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604 lines
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<title>FFI Tutorial</title>
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<meta name="Author" content="Mike Pall">
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<meta name="Copyright" content="Copyright (C) 2005-2017, Mike Pall">
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<div id="site">
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<a href="http://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
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</div>
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<div id="head">
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<h1>FFI Tutorial</h1>
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</div>
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<div id="nav">
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<ul><li>
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<a href="luajit.html">LuaJIT</a>
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<a href="http://luajit.org/download.html">Download <span class="ext">»</span></a>
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<a href="extensions.html">Extensions</a>
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<ul><li>
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<a href="ext_ffi.html">FFI Library</a>
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<ul><li>
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<a class="current" href="ext_ffi_tutorial.html">FFI Tutorial</a>
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<a href="ext_ffi_api.html">ffi.* API</a>
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<a href="ext_ffi_semantics.html">FFI Semantics</a>
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<a href="ext_jit.html">jit.* Library</a>
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</div>
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<div id="main">
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<p>
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This page is intended to give you an overview of the features of the FFI
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library by presenting a few use cases and guidelines.
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</p>
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<p>
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This page makes no attempt to explain all of the FFI library, though.
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You'll want to have a look at the <a href="ext_ffi_api.html">ffi.* API
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function reference</a> and the <a href="ext_ffi_semantics.html">FFI
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semantics</a> to learn more.
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</p>
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<h2 id="load">Loading the FFI Library</h2>
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<p>
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The FFI library is built into LuaJIT by default, but it's not loaded
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and initialized by default. The suggested way to use the FFI library
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is to add the following to the start of every Lua file that needs one
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of its functions:
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</p>
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<pre class="code">
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local ffi = require("ffi")
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</pre>
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<p>
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Please note this doesn't define an <tt>ffi</tt> variable in the table
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of globals — you really need to use the local variable. The
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<tt>require</tt> function ensures the library is only loaded once.
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</p>
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<p style="font-size: 8pt;">
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Note: If you want to experiment with the FFI from the interactive prompt
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of the command line executable, omit the <tt>local</tt>, as it doesn't
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preserve local variables across lines.
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</p>
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<h2 id="sleep">Accessing Standard System Functions</h2>
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<p>
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The following code explains how to access standard system functions.
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We slowly print two lines of dots by sleeping for 10 milliseconds
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after each dot:
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</p>
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<pre class="code mark">
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<span class="codemark">
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①
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②
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③
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④
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⑤
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⑥</span>local ffi = require("ffi")
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ffi.cdef[[
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<span style="color:#00a000;">void Sleep(int ms);
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int poll(struct pollfd *fds, unsigned long nfds, int timeout);</span>
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]]
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local sleep
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if ffi.os == "Windows" then
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function sleep(s)
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ffi.C.Sleep(s*1000)
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end
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else
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function sleep(s)
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ffi.C.poll(nil, 0, s*1000)
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end
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end
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for i=1,160 do
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io.write("."); io.flush()
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sleep(0.01)
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end
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io.write("\n")
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</pre>
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<p>
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Here's the step-by-step explanation:
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</p>
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<p>
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<span class="mark">①</span> This defines the
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C library functions we're going to use. The part inside the
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double-brackets (in green) is just standard C syntax. You can
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usually get this info from the C header files or the
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documentation provided by each C library or C compiler.
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</p>
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<p>
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<span class="mark">②</span> The difficulty we're
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facing here, is that there are different standards to choose from.
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Windows has a simple <tt>Sleep()</tt> function. On other systems there
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are a variety of functions available to achieve sub-second sleeps, but
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with no clear consensus. Thankfully <tt>poll()</tt> can be used for
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this task, too, and it's present on most non-Windows systems. The
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check for <tt>ffi.os</tt> makes sure we use the Windows-specific
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function only on Windows systems.
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</p>
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<p>
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<span class="mark">③</span> Here we're wrapping the
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call to the C function in a Lua function. This isn't strictly
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necessary, but it's helpful to deal with system-specific issues only
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in one part of the code. The way we're wrapping it ensures the check
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for the OS is only done during initialization and not for every call.
