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FFI: Finish docs on FFI semantics. Phew.

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Mike Pall 2011-02-10 03:12:09 +01:00
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@ -8,6 +8,12 @@
<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.convtable { line-height: 1.2; }
tr.convhead td { font-weight: bold; }
td.convin { width: 11em; }
td.convop { font-style: italic; width: 16em; }
</style>
</head>
<body>
<div id="site">
@ -63,10 +69,25 @@ and its interaction with both Lua and C&nbsp;code.
<p>
Given that the FFI library is designed to interface with C&nbsp;code
and that declarations can be written in plain C&nbsp;syntax, it
closely follows the C&nbsp;language semantics wherever possible. Some
concessions are needed for smoother interoperation with Lua language
semantics. But it should be straightforward to write applications
using the LuaJIT FFI for developers with a C or C++ background.
closely follows the C&nbsp;language semantics, wherever possible. Some
minor concessions are needed for smoother interoperation with Lua
language semantics.
</p>
<p>
Please don't be overwhelmed by the contents of this page &mdash; this
is a reference and you may need to consult it, if in doubt. It doesn't
hurt to skim this page, but most of the semantics "just work" as you'd
expect them to work. It should be straightforward to write
applications using the LuaJIT FFI for developers with a C or C++
background.
</p>
<p class="indent" style="color: #c00000;">
Please note: this is the first public release of the FFI library. This
does not comprise the final specification for the FFI semantics, yet.
Some of the semantics may need to be changed, based on feedback from
developers. Please <a href="contact.html">report</a> any problems
you've encountered or any improvements you'd like to see &mdash; thank
you!
</p>
<h2 id="clang">C Language Support</h2>
@ -93,12 +114,8 @@ the following extensions:
</p>
<ul>
<li>C++-style comments (<tt>//</tt>).</li>
<li>The <tt>'\e'</tt> escape in character and string literals.</li>
<li>The <tt>long long</tt> 64&nbsp;bit integer type.</tt>
<li>The C99/C++ boolean type, declared with the keywords <tt>bool</tt>
or <tt>_Bool</tt>.</li>
@ -122,14 +139,16 @@ inside a <tt>struct</tt>/<tt>union</tt>.</li>
<tt>enum</tt>, except that declared constants are visible in the
global namespace, too.</li>
<li>C++-style scoped <tt>static&nbsp;const</tt> declarations inside a
<tt>struct</tt>/<tt>union</tt>.</li>
<li>Scoped <tt>static&nbsp;const</tt> declarations inside a
<tt>struct</tt>/<tt>union</tt> (from C++).</li>
<li>Zero-length arrays (<tt>[0]</tt>), empty
<tt>struct</tt>/<tt>union</tt>, variable-length arrays (VLA,
<tt>[?]</tt>) and variable-length structs (VLS, with a trailing
VLA).</li>
<li>C++ reference types (<tt>int&nbsp;&amp;x</tt>).</li>
<li>Alternate GCC keywords with '<tt>__</tt>', e.g.
<tt>__const__</tt>.</li>
@ -142,7 +161,7 @@ VLA).</li>
<tt>__alignof__</tt> operator.</li>
<li>GCC <tt>__asm__("symname")</tt> symbol name redirection for
function declarations.</tt>
function declarations.</li>
<li>MSVC keywords for fixed-length types: <tt>__int8</tt>,
<tt>__int16</tt>, <tt>__int32</tt> and <tt>__int64</tt>.</li>
@ -204,18 +223,234 @@ not implemented.</li>
</ul>
<h2 id="convert">C Type Conversion Rules</h2>
<h3 id="convert_tolua">Conversions from C&nbsp;types to Lua objects</h3>
<p>
TODO
These conversion rules apply for <em>read accesses</em> to
C&nbsp;types: indexing pointers, arrays or
<tt>struct</tt>/<tt>union</tt> types; reading external variables or
constant values; retrieving return values from C&nbsp;calls:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin"><tt>int8_t</tt>, <tt>int16_t</tt></td><td class="convop">&rarr;<sup>sign-ext</sup> <tt>int32_t</tt> &rarr; <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="even">
<td class="convin"><tt>uint8_t</tt>, <tt>uint16_t</tt></td><td class="convop">&rarr;<sup>zero-ext</sup> <tt>int32_t</tt> &rarr; <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="odd">
