You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
604 lines
22 KiB
HTML
604 lines
22 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
|
|
<html>
|
|
<head>
|
|
<title>FFI Tutorial</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) 2005-2016, 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">
|
|
<style type="text/css">
|
|
table.idiomtable { font-size: 90%; line-height: 1.2; }
|
|
table.idiomtable tt { font-size: 100%; }
|
|
table.idiomtable td { vertical-align: top; }
|
|
tr.idiomhead td { font-weight: bold; }
|
|
td.idiomlua b { font-weight: normal; color: #2142bf; }
|
|
</style>
|
|
</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 Tutorial</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">»</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 href="ext_ffi.html">FFI Library</a>
|
|
<ul><li>
|
|
<a class="current" 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><li>
|
|
<a href="ext_profiler.html">Profiler</a>
|
|
</li></ul>
|
|
</li><li>
|
|
<a href="status.html">Status</a>
|
|
<ul><li>
|
|
<a href="changes.html">Changes</a>
|
|
</li></ul>
|
|
</li><li>
|
|
<a href="faq.html">FAQ</a>
|
|
</li><li>
|
|
<a href="http://luajit.org/performance.html">Performance <span class="ext">»</span></a>
|
|
</li><li>
|
|
<a href="http://wiki.luajit.org/">Wiki <span class="ext">»</span></a>
|
|
</li><li>
|
|
<a href="http://luajit.org/list.html">Mailing List <span class="ext">»</span></a>
|
|
</li></ul>
|
|
</div>
|
|
<div id="main">
|
|
<p>
|
|
This page is intended to give you an overview of the features of the FFI
|
|
library by presenting a few use cases and guidelines.
|
|
</p>
|
|
<p>
|
|
This page makes no attempt to explain all of the FFI library, though.
|
|
You'll want to have a look at the <a href="ext_ffi_api.html">ffi.* API
|
|
function reference</a> and the <a href="ext_ffi_semantics.html">FFI
|
|
semantics</a> to learn more.
|
|
</p>
|
|
|
|
<h2 id="load">Loading the FFI Library</h2>
|
|
<p>
|
|
The FFI library is built into LuaJIT by default, but it's not loaded
|
|
and initialized by default. The suggested way to use the FFI library
|
|
is to add the following to the start of every Lua file that needs one
|
|
of its functions:
|
|
</p>
|
|
<pre class="code">
|
|
local ffi = require("ffi")
|
|
</pre>
|
|
<p>
|
|
Please note this doesn't define an <tt>ffi</tt> variable in the table
|
|
of globals — you really need to use the local variable. The
|
|
<tt>require</tt> function ensures the library is only loaded once.
|
|
</p>
|
|
<p style="font-size: 8pt;">
|
|
Note: If you want to experiment with the FFI from the interactive prompt
|
|
of the command line executable, omit the <tt>local</tt>, as it doesn't
|
|
preserve local variables across lines.
|
|
</p>
|
|
|
|
<h2 id="sleep">Accessing Standard System Functions</h2>
|
|
<p>
|
|
The following code explains how to access standard system functions.
|
|
We slowly print two lines of dots by sleeping for 10 milliseconds
|
|
after each dot:
|
|
</p>
|
|
<pre class="code mark">
|
|
<span class="codemark">
|
|
①
|
|
|
|
|
|
|
|
|
|
|
|
②
|
|
③
|
|
④
|
|
|
|
|
|
|
|
⑤
|
|
|
|
|
|
|
|
|
|
|
|
⑥</span>local ffi = require("ffi")
|
|
ffi.cdef[[
|
|
<span style="color:#00a000;">void Sleep(int ms);
|
|
int poll(struct pollfd *fds, unsigned long nfds, int timeout);</span>
|
|
]]
|
|
|
|
local sleep
|
|
if ffi.os == "Windows" then
|
|
function sleep(s)
|
|
ffi.C.Sleep(s*1000)
|
|
end
|
|
else
|
|
function sleep(s)
|
|
ffi.C.poll(nil, 0, s*1000)
|
|
end
|
|
end
|
|
|
|
for i=1,160 do
|
|
io.write("."); io.flush()
|
|
sleep(0.01)
|
|
end
|
|
io.write("\n")
|
|
</pre>
|
|
<p>
|
|
Here's the step-by-step explanation:
|
|
</p>
|
|
<p>
|
|
<span class="mark">①</span> This defines the
|
|
C library functions we're going to use. The part inside the
|
|
double-brackets (in green) is just standard C syntax. You can
|
|
usually get this info from the C header files or the
|
|
documentation provided by each C library or C compiler.