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</p>
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<p>
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<span class="mark">④</span> A more subtle point is
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that we defined our <tt>sleep()</tt> function (for the sake of this
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example) as taking the number of seconds, but accepting fractional
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seconds. Multiplying this by 1000 gets us milliseconds, but that still
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leaves it a Lua number, which is a floating-point value. Alas, the
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<tt>Sleep()</tt> function only accepts an integer value. Luckily for
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us, the FFI library automatically performs the conversion when calling
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the function (truncating the FP value towards zero, like in C).
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</p>
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<p style="font-size: 8pt;">
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Some readers will notice that <tt>Sleep()</tt> is part of
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<tt>KERNEL32.DLL</tt> and is also a <tt>stdcall</tt> function. So how
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can this possibly work? The FFI library provides the <tt>ffi.C</tt>
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default C library namespace, which allows calling functions from
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the default set of libraries, like a C compiler would. Also, the
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FFI library automatically detects <tt>stdcall</tt> functions, so you
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don't need to declare them as such.
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</p>
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<p>
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<span class="mark">⑤</span> The <tt>poll()</tt>
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function takes a couple more arguments we're not going to use. You can
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simply use <tt>nil</tt> to pass a <tt>NULL</tt> pointer and <tt>0</tt>
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for the <tt>nfds</tt> parameter. Please note that the
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number <tt>0</tt> <em>does not convert to a pointer value</em>,
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unlike in C++. You really have to pass pointers to pointer arguments
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and numbers to number arguments.
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</p>
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<p style="font-size: 8pt;">
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The page on <a href="ext_ffi_semantics.html">FFI semantics</a> has all
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of the gory details about
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<a href="ext_ffi_semantics.html#convert">conversions between Lua
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objects and C types</a>. For the most part you don't have to deal
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with this, as it's performed automatically and it's carefully designed
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to bridge the semantic differences between Lua and C.
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</p>
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<p>
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<span class="mark">⑥</span> Now that we have defined
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our own <tt>sleep()</tt> function, we can just call it from plain Lua
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code. That wasn't so bad, huh? Turning these boring animated dots into
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a fascinating best-selling game is left as an exercise for the reader.
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:-)
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</p>
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<h2 id="zlib">Accessing the zlib Compression Library</h2>
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<p>
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The following code shows how to access the <a
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href="http://zlib.net/">zlib</a> compression library from Lua code.
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We'll define two convenience wrapper functions that take a string and
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compress or uncompress it to another string:
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</p>
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<pre class="code mark">
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<span class="codemark">
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①
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②
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③
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④
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⑤
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⑥
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⑦</span>local ffi = require("ffi")
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ffi.cdef[[
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<span style="color:#00a000;">unsigned long compressBound(unsigned long sourceLen);
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int compress2(uint8_t *dest, unsigned long *destLen,
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const uint8_t *source, unsigned long sourceLen, int level);
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int uncompress(uint8_t *dest, unsigned long *destLen,
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const uint8_t *source, unsigned long sourceLen);</span>
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]]
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local zlib = ffi.load(ffi.os == "Windows" and "zlib1" or "z")
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local function compress(txt)
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local n = zlib.compressBound(#txt)
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local buf = ffi.new("uint8_t[?]", n)
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local buflen = ffi.new("unsigned long[1]", n)
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local res = zlib.compress2(buf, buflen, txt, #txt, 9)
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assert(res == 0)
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return ffi.string(buf, buflen[0])
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end
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local function uncompress(comp, n)
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local buf = ffi.new("uint8_t[?]", n)
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local buflen = ffi.new("unsigned long[1]", n)
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local res = zlib.uncompress(buf, buflen, comp, #comp)
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assert(res == 0)
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return ffi.string(buf, buflen[0])
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end
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-- Simple test code.
