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| author | sanine <sanine.not@pm.me> | 2023-03-11 15:58:20 -0600 |
|---|---|---|
| committer | sanine <sanine.not@pm.me> | 2023-03-11 15:58:20 -0600 |
| commit | ebc50b387ab209c9f9a0d92e340ac293d5697274 (patch) | |
| tree | ea8c8b3677a18c994d2b9d33dbef3461dcf18113 /libs/luajit-cmake/luajit/doc/ext_ffi_semantics.html | |
| parent | c2329b4c8258baa9429c77566c9def97d00e96d7 (diff) | |
build & link with luajit instead of lua5.1
Diffstat (limited to 'libs/luajit-cmake/luajit/doc/ext_ffi_semantics.html')
| -rw-r--r-- | libs/luajit-cmake/luajit/doc/ext_ffi_semantics.html | 1256 |
1 files changed, 1256 insertions, 0 deletions
diff --git a/libs/luajit-cmake/luajit/doc/ext_ffi_semantics.html b/libs/luajit-cmake/luajit/doc/ext_ffi_semantics.html new file mode 100644 index 0000000..603f995 --- /dev/null +++ b/libs/luajit-cmake/luajit/doc/ext_ffi_semantics.html @@ -0,0 +1,1256 @@ +<!DOCTYPE html> +<html> +<head> +<title>FFI Semantics</title> +<meta charset="utf-8"> +<meta name="Copyright" content="Copyright (C) 2005-2022"> +<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.convop { font-style: italic; width: 40%; } +</style> +</head> +<body> +<div id="site"> +<a href="https://luajit.org"><span>Lua<span id="logo">JIT</span></span></a> +</div> +<div id="head"> +<h1>FFI Semantics</h1> +</div> +<div id="nav"> +<ul><li> +<a href="luajit.html">LuaJIT</a> +<ul><li> +<a href="https://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 href="ext_ffi_tutorial.html">FFI Tutorial</a> +</li><li> +<a href="ext_ffi_api.html">ffi.* API</a> +</li><li> +<a class="current" href="ext_ffi_semantics.html">FFI Semantics</a> +</li></ul> +</li><li> +<a href="ext_buffer.html">String Buffers</a> +</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> +</li><li> +<a href="faq.html">FAQ</a> +</li><li> +<a href="https://luajit.org/list.html">Mailing List <span class="ext">»</span></a> +</li></ul> +</div> +<div id="main"> +<p> +This page describes the detailed semantics underlying the FFI library +and its interaction with both Lua and C code. +</p> +<p> +Given that the FFI library is designed to interface with C code +and that declarations can be written in plain C syntax, <b>it +closely follows the C language semantics</b>, 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 — 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> + +<h2 id="clang">C Language Support</h2> +<p> +The FFI library has a built-in C parser with a minimal memory +footprint. It's used by the <a href="ext_ffi_api.html">ffi.* library +functions</a> to declare C types or external symbols. +</p> +<p> +Its only purpose is to parse C declarations, as found e.g. in +C header files. Although it does evaluate constant expressions, +it's <em>not</em> a C compiler. The body of <tt>inline</tt> +C function definitions is simply ignored. +</p> +<p> +Also, this is <em>not</em> a validating C parser. It expects and +accepts correctly formed C declarations, but it may choose to +ignore bad declarations or show rather generic error messages. If in +doubt, please check the input against your favorite C compiler. +</p> +<p> +The C parser complies to the <b>C99 language standard</b> plus +the following extensions: +</p> +<ul> + +<li>The <tt>'\e'</tt> escape in character and string literals.</li> + +<li>The C99/C++ boolean type, declared with the keywords <tt>bool</tt> +or <tt>_Bool</tt>.</li> + +<li>Complex numbers, declared with the keywords <tt>complex</tt> or +<tt>_Complex</tt>.</li> + +<li>Two complex number types: <tt>complex</tt> (aka +<tt>complex double</tt>) and <tt>complex float</tt>.</li> + +<li>Vector types, declared with the GCC <tt>mode</tt> or +<tt>vector_size</tt> attribute.</li> + +<li>Unnamed ('transparent') <tt>struct</tt>/<tt>union</tt> fields +inside a <tt>struct</tt>/<tt>union</tt>.</li> + +<li>Incomplete <tt>enum</tt> declarations, handled like incomplete +<tt>struct</tt> declarations.</li> + +<li>Unnamed <tt>enum</tt> fields inside a +<tt>struct</tt>/<tt>union</tt>. This is similar to a scoped C++ +<tt>enum</tt>, except that declared constants are visible in the +global namespace, too.</li> + +<li>Scoped <tt>static 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 &x</tt>).</li> + +<li>Alternate GCC keywords with '<tt>__</tt>', e.g. +<tt>__const__</tt>.</li> + +<li>GCC <tt>__attribute__</tt> with the following attributes: +<tt>aligned</tt>, <tt>packed</tt>, <tt>mode</tt>, +<tt>vector_size</tt>, <tt>cdecl</tt>, <tt>fastcall</tt>, +<tt>stdcall</tt>, <tt>thiscall</tt>.</li> + +<li>The GCC <tt>__extension__</tt> keyword and the GCC +<tt>__alignof__</tt> operator.</li> + +<li>GCC <tt>__asm__("symname")</tt> symbol name redirection for +function declarations.</li> + +<li>MSVC keywords for fixed-length types: <tt>__int8</tt>, +<tt>__int16</tt>, <tt>__int32</tt> and <tt>__int64</tt>.</li> + +<li>MSVC <tt>__cdecl</tt>, <tt>__fastcall</tt>, <tt>__stdcall</tt>, +<tt>__thiscall</tt>, <tt>__ptr32</tt>, <tt