<td class="convin"><tt>int32_t</tt>, <tt>uint32_t</tt></td><td class="convop">&rarr; <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="even">
<td class="convin"><tt>int64_t</tt>, <tt>uint64_t</tt></td><td class="convop">boxed value</td><td class="convout">64 bit int cdata</td></tr>
<tr class="odd separate">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">&rarr; <tt>double</tt></td><td class="convout">number</td></tr>
<tr class="even separate">
<td class="convin"><tt>bool</tt></td><td class="convop">0 &rarr; <tt>true</tt>, otherwise <tt>false</tt></td><td class="convout">boolean</td></tr>
<tr class="odd separate">
<td class="convin">Complex number</td><td class="convop">boxed value</td><td class="convout">complex cdata</td></tr>
<tr class="even">
<td class="convin">Vector</td><td class="convop">boxed value</td><td class="convout">vector cdata</td></tr>
<tr class="odd">
<td class="convin">Pointer</td><td class="convop">boxed value</td><td class="convout">pointer cdata</td></tr>
<tr class="even separate">
<td class="convin">Array</td><td class="convop">boxed reference</td><td class="convout">reference cdata</td></tr>
<tr class="odd">
<td class="convin"><tt>struct</tt>/<tt>union</tt></td><td class="convop">boxed reference</td><td class="convout">reference cdata</td></tr>
</table>
<p>
Bitfields or <tt>enum</tt> types are treated like their underlying
type.
</p>
<p>
Reference types are dereferenced <em>before</em> a conversion can take
place &mdash; the conversion is applied to the C&nbsp;type pointed to
by the reference.
</p>
<h3 id="convert_fromlua">Conversions from Lua objects to C&nbsp;types</h3>
<p>
These conversion rules apply for <em>write accesses</em> to
C&nbsp;types: indexing pointers, arrays or
<tt>struct</tt>/<tt>union</tt> types; initializing cdata objects;
casts to C&nbsp;types; writing to external variables; passing
arguments to C&nbsp;calls:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin">number</td><td class="convop">&rarr;</td><td class="convout"><tt>double</tt></td></tr>
<tr class="even">
<td class="convin">boolean</td><td class="convop"><tt>false</tt> &rarr; 0, <tt>true</tt> &rarr; 1</td><td class="convout"><tt>bool</tt></td></tr>
<tr class="odd separate">
<td class="convin">nil</td><td class="convop"><tt>NULL</tt> &rarr;</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even">
<td class="convin">userdata</td><td class="convop">userdata payload &rarr;</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="odd">
<td class="convin">lightuserdata</td><td class="convop">lightuserdata address &rarr;</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even separate">
<td class="convin">string</td><td class="convop">match against <tt>enum</tt> constant</td><td class="convout"><tt>enum</tt></td></tr>
<tr class="odd">
<td class="convin">string</td><td class="convop">copy string data + zero-byte</td><td class="convout"><tt>int8_t[]</tt>, <tt>uint8_t[]</tt></td></tr>
<tr class="even">
<td class="convin">string</td><td class="convop">string data &rarr;</td><td class="convout"><tt>const char[]</tt></td></tr>
<tr class="odd separate">
<td class="convin">table</td><td class="convop"><a href="#init_table">table initializer</a></td><td class="convout">Array</td></tr>
<tr class="even">
<td class="convin">table</td><td class="convop"><a href="#init_table">table initializer</a></td><td class="convout"><tt>struct</tt>/<tt>union</tt></td></tr>
<tr class="odd separate">
<td class="convin">cdata</td><td class="convop">cdata payload &rarr;</td><td class="convout">C type</td></tr>
</table>
<p>
If the result type of this conversion doesn't match the
C&nbsp;type of the destination, the
<a href="#convert_between">conversion rules between C&nbsp;types</a>
are applied.
</p>
<p>
Reference types are immutable after initialization ("no reseating of
references"). For initialization purposes or when passing values to
reference parameters, they are treated like pointers. Note that unlike
in C++, there's no way to implement automatic reference generation of
variables under the Lua language semantics. If you want to call a
function with a reference parameter, you need to explicitly pass a
one-element array.