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local txt = string.rep("abcd", 1000)
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print("Uncompressed size: ", #txt)
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local c = compress(txt)
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print("Compressed size: ", #c)
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local txt2 = uncompress(c, #txt)
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assert(txt2 == txt)
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</pre>
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<p>
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Here's the step-by-step explanation:
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</p>
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<p>
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<span class="mark">①</span> This defines some of the
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C functions provided by zlib. For the sake of this example, some
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type indirections have been reduced and it uses the pre-defined
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fixed-size integer types, while still adhering to the zlib API/ABI.
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</p>
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<p>
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<span class="mark">②</span> This loads the zlib shared
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library. On POSIX systems it's named <tt>libz.so</tt> and usually
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comes pre-installed. Since <tt>ffi.load()</tt> automatically adds any
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missing standard prefixes/suffixes, we can simply load the
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<tt>"z"</tt> library. On Windows it's named <tt>zlib1.dll</tt> and
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you'll have to download it first from the
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<a href="http://zlib.net/"><span class="ext">»</span> zlib site</a>. The check for
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<tt>ffi.os</tt> makes sure we pass the right name to
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<tt>ffi.load()</tt>.
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</p>
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<p>
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<span class="mark">③</span> First, the maximum size of
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the compression buffer is obtained by calling the
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<tt>zlib.compressBound</tt> function with the length of the
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uncompressed string. The next line allocates a byte buffer of this
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size. The <tt>[?]</tt> in the type specification indicates a
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variable-length array (VLA). The actual number of elements of this
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array is given as the 2nd argument to <tt>ffi.new()</tt>.
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</p>
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<p>
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<span class="mark">④</span> This may look strange at
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first, but have a look at the declaration of the <tt>compress2</tt>
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function from zlib: the destination length is defined as a pointer!
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This is because you pass in the maximum buffer size and get back the
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actual length that was used.
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</p>
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<p>
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In C you'd pass in the address of a local variable
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(<tt>&buflen</tt>). But since there's no address-of operator in
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Lua, we'll just pass in a one-element array. Conveniently it can be
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initialized with the maximum buffer size in one step. Calling the
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actual <tt>zlib.compress2</tt> function is then straightforward.
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</p>
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<p>
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<span class="mark">⑤</span> We want to return the
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compressed data as a Lua string, so we'll use <tt>ffi.string()</tt>.
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It needs a pointer to the start of the data and the actual length. The
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length has been returned in the <tt>buflen</tt> array, so we'll just
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get it from there.
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</p>
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<p style="font-size: 8pt;">
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Note that since the function returns now, the <tt>buf</tt> and
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<tt>buflen</tt> variables will eventually be garbage collected. This
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is fine, because <tt>ffi.string()</tt> has copied the contents to a
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newly created (interned) Lua string. If you plan to call this function
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lots of times, consider reusing the buffers and/or handing back the
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results in buffers instead of strings. This will reduce the overhead
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for garbage collection and string interning.
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</p>
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<p>
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<span class="mark">⑥</span> The <tt>uncompress</tt>
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functions does the exact opposite of the <tt>compress</tt> function.
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The compressed data doesn't include the size of the original string,
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so this needs to be passed in. Otherwise no surprises here.
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</p>
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<p>
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<span class="mark">⑦</span> The code, that makes use
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of the functions we just defined, is just plain Lua code. It doesn't
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need to know anything about the LuaJIT FFI — the convenience
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wrapper functions completely hide it.
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</p>
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<p>
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One major advantage of the LuaJIT FFI is that you are now able to
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write those wrappers <em>in Lua</em>. And at a fraction of the time it
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would cost you to create an extra C module using the Lua/C API.
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Many of the simpler C functions can probably be used directly
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from your Lua code, without any wrappers.
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</p>
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<p style="font-size: 8pt;">
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Side note: the zlib API uses the <tt>long</tt> type for passing
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lengths and sizes around. But all those zlib functions actually only
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deal with 32 bit values. This is an unfortunate choice for a
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public API, but may be explained by zlib's history — we'll just
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have to deal with it.
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</p>
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<p style="font-size: 8pt;">
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First, you should know that a <tt>long</tt> is a 64 bit type e.g.