|
|
</p>
|
|
<p>
|
|
<span class="mark">②</span> The difficulty we're
|
|
facing here, is that there are different standards to choose from.
|
|
Windows has a simple <tt>Sleep()</tt> function. On other systems there
|
|
are a variety of functions available to achieve sub-second sleeps, but
|
|
with no clear consensus. Thankfully <tt>poll()</tt> can be used for
|
|
this task, too, and it's present on most non-Windows systems. The
|
|
check for <tt>ffi.os</tt> makes sure we use the Windows-specific
|
|
function only on Windows systems.
|
|
</p>
|
|
<p>
|
|
<span class="mark">③</span> Here we're wrapping the
|
|
call to the C function in a Lua function. This isn't strictly
|
|
necessary, but it's helpful to deal with system-specific issues only
|
|
in one part of the code. The way we're wrapping it ensures the check
|
|
for the OS is only done during initialization and not for every call.
|
|
</p>
|
|
<p>
|
|
<span class="mark">④</span> A more subtle point is
|
|
that we defined our <tt>sleep()</tt> function (for the sake of this
|
|
example) as taking the number of seconds, but accepting fractional
|
|
seconds. Multiplying this by 1000 gets us milliseconds, but that still
|
|
leaves it a Lua number, which is a floating-point value. Alas, the
|
|
<tt>Sleep()</tt> function only accepts an integer value. Luckily for
|
|
us, the FFI library automatically performs the conversion when calling
|
|
the function (truncating the FP value towards zero, like in C).
|
|
</p>
|
|
<p style="font-size: 8pt;">
|
|
Some readers will notice that <tt>Sleep()</tt> is part of
|
|
<tt>KERNEL32.DLL</tt> and is also a <tt>stdcall</tt> function. So how
|
|
can this possibly work? The FFI library provides the <tt>ffi.C</tt>
|
|
default C library namespace, which allows calling functions from
|
|
the default set of libraries, like a C compiler would. Also, the
|
|
FFI library automatically detects <tt>stdcall</tt> functions, so you
|
|
don't need to declare them as such.
|
|
</p>
|
|
<p>
|
|
<span class="mark">⑤</span> The <tt>poll()</tt>
|
|
function takes a couple more arguments we're not going to use. You can
|
|
simply use <tt>nil</tt> to pass a <tt>NULL</tt> pointer and <tt>0</tt>
|
|
for the <tt>nfds</tt> parameter. Please note that the
|
|
number <tt>0</tt> <em>does not convert to a pointer value</em>,
|
|
unlike in C++. You really have to pass pointers to pointer arguments
|
|
and numbers to number arguments.
|
|
</p>
|
|
<p style="font-size: 8pt;">
|
|
The page on <a href="ext_ffi_semantics.html">FFI semantics</a> has all
|
|
of the gory details about
|
|
<a href="ext_ffi_semantics.html#convert">conversions between Lua
|
|
objects and C types</a>. For the most part you don't have to deal
|
|
with this, as it's performed automatically and it's carefully designed
|
|
to bridge the semantic differences between Lua and C.
|
|
</p>
|
|
<p>
|
|
<span class="mark">⑥</span> Now that we have defined
|
|
our own <tt>sleep()</tt> function, we can just call it from plain Lua
|
|
code. That wasn't so bad, huh? Turning these boring animated dots into
|
|
a fascinating best-selling game is left as an exercise for the reader.
|
|
:-)
|
|
</p>
|
|
|
|
<h2 id="zlib">Accessing the zlib Compression Library</h2>
|
|
<p>
|
|
The following code shows how to access the <a
|
|
href="http://zlib.net/">zlib</a> compression library from Lua code.
|
|
We'll define two convenience wrapper functions that take a string and
|
|
compress or uncompress it to another string:
|
|
</p>
|
|
<pre class="code mark">
|
|
<span class="codemark">
|
|
①
|
|
|
|
|
|
|
|
|
|
|
|
|
|
②
|
|
|
|
|
|
③
|
|
|
|
④
|
|
|
|
|
|
⑤
|
|
|
|
|
|
⑥
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
⑦</span>local ffi = require("ffi")
|
|
ffi.cdef[[
|
|
<span style="color:#00a000;">unsigned long compressBound(unsigned long sourceLen);
|
|
int compress2(uint8_t *dest, unsigned long *destLen,
|
|
const uint8_t *source, unsigned long sourceLen, int level);
|
|
int uncompress(uint8_t *dest, unsigned long *destLen,
|
|
const uint8_t *source, unsigned long sourceLen);</span>
|
|
]]
|
|
local zlib = ffi.load(ffi.os == "Windows" and "zlib1" or "z")
|
|
|
|
local function compress(txt)
|
|
local n = zlib.compressBound(#txt)
|
|
local buf = ffi.new("uint8_t[?]", n)
|
|
local buflen = ffi.new("unsigned long[1]", n)
|
|
local res = zlib.compress2(buf, buflen, txt, #txt, 9)
|
|
assert(res == 0)
|
|
return ffi.string(buf, buflen[0])
|
|
end
|
|
|
|
local function uncompress(comp, n)
|
|
local buf = ffi.new("uint8_t[?]", n)
|
|
local buflen = ffi.new("unsigned long[1]", n)
|
|
local res = zlib.uncompress(buf, buflen, comp, #comp)
|
|
assert(res == 0)
|
|
return ffi.string(buf, buflen[0])
|
|
end
|
|
|
|
-- Simple test code.