</p>
<h3 id="convert_between">Conversions between C&nbsp;types</h3>
<p>
These conversion rules are more or less the same as the standard
C&nbsp;conversion rules. Some rules only apply to casts, or require
pointer or type compatibility:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin">Signed integer</td><td class="convop">&rarr;<sup>narrow or sign-extend</sup></td><td class="convout">Integer</td></tr>
<tr class="even">
<td class="convin">Unsigned integer</td><td class="convop">&rarr;<sup>narrow or zero-extend</sup></td><td class="convout">Integer</td></tr>
<tr class="odd">
<td class="convin">Integer</td><td class="convop">&rarr;<sup>round</sup></td><td class="convout"><tt>double</tt>, <tt>float</tt></td></tr>
<tr class="even">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">&rarr;<sup>trunc</sup> <tt>int32_t</tt> &rarr;<sup>narrow</sup></td><td class="convout"><tt>(u)int8_t</tt>, <tt>(u)int16_t</tt></td></tr>
<tr class="odd">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">&rarr;<sup>trunc</sup></td><td class="convout"><tt>(u)int32_t</tt>, <tt>(u)int64_t</tt></td></tr>
<tr class="even">
<td class="convin"><tt>double</tt>, <tt>float</tt></td><td class="convop">&rarr;<sup>round</sup></td><td class="convout"><tt>float</tt>, <tt>double</tt></td></tr>
<tr class="odd separate">
<td class="convin">Number</td><td class="convop">n == 0 &rarr; 0, otherwise 1</td><td class="convout"><tt>bool</tt></td></tr>
<tr class="even">
<td class="convin"><tt>bool</tt></td><td class="convop"><tt>false</tt> &rarr; 0, <tt>true</tt> &rarr; 1</td><td class="convout">Number</td></tr>
<tr class="odd separate">
<td class="convin">Complex number</td><td class="convop">convert real part</td><td class="convout">Number</td></tr>
<tr class="even">
<td class="convin">Number</td><td class="convop">convert real part, imag = 0</td><td class="convout">Complex number</td></tr>
<tr class="odd">
<td class="convin">Complex number</td><td class="convop">convert real and imag part</td><td class="convout">Complex number</td></tr>
<tr class="even separate">
<td class="convin">Number</td><td class="convop">convert scalar and replicate</td><td class="convout">Vector</td></tr>
<tr class="odd">
<td class="convin">Vector</td><td class="convop">copy (same size)</td><td class="convout">Vector</td></tr>
<tr class="even separate">
<td class="convin"><tt>struct</tt>/<tt>union</tt></td><td class="convop">take base address (compat)</td><td class="convout">Pointer</td></tr>
<tr class="odd">
<td class="convin">Array</td><td class="convop">take base address (compat)</td><td class="convout">Pointer</td></tr>
<tr class="even">
<td class="convin">Function</td><td class="convop">take function address</td><td class="convout">Function pointer</td></tr>
<tr class="odd separate">
<td class="convin">Number</td><td class="convop">convert via <tt>uintptr_t</tt> (cast)</td><td class="convout">Pointer</td></tr>
<tr class="even">
<td class="convin">Pointer</td><td class="convop">convert address (compat/cast)</td><td class="convout">Pointer</td></tr>
<tr class="odd">
<td class="convin">Pointer</td><td class="convop">convert address (cast)</td><td class="convout">Integer</td></tr>
<tr class="even">
<td class="convin">Array</td><td class="convop">convert base address (cast)</td><td class="convout">Integer</td></tr>
<tr class="odd separate">
<td class="convin">Array</td><td class="convop">copy (compat)</td><td class="convout">Array</td></tr>
<tr class="even">
<td class="convin"><tt>struct</tt>/<tt>union</tt></td><td class="convop">copy (identical type)</td><td class="convout"><tt>struct</tt>/<tt>union</tt></td></tr>
</table>
<p>
Bitfields or <tt>enum</tt> types are treated like their underlying
type.
</p>
<p>
Conversions not listed above will raise an error. E.g. it's not
possible to convert a pointer to a complex number or vice versa.