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on POSIX/x64 systems, but a 32 bit type on Windows/x64 and on
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32 bit systems. Thus a <tt>long</tt> result can be either a plain
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Lua number or a boxed 64 bit integer cdata object, depending on
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the target system.
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</p>
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<p style="font-size: 8pt;">
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Ok, so the <tt>ffi.*</tt> functions generally accept cdata objects
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wherever you'd want to use a number. That's why we get a away with
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passing <tt>n</tt> to <tt>ffi.string()</tt> above. But other Lua
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library functions or modules don't know how to deal with this. So for
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maximum portability one needs to use <tt>tonumber()</tt> on returned
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<tt>long</tt> results before passing them on. Otherwise the
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application might work on some systems, but would fail in a POSIX/x64
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environment.
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</p>
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<h2 id="metatype">Defining Metamethods for a C Type</h2>
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<p>
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The following code explains how to define metamethods for a C type.
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We define a simple point type and add some operations to it:
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</p>
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<pre class="code mark">
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<span class="codemark">
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①
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②
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③
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④
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⑤
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⑥</span>local ffi = require("ffi")
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ffi.cdef[[
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<span style="color:#00a000;">typedef struct { double x, y; } point_t;</span>
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]]
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local point
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local mt = {
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__add = function(a, b) return point(a.x+b.x, a.y+b.y) end,
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__len = function(a) return math.sqrt(a.x*a.x + a.y*a.y) end,
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__index = {
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area = function(a) return a.x*a.x + a.y*a.y end,
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},
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}
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point = ffi.metatype("point_t", mt)
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local a = point(3, 4)
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print(a.x, a.y) --> 3 4
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print(#a) --> 5
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print(a:area()) --> 25
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local b = a + point(0.5, 8)
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print(#b) --> 12.5
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</pre>
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<p>
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Here's the step-by-step explanation:
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</p>
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<p>
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<span class="mark">①</span> This defines the C type for a
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two-dimensional point object.
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</p>
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<p>
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<span class="mark">②</span> We have to declare the variable
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holding the point constructor first, because it's used inside of a
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metamethod.
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</p>
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<p>
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<span class="mark">③</span> Let's define an <tt>__add</tt>
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metamethod which adds the coordinates of two points and creates a new
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point object. For simplicity, this function assumes that both arguments
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are points. But it could be any mix of objects, if at least one operand
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is of the required type (e.g. adding a point plus a number or vice
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versa). Our <tt>__len</tt> metamethod returns the distance of a point to
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the origin.
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</p>
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<p>
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<span class="mark">④</span> If we run out of operators, we can
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define named methods, too. Here the <tt>__index</tt> table defines an
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<tt>area</tt> function. For custom indexing needs, one might want to
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define <tt>__index</tt> and <tt>__newindex</tt> <em>functions</em> instead.
|
|
</p>
|
|
<p>
|
|
<span class="mark">⑤</span> This associates the metamethods with
|
|
our C type. This only needs to be done once. For convenience, a
|
|
constructor is returned by
|
|
<a href="ext_ffi_api.html#ffi_metatype"><tt>ffi.metatype()</tt></a>.
|
|
We're not required to use it, though. The original C type can still
|
|
be used e.g. to create an array of points. The metamethods automatically
|
|
apply to any and all uses of this type.
|
|
</p>
|
|
<p>
|
|
Please note that the association with a metatable is permanent and
|
|
<b>the metatable must not be modified afterwards!</b> Ditto for the
|
|
<tt>__index</tt> table.
|
|
</p>
|
|
<p>
|
|
<span class="mark">⑥</span> Here are some simple usage examples
|
|
for the point type and their expected results. The pre-defined
|
|
operations (such as <tt>a.x</tt>) can be freely mixed with the newly
|
|
defined metamethods. Note that <tt>area</tt> is a method and must be
|
|
called with the Lua syntax for methods: <tt>a:area()</tt>, not
|
|
<tt>a.area()</tt>.