|
|
local txt = string.rep("abcd", 1000)
|
|
print("Uncompressed size: ", #txt)
|
|
local c = compress(txt)
|
|
print("Compressed size: ", #c)
|
|
local txt2 = uncompress(c, #txt)
|
|
assert(txt2 == txt)
|
|
</pre>
|
|
<p>
|
|
Here's the step-by-step explanation:
|
|
</p>
|
|
<p>
|
|
<span class="mark">①</span> This defines some of the
|
|
C functions provided by zlib. For the sake of this example, some
|
|
type indirections have been reduced and it uses the pre-defined
|
|
fixed-size integer types, while still adhering to the zlib API/ABI.
|
|
</p>
|
|
<p>
|
|
<span class="mark">②</span> This loads the zlib shared
|
|
library. On POSIX systems it's named <tt>libz.so</tt> and usually
|
|
comes pre-installed. Since <tt>ffi.load()</tt> automatically adds any
|
|
missing standard prefixes/suffixes, we can simply load the
|
|
<tt>"z"</tt> library. On Windows it's named <tt>zlib1.dll</tt> and
|
|
you'll have to download it first from the
|
|
<a href="http://zlib.net/"><span class="ext">»</span> zlib site</a>. The check for
|
|
<tt>ffi.os</tt> makes sure we pass the right name to
|
|
<tt>ffi.load()</tt>.
|
|
</p>
|
|
<p>
|
|
<span class="mark">③</span> First, the maximum size of
|
|
the compression buffer is obtained by calling the
|
|
<tt>zlib.compressBound</tt> function with the length of the
|
|
uncompressed string. The next line allocates a byte buffer of this
|
|
size. The <tt>[?]</tt> in the type specification indicates a
|
|
variable-length array (VLA). The actual number of elements of this
|
|
array is given as the 2nd argument to <tt>ffi.new()</tt>.
|
|
</p>
|
|
<p>
|
|
<span class="mark">④</span> This may look strange at
|
|
first, but have a look at the declaration of the <tt>compress2</tt>
|
|
function from zlib: the destination length is defined as a pointer!
|
|
This is because you pass in the maximum buffer size and get back the
|
|
actual length that was used.
|
|
</p>
|
|
<p>
|
|
In C you'd pass in the address of a local variable
|
|
(<tt>&buflen</tt>). But since there's no address-of operator in
|
|
Lua, we'll just pass in a one-element array. Conveniently it can be
|
|
initialized with the maximum buffer size in one step. Calling the
|
|
actual <tt>zlib.compress2</tt> function is then straightforward.
|
|
</p>
|
|
<p>
|
|
<span class="mark">⑤</span> We want to return the
|
|
compressed data as a Lua string, so we'll use <tt>ffi.string()</tt>.
|
|
It needs a pointer to the start of the data and the actual length. The
|
|
length has been returned in the <tt>buflen</tt> array, so we'll just
|
|
get it from there.
|
|
</p>
|
|
<p style="font-size: 8pt;">
|
|
Note that since the function returns now, the <tt>buf</tt> and
|
|
<tt>buflen</tt> variables will eventually be garbage collected. This
|
|
is fine, because <tt>ffi.string()</tt> has copied the contents to a
|
|
newly created (interned) Lua string. If you plan to call this function
|
|
lots of times, consider reusing the buffers and/or handing back the
|
|
results in buffers instead of strings. This will reduce the overhead
|
|
for garbage collection and string interning.
|
|
</p>
|
|
<p>
|
|
<span class="mark">⑥</span> The <tt>uncompress</tt>
|
|
functions does the exact opposite of the <tt>compress</tt> function.
|
|
The compressed data doesn't include the size of the original string,
|
|
so this needs to be passed in. Otherwise no surprises here.