</p>
<h3 id="convert_vararg">Conversions for vararg C&nbsp;function arguments</h3>
<p>
The following default conversion rules apply when passing Lua objects
to the variable argument part of vararg C&nbsp;functions:
</p>
<table class="convtable">
<tr class="convhead">
<td class="convin">Input</td>
<td class="convop">Conversion</td>
<td class="convout">Output</td>
</tr>
<tr class="odd separate">
<td class="convin">number</td><td class="convop">&rarr;</td><td class="convout"><tt>double</tt></td></tr>
<tr class="even">
<td class="convin">boolean</td><td class="convop"><tt>false</tt> &rarr; 0, <tt>true</tt> &rarr; 1</td><td class="convout"><tt>bool</tt></td></tr>
<tr class="odd separate">
<td class="convin">nil</td><td class="convop"><tt>NULL</tt> &rarr;</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even">
<td class="convin">userdata</td><td class="convop">userdata payload &rarr;</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="odd">
<td class="convin">lightuserdata</td><td class="convop">lightuserdata address &rarr;</td><td class="convout"><tt>(void *)</tt></td></tr>
<tr class="even separate">
<td class="convin">string</td><td class="convop">string data &rarr;</td><td class="convout"><tt>const char *</tt></td></tr>
<tr class="odd separate">
<td class="convin"><tt>float</tt> cdata</td><td class="convop">&rarr;</td><td class="convout"><tt>double</tt></td></tr>
<tr class="even">
<td class="convin">Array cdata</td><td class="convop">take base address</td><td class="convout">Element pointer</td></tr>
<tr class="odd">
<td class="convin"><tt>struct</tt>/<tt>union</tt> cdata</td><td class="convop">take base address</td><td class="convout"><tt>struct</tt>/<tt>union</tt> pointer</td></tr>
<tr class="even">
<td class="convin">Function cdata</td><td class="convop">take function address</td><td class="convout">Function pointer</td></tr>
<tr class="odd">
<td class="convin">Any other cdata</td><td class="convop">no conversion</td><td class="convout">C type</td></tr>
</table>
<p>
To pass a Lua object, other than a cdata object, as a specific type,
you need to override the conversion rules: create a temporary cdata
object with a constructor or a cast and initialize it with the value
to pass:
</p>
<p>
Assuming <tt>x</tt> is a Lua number, here's how to pass it as an
integer to a vararg function:
</p>
<pre class="code">
ffi.cdef[[
int printf(const char *fmt, ...);
]]
ffi.C.printf("integer value: %d\n", ffi.new("int", x))
</pre>
<p>
If you don't do this, the default Lua number &rarr; <tt>double</tt>
conversion rule applies. A vararg C&nbsp;function expecting an integer
will see a garbled or uninitialized value.
</p>
<h3 id="convert_tolua">Conversions from C&nbsp;types to Lua objects</h2>
<h3 id="convert_fromlua">Conversions from Lua objects to C&nbsp;types</h2>
<h3 id="convert_between">Conversions between C&nbsp;types</h2>
<h2 id="init">Initializers</h2>
<p>
Creating a cdata object with <a href="ffi_ext_api.html#ffi_new">ffi.new()</a>
or the equivalent constructor syntax always initializes its contents,
too. Different rules apply, depending on the number of optional
Creating a cdata object with
<a href="ext_ffi_api.html#ffi_new"><tt>ffi.new()</tt></a> or the
equivalent constructor syntax always initializes its contents, too.
Different rules apply, depending on the number of optional
initializers and the C&nbsp;types involved:
</p>
<ul>
@ -230,8 +465,8 @@ when a single initializer is given. Otherwise they are treated like
regular arrays.</li>
<li>Aggregate types (arrays and structs) accept either a single
compound initializer (Lua table or string) or a flat list of
initializers.</li>
<a href="#init_table">table initializer</a> or a flat list of initializers.
Byte arrays can be initialized with a Lua string, too.</li>
<li>The elements of an array are initialized, starting at index zero.