|
|
</p>
|
|
<p>
|
|
The C type metamethod mechanism is most useful when used in
|
|
conjunction with C libraries that are written in an object-oriented
|
|
style. Creators return a pointer to a new instance and methods take an
|
|
instance pointer as the first argument. Sometimes you can just point
|
|
<tt>__index</tt> to the library namespace and <tt>__gc</tt> to the
|
|
destructor and you're done. But often enough you'll want to add
|
|
convenience wrappers, e.g. to return actual Lua strings or when
|
|
returning multiple values.
|
|
</p>
|
|
<p>
|
|
Some C libraries only declare instance pointers as an opaque
|
|
<tt>void *</tt> type. In this case you can use a fake type for all
|
|
declarations, e.g. a pointer to a named (incomplete) struct will do:
|
|
<tt>typedef struct foo_type *foo_handle</tt>. The C side doesn't
|
|
know what you declare with the LuaJIT FFI, but as long as the underlying
|
|
types are compatible, everything still works.
|
|
</p>
|
|
|
|
<h2 id="idioms">Translating C Idioms</h2>
|
|
<p>
|
|
Here's a list of common C idioms and their translation to the
|
|
LuaJIT FFI:
|
|
</p>
|
|
<table class="idiomtable">
|
|
<tr class="idiomhead">
|
|
<td class="idiomdesc">Idiom</td>
|
|
<td class="idiomc">C code</td>
|
|
<td class="idiomlua">Lua code</td>
|
|
</tr>
|
|
<tr class="odd separate">
|
|
<td class="idiomdesc">Pointer dereference<br><tt>int *p;</tt></td><td class="idiomc"><tt>x = *p;<br>*p = y;</tt></td><td class="idiomlua"><tt>x = <b>p[0]</b><br><b>p[0]</b> = y</tt></td></tr>
|
|
<tr class="even">
|
|
<td class="idiomdesc">Pointer indexing<br><tt>int i, *p;</tt></td><td class="idiomc"><tt>x = p[i];<br>p[i+1] = y;</tt></td><td class="idiomlua"><tt>x = p[i]<br>p[i+1] = y</tt></td></tr>
|
|
<tr class="odd">
|
|
<td class="idiomdesc">Array indexing<br><tt>int i, a[];</tt></td><td class="idiomc"><tt>x = a[i];<br>a[i+1] = y;</tt></td><td class="idiomlua"><tt>x = a[i]<br>a[i+1] = y</tt></td></tr>
|
|
<tr class="even separate">
|
|
<td class="idiomdesc"><tt>struct</tt>/<tt>union</tt> dereference<br><tt>struct foo s;</tt></td><td class="idiomc"><tt>x = s.field;<br>s.field = y;</tt></td><td class="idiomlua"><tt>x = s.field<br>s.field = y</tt></td></tr>
|
|
<tr class="odd">
|
|
<td class="idiomdesc"><tt>struct</tt>/<tt>union</tt> pointer deref.<br><tt>struct foo *sp;</tt></td><td class="idiomc"><tt>x = sp->field;<br>sp->field = y;</tt></td><td class="idiomlua"><tt>x = <b>s.field</b><br><b>s.field</b> = y</tt></td></tr>
|
|
<tr class="even separate">
|
|
<td class="idiomdesc">Pointer arithmetic<br><tt>int i, *p;</tt></td><td class="idiomc"><tt>x = p + i;<br>y = p - i;</tt></td><td class="idiomlua"><tt>x = p + i<br>y = p - i</tt></td></tr>
|
|
<tr class="odd">
|
|
<td class="idiomdesc">Pointer difference<br><tt>int *p1, *p2;</tt></td><td class="idiomc"><tt>x = p1 - p2;</tt></td><td class="idiomlua"><tt>x = p1 - p2</tt></td></tr>
|
|
<tr class="even">
|
|
<td class="idiomdesc">Array element pointer<br><tt>int i, a[];</tt></td><td class="idiomc"><tt>x = &a[i];</tt></td><td class="idiomlua"><tt>x = <b>a+i</b></tt></td></tr>
|
|
<tr class="odd">
|
|
<td class="idiomdesc">Cast pointer to address<br><tt>int *p;</tt></td><td class="idiomc"><tt>x = (intptr_t)p;</tt></td><td class="idiomlua"><tt>x = <b>tonumber(<br> ffi.cast("intptr_t",<br> p))</b></tt></td></tr>