|
|
</p>
|
|
<p>
|
|
<span class="mark">⑦</span> The code, that makes use
|
|
of the functions we just defined, is just plain Lua code. It doesn't
|
|
need to know anything about the LuaJIT FFI — the convenience
|
|
wrapper functions completely hide it.
|
|
</p>
|
|
<p>
|
|
One major advantage of the LuaJIT FFI is that you are now able to
|
|
write those wrappers <em>in Lua</em>. And at a fraction of the time it
|
|
would cost you to create an extra C module using the Lua/C API.
|
|
Many of the simpler C functions can probably be used directly
|
|
from your Lua code, without any wrappers.
|
|
</p>
|
|
<p style="font-size: 8pt;">
|
|
Side note: the zlib API uses the <tt>long</tt> type for passing
|
|
lengths and sizes around. But all those zlib functions actually only
|
|
deal with 32 bit values. This is an unfortunate choice for a
|
|
public API, but may be explained by zlib's history — we'll just
|
|
have to deal with it.
|
|
</p>
|
|
<p style="font-size: 8pt;">
|
|
First, you should know that a <tt>long</tt> is a 64 bit type e.g.
|
|
on POSIX/x64 systems, but a 32 bit type on Windows/x64 and on
|
|
32 bit systems. Thus a <tt>long</tt> result can be either a plain
|
|
Lua number or a boxed 64 bit integer cdata object, depending on
|
|
the target system.
|
|
</p>
|
|
<p style="font-size: 8pt;">
|
|
Ok, so the <tt>ffi.*</tt> functions generally accept cdata objects
|
|
wherever you'd want to use a number. That's why we get a away with
|
|
passing <tt>n</tt> to <tt>ffi.string()</tt> above. But other Lua
|
|
library functions or modules don't know how to deal with this. So for
|
|
maximum portability one needs to use <tt>tonumber()</tt> on returned
|
|
<tt>long</tt> results before passing them on. Otherwise the
|
|
application might work on some systems, but would fail in a POSIX/x64
|
|
environment.
|
|
</p>
|
|
|
|
<h2 id="metatype">Defining Metamethods for a C Type</h2>
|
|
<p>
|
|
The following code explains how to define metamethods for a C type.
|
|
We define a simple point type and add some operations to it:
|
|
</p>
|
|
<pre class="code mark">
|
|
<span class="codemark">
|
|
①
|
|
|
|
|
|
|
|
②
|
|
|
|
③
|
|
|
|
④
|
|
|
|
|
|
|
|
⑤
|
|
|
|
⑥</span>local ffi = require("ffi")
|
|
ffi.cdef[[
|
|
<span style="color:#00a000;">typedef struct { double x, y; } point_t;</span>
|
|
]]
|
|
|
|
local point
|
|
local mt = {
|
|
__add = function(a, b) return point(a.x+b.x, a.y+b.y) end,
|
|
__len = function(a) return math.sqrt(a.x*a.x + a.y*a.y) end,
|
|
__index = {
|
|
area = function(a) return a.x*a.x + a.y*a.y end,
|
|
},
|
|
}
|
|
point = ffi.metatype("point_t", mt)
|
|
|
|
local a = point(3, 4)
|
|
print(a.x, a.y) --> 3 4
|
|
print(#a) --> 5
|
|
print(a:area()) --> 25
|
|
local b = a + point(0.5, 8)
|
|
print(#b) --> 12.5
|
|
</pre>
|
|
<p>
|
|
Here's the step-by-step explanation:
|
|
</p>
|
|
<p>
|
|
<span class="mark">①</span> This defines the C type for a
|
|
two-dimensional point object.
|
|
</p>
|
|
<p>
|
|
<span class="mark">②</span> We have to declare the variable
|
|
holding the point constructor first, because it's used inside of a
|
|
metamethod.
|
|
</p>
|
|
<p>
|
|
<span class="mark">③</span> Let's define an <tt>__add</tt>
|
|
metamethod which adds the coordinates of two points and creates a new
|
|
point object. For simplicity, this function assumes that both arguments
|
|
are points. But it could be any mix of objects, if at least one operand
|
|
is of the required type (e.g. adding a point plus a number or vice
|
|
versa). Our <tt>__len</tt> metamethod returns the distance of a point to
|
|
the origin.
|
|
</p>
|
|
<p>
|
|
<span class="mark">④</span> If we run out of operators, we can
|
|
define named methods, too. Here the <tt>__index</tt> table defines an
|
|
<tt>area</tt> function. For custom indexing needs, one might want to
|
|
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 © 2005-2016 Mike Pall
|
|
<span class="noprint">
|
|
·
|
|
<a href="contact.html">Contact</a>
|
|
</span>
|
|
</div>
|
|
</body>
|
|
</html>
|