If a single initializer is given for an array, it's repeated for all
@ -247,15 +482,272 @@ bytes.</li>
flat initializer.</li>
<li>Elements or fields which are aggregates themselves are initialized
with a <em>single</em> initializer, but this may be a compound
initializer or a compatible aggregate, of course.</li>
with a <em>single</em> initializer, but this may be a table
initializer or a compatible aggregate.</li>
<li>Excess initializers cause an error.</li>
</ul>
<h2 id="ops">Operations on cdata Objects</h2>
<h2 id="init_table">Table Initializers</h2>
<p>
TODO
The following rules apply if a Lua table is used to initialize an
Array or a <tt>struct</tt>/<tt>union</tt>:
</p>
<ul>
<li>If the table index <tt>[0]</tt> is non-<tt>nil</tt>, then the
table is assumed to be zero-based. Otherwise it's assumed to be
one-based.</li>
<li>Array elements, starting at index zero, are initialized one-by-one
with the consecutive table elements, starting at either index
<tt>[0]</tt> or <tt>[1]</tt>. This process stops at the first
<tt>nil</tt> table element.</li>
<li>If exactly one array element was initialized, it's repeated for
all the remaining elements. Otherwise all remaining uninitialized
elements are filled with zero bytes.</li>
<li>If the table has a non-empty hash part, a
<tt>struct</tt>/<tt>union</tt> is initialized by looking up each field
name (as a string key) in the table. Each non-<tt>nil</tt> value is
used to initialize the corresponding field.</li>
<li>Otherwise a <tt>struct</tt>/<tt>union</tt> is initialized in the
order of the declaration of its fields. Each field is initialized with
the consecutive table elements, starting at either index <tt>[0]</tt>
or <tt>[1]</tt>. This process stops at the first <tt>nil</tt> table
element.</li>
<li>Uninitialized fields of a <tt>struct</tt> are filled with zero
bytes.</li>
<li>Initialization of a <tt>union</tt> stops after one field has been
initialized. If no field has been initialized, the <tt>union</tt> is
filled with zero bytes.</li>
<li>Elements or fields which are aggregates themselves are initialized
with a <em>single</em> initializer, but this may be a nested table
initializer (or a compatible aggregate).</li>
<li>Excess initializers for an array cause an error. Excess
initializers for a <tt>struct</tt>/<tt>union</tt> are ignored.
Unrelated table entries are ignored, too.</li>
</ul>
<p>
Example:
</p>
<pre class="code">
local ffi = require("ffi")
ffi.cdef[[
struct foo { int a, b; };
union bar { int i; double d; };
struct nested { int x; struct foo y; };
]]
ffi.new("int[3]", {}) --> 0, 0, 0
ffi.new("int[3]", {1}) --> 1, 1, 1
ffi.new("int[3]", {1,2}) --> 1, 2, 0
ffi.new("int[3]", {1,2,3}) --> 1, 2, 3
ffi.new("int[3]", {[0]=1}) --> 1, 1, 1
ffi.new("int[3]", {[0]=1,2}) --> 1, 2, 0
ffi.new("int[3]", {[0]=1,2,3}) --> 1, 2, 3
ffi.new("int[3]", {[0]=1,2,3,4}) --> error: too many initializers
ffi.new("struct foo", {}) --> a = 0, b = 0
ffi.new("struct foo", {1}) --> a = 1, b = 0
ffi.new("struct foo", {1,2}) --> a = 1, b = 2
ffi.new("struct foo", {[0]=1,2}) --> a = 1, b = 2
ffi.new("struct foo", {b=2}) --> a = 0, b = 2
ffi.new("struct foo", {a=1,b=2,c=3}) --> a = 1, b = 2 'c' is ignored
ffi.new("union bar", {}) --> i = 0, d = 0.0
ffi.new("union bar", {1}) --> i = 1, d = ?
ffi.new("union bar", {[0]=1,2}) --> i = 1, d = ? '2' is ignored
ffi.new("union bar", {d=2}) --> i = ?, d = 2.0
ffi.new("struct nested", {1,{2,3}}) --> x = 1, y.a = 2, y.b = 3
ffi.new("struct nested", {x=1,y={2,3}}) --> x = 1, y.a = 2, y.b = 3
</pre>
<h2 id="cdata_ops">Operations on cdata Objects</h2>
<p>
All of the standard Lua operators can be applied to cdata objects or a
mix of a cdata object and another Lua object. The following list shows
the valid combinations. All other combinations currently raise an
error.
</p>
<p>
Reference types are dereferenced <em>before</em> performing each of
the operations below &mdash; the operation is applied to the
C&nbsp;type pointed to by the reference.