|
|
<tr class="even separate">
|
|
<td class="idiomdesc">Functions with outargs<br><tt>void foo(int *inoutlen);</tt></td><td class="idiomc"><tt>int len = x;<br>foo(&len);<br>y = len;</tt></td><td class="idiomlua"><tt><b>local len =<br> ffi.new("int[1]", x)<br>foo(len)<br>y = len[0]</b></tt></td></tr>
|
|
<tr class="odd">
|
|
<td class="idiomdesc"><a href="ext_ffi_semantics.html#convert_vararg">Vararg conversions</a><br><tt>int printf(char *fmt, ...);</tt></td><td class="idiomc"><tt>printf("%g", 1.0);<br>printf("%d", 1);<br> </tt></td><td class="idiomlua"><tt>printf("%g", 1);<br>printf("%d",<br> <b>ffi.new("int", 1)</b>)</tt></td></tr>
|
|
</table>
|
|
|
|
<h2 id="cache">To Cache or Not to Cache</h2>
|
|
<p>
|
|
It's a common Lua idiom to cache library functions in local variables
|
|
or upvalues, e.g.:
|
|
</p>
|
|
<pre class="code">
|
|
local byte, char = string.byte, string.char
|
|
local function foo(x)
|
|
return char(byte(x)+1)
|
|
end
|
|
</pre>
|
|
<p>
|
|
This replaces several hash-table lookups with a (faster) direct use of
|
|
a local or an upvalue. This is less important with LuaJIT, since the
|
|
JIT compiler optimizes hash-table lookups a lot and is even able to
|
|
hoist most of them out of the inner loops. It can't eliminate
|
|
<em>all</em> of them, though, and it saves some typing for often-used
|
|
functions. So there's still a place for this, even with LuaJIT.
|
|
</p>
|
|
<p>
|
|
The situation is a bit different with C function calls via the
|
|
FFI library. The JIT compiler has special logic to eliminate <em>all
|
|
of the lookup overhead</em> for functions resolved from a
|
|
<a href="ext_ffi_semantics.html#clib">C library namespace</a>!
|
|
Thus it's not helpful and actually counter-productive to cache
|
|
individual C functions like this:
|
|
</p>
|
|
<pre class="code">
|
|
local <b>funca</b>, <b>funcb</b> = ffi.C.funca, ffi.C.funcb -- <span style="color:#c00000;">Not helpful!</span>
|
|
local function foo(x, n)
|
|
for i=1,n do <b>funcb</b>(<b>funca</b>(x, i), 1) end
|
|
end
|
|
</pre>
|
|
<p>
|
|
This turns them into indirect calls and generates bigger and slower
|
|
machine code. Instead you'll want to cache the namespace itself and
|
|
rely on the JIT compiler to eliminate the lookups:
|
|
</p>
|
|
<pre class="code">
|
|
local <b>C</b> = ffi.C -- <span style="color:#00a000;">Instead use this!</span>
|
|
local function foo(x, n)
|
|
for i=1,n do <b>C.funcb</b>(<b>C.funca</b>(x, i), 1) end
|
|
end
|
|
</pre>
|
|
<p>
|
|
This generates both shorter and faster code. So <b>don't cache
|
|
C functions</b>, but <b>do</b> cache namespaces! Most often the
|
|
namespace is already in a local variable at an outer scope, e.g. from
|
|
<tt>local lib = ffi.load(...)</tt>. Note that copying
|
|
it to a local variable in the function scope is unnecessary.
|
|
</p>
|
|
<br class="flush">
|
|
</div>
|
|
<div id="foot">
|
|
<hr class="hide">
|
|
Copyright © 2005-2017 Mike Pall
|
|
<span class="noprint">
|
|
·
|
|
<a href="contact.html">Contact</a>
|
|
</span>
|
|
</div>
|
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|
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