img = ffi.new("rgba_pixel[?]", n)</b>
- local f = 255/(n-1)
- for i=<b>0,n-1</b> do
- <b>img[i].green = i*f</b>
- <b>img[i].alpha = 255</b>
- end
- return img
-end
-
-local function image_to_grey(img, n)
- for i=<b>0,n-1</b> do
- local y = <b>0.3*img[i].red + 0.59*img[i].green + 0.11*img[i].blue</b>
- img[i].red = y; img[i].green = y; img[i].blue = y
- end
-end
-
-local N = 400*400
-local img = image_ramp_green(N)
-for i=1,1000 do
- image_to_grey(img, N)
-end
-</pre>
-<p>
-Ok, so that wasn't too difficult:
-</p>
-<p>
-<span class="mark">&#9312;</span> First, load the FFI
-library and declare the low-level data type. Here we choose a
-<tt>struct</tt> which holds four byte fields, one for each component
-of a 4x8&nbsp;bit RGBA pixel.
-</p>
-<p>
-<span class="mark">&#9313;</span> Creating the data
-structure with <tt>ffi.new()</tt> is straightforward &mdash; the
-<tt>'?'</tt> is a placeholder for the number of elements of a
-variable-length array.
-</p>
-<p>
-<span class="mark">&#9314;</span> C&nbsp;arrays are
-zero-based, so the indexes have to run from <tt>0</tt> to
-<tt>n-1</tt>. One might want to allocate one more element instead to
-simplify converting legacy code.
-</p>
-<p>
-<span class="mark">&#9315;</span> Since <tt>ffi.new()</tt>
-zero-fills the array by default, we only need to set the green and the
-alpha fields.
-</p>
-<p>
-<span class="mark">&#9316;</span> The calls to
-<tt>math.floor()</tt> can be omitted here, because floating-point
-numbers are already truncated towards zero when converting them to an
-integer. This happens implicitly when the number is stored in the
-fields of each pixel.
-</p>
-<p>
-Now let's have a look at the impact of the changes: first, memory
-consumption for the image is down from 22&nbsp;Megabytes to
-640&nbsp;Kilobytes (400*400*4 bytes). That's a factor of 35x less! So,
-yes, tables do have a noticeable overhead. BTW: The original program
-would consume 40&nbsp;Megabytes in plain Lua (on x64).
-</p>
-<p>
-Next, performance: the pure Lua version runs in 9.57 seconds (52.9
-seconds with the Lua interpreter) and the FFI version runs in 0.48
-seconds on my machine (YMMV). That's a factor of 20x faster (110x
-faster than the Lua interpreter).
-</p>
-<p style="font-size: 8pt;">
-The avid reader may notice that converting the pure Lua version over
-to use array indexes for the colors (<tt>[1]</tt> instead of
-<tt>.red</tt>, <tt>[2]</tt> instead of <tt>.green</tt> etc.) ought to
-be more compact and faster. This is certainly true (by a factor of
-~1.7x). Switching to a struct-of-arrays would help, too.
-</p>
-<p style="font-size: 8pt;">
-However the resulting code would be less idiomatic and rather
-error-prone. And it still doesn't get even close to the performance of
-the FFI version of the code. Also, high-level data structures cannot
-be easily passed to other C&nbsp;functions, especially I/O functions,
-without undue conversion penalties.
-</p>
-<br class="flush">
-</div>
-<div id="foot">
-<hr class="hide">
-Copyright &copy; 2005-2013 Mike Pall
-<span class="noprint">
-&middot;
-<a href="contact.html">Contact</a>
-</span>
-</div>
-</body>
-</html>