</p>
<h3 id="cdata_array">Indexing a cdata object</h3>
<ul>
<li><b>Indexing a pointer/array</b>: a cdata pointer/array can be
indexed by a cdata number or a Lua number. The element address is
computed as the base address plus the number value multiplied by the
element size in bytes. A read access loads the element value and
<a href="#convert_tolua">converts it to a Lua object</a>. A write
access <a href="#convert_fromlua">converts a Lua object to the element
type</a> and stores the converted value to the element. An error is
raised if the element size is undefined or a write access to a
constant element is attempted.</li>
<li><b>Dereferencing a <tt>struct</tt>/<tt>union</tt> field</b>: a
cdata <tt>struct</tt>/<tt>union</tt> or a pointer to a
<tt>struct</tt>/<tt>union</tt> can be dereferenced by a string key,
giving the field name. The field address is computed as the base
address plus the relative offset of the field. A read access loads the
field value and <a href="#convert_tolua">converts it to a Lua
object</a>. A write access <a href="#convert_fromlua">converts a Lua
object to the field type</a> and stores the converted value to the
field. An error is raised if a write access to a constant
<tt>struct</tt>/<tt>union</tt> or a constant field is attempted.</li>
<li><b>Indexing a complex number</b>: a complex number can be indexed
either by a cdata number or a Lua number with the values 0 or 1, or by
the strings <tt>"re"</tt> or <tt>"im"</tt>. A read access loads the
real part (<tt>[0]</tt>, <tt>.re</tt>) or the imaginary part
(<tt>[1]</tt>, <tt>.im</tt>) part of a complex number and
<a href="#convert_tolua">converts it to a Lua number</a>. The
sub-parts of a complex number are immutable &mdash; assigning to an
index of a complex number raises an error. Accessing out-of-bound
indexes returns unspecified results, but is guaranteed not to trigger
memory access violations.</li>
<li><b>Indexing a vector</b>: a vector is treated like an array for
indexing purposes, except the vector elements are immutable &mdash;
assigning to an index of a vector raises an error.</li>
</ul>
<p>
Note: since there's (deliberately) no address-of operator, a cdata
object holding a value type is effectively immutable after
initialization. The JIT compiler benefits from this fact when applying
certain optimizations.
</p>
<p>
As a consequence of this, the <em>elements</em> of complex numbers and
vectors are immutable. But the elements of an aggregate holding these
types <em>may</em> be modified of course. I.e. you cannot assign to
<tt>foo.c.im</tt>, but you can assign a (newly created) complex number
to <tt>foo.c</tt>.
</p>
<h3 id="cdata_call">Calling a cdata object</h3>
<ul>
<li><b>Constructor</b>: a ctype object can be called and used as a
<a href="ext_ffi_api.html#ffi_new">constructor</a>.</li>
<li><b>C&nbsp;function call</b>: a cdata function or cdata function
pointer can be called. The passed arguments are
<a href="#convert_fromlua">converted to the C&nbsp;types</a> of the
parameters given by the function declaration. Arguments passed to the
variable argument part of vararg C&nbsp;function use
<a href="#convert_vararg">special conversion rules</a>. This
C&nbsp;function is called and the return value (if any) is
<a href="#convert_tolua">converted to a Lua object</a>.</li>
</ul>
<h3 id="cdata_arith">Arithmetic on cdata objects</h3>
<ul>
<li><b>Pointer arithmetic</b>: a cdata pointer/array and a cdata
number or a Lua number can be added or subtracted. The number must be
on the right hand side for a subtraction. The result is a pointer of
the same type with an address plus or minus the number value
multiplied by the element size in bytes. An error is raised if the
element size is undefined.</li>
<li><b>Pointer difference</b>: two compatible cdata pointers/arrays
can be subtracted. The result is the difference between their
addresses, divided by the element size in bytes. An error is raised if
the element size is undefined or zero.</li>
<li><a href="ext_ffi_int64.html">64&nbsp;bit integer arithmetic</a>:
the standard arithmetic operators
(<tt>+&nbsp;-&nbsp;*&nbsp;/&nbsp;%&nbsp;^</tt> and unary <tt>-</tt>)
can be applied to two cdata numbers, or a cdata number and a Lua
number. If one of them is an <tt>uint64_t</tt>, the other side is
converted to an <tt>uint64_t</tt> and an unsigned arithmetic operation
is performed. Otherwise both sides are converted to an
<tt>int64_t</tt> and a signed arithmetic operation is performed. The
result is a boxed 64&nbsp;bit cdata object.</li>
</ul>
<h3 id="cdata_comp">Comparisons of cdata objects</h3>
<ul>
<li><b>Pointer comparison</b>: two compatible cdata pointers/arrays
can be compared. The result is the same as an unsigned comparison of
their addresses. <tt>nil</tt> is treated like a <tt>NULL</tt> pointer,
which is compatible with any other pointer type.</li>
<li><a href="ext_ffi_int64.html">64&nbsp;bit integer comparison</a>:
two cdata numbers, or a cdata number and a Lua number can be compared
with each other. If one of them is an <tt>uint64_t</tt>, the other
side is converted to an <tt>uint64_t</tt> and an unsigned comparison
is performed. Otherwise both sides are converted to an
<tt>int64_t</tt> and a signed comparison is performed.</li>
</ul>
<h3 id="cdata_key">cdata objects as table keys</h3>
<p>
Lua tables may be indexed by cdata objects, but this doesn't provide
any useful semantics &mdash; <b>cdata objects are unsuitable as table
keys!</b>
</p>
<p>
A cdata object is treated like any other garbage-collected object and
is hashed and compared by its address for table indexing. Since
there's no interning for cdata value types, the same value may be
boxed in different cdata objects with different addresses. Thus
<tt>t[1LL+1LL]</tt> and <tt>t[2LL]</tt> usually <b>do not</b> point to
the same hash slot and they certainly <b>do not</b> point to the same
hash slot as <tt>t[2]</tt>.
</p>
<p>
It would seriously drive up implementation complexity and slow down
the common case, if one were to add extra handling for by-value
hashing and comparisons to Lua tables. Given the ubiquity of their use
inside the VM, this is not acceptable.
</p>
<p>
There are three viable alternatives, if you really need to use cdata
objects as keys:
</p>
<ul>
<li>If you can get by with the precision of Lua numbers
(52&nbsp;bits), then use <tt>tonumber()</tt> on a cdata number or
combine multiple fields of a cdata aggregate to a Lua number. Then use
the resulting Lua number as a key when indexing tables.<br>
One obvious benefit: <tt>t[tonumber(2LL)]</tt> <b>does</b> point to
the same slot as <tt>t[2]</tt>.</li>
<li>Otherwise use either <tt>tostring()</tt> on 64&nbsp;bit integers
or complex numbers or combine multiple fields of a cdata aggregate to
a Lua string (e.g. with
<a href="ext_ffi_api.html#ffi_string"><tt>ffi.string()</tt></a>). Then
use the resulting Lua string as a key when indexing tables.</li>
<li>Create your own specialized hash table implementation using the
C&nbsp;types provided by the FFI library, just like you would in
C&nbsp;code. Ultimately this may give much better performance than the
other alternatives or what a generic by-value hash table could
possibly provide.</li>
</ul>
<h2 id="gc">Garbage Collection of cdata Objects</h2>
<p>
@ -297,14 +789,9 @@ is not garbage collected.
Objects which are passed as an argument to an external C&nbsp;function
are kept alive until the call returns. So it's generally safe to
create temporary cdata objects in argument lists. This is a common
idiom for passing specific C&nbsp;types to vararg functions:
idiom for <a href="#convert_vararg">passing specific C&nbsp;types to
vararg functions</a>.
</p>
<pre class="code">
ffi.cdef[[
int printf(const char *fmt, ...);
]]
ffi.C.printf("integer value: %d\n", ffi.new("int", x)) -- <span style="color:#00a000;">OK</span>
</pre>
<p>
Memory areas returned by C functions (e.g. from <tt>malloc()</tt>)
must be manually managed, of course. Pointers to cdata objects are
@ -468,12 +955,12 @@ suboptimal performance, especially when used in inner loops:
<li>Array/<tt>struct</tt> copies and bulk initializations.</li>
<li>Bitfield accesses and initializations.</li>
<li>Vector operations.</li>
<li>Lua tables as compound initializers.</li>
<li>Table initializers.</li>
<li>Initialization of nested <tt>struct</tt>/<tt>union</tt> types.</li>
<li>Allocations of variable-length arrays or structs.</li>
<li>Allocations of C&nbsp;types with a size &gt; 64&nbsp;bytes or an
alignment &gt; 8&nbsp;bytes.</li>
<li>Conversions from <tt>lightuserdata</tt> to <tt>void&nbsp;*</tt>.</li>
<li>Conversions from lightuserdata to <tt>void&nbsp;*</tt>.</li>
<li>Pointer differences for element sizes that are not a power of
two.</li>
<li>Calls to non-cdecl or vararg C&nbsp;functions.</li>
@ -496,7 +983,7 @@ Other missing features:
<li>Callbacks from C&nbsp;code to Lua functions.</li>
<li>Atomic handling of <tt>errno</tt>.</li>
<li>Passing structs by value to vararg C&nbsp;functions.</li>
<li><a href="extensions.html#exceptions">C++ exception interoperability<a/>
<li><a href="extensions.html#exceptions">C++ exception interoperability</a>
does not extend to C&nbsp;functions called via the FFI.</li>
</ul>
<br class="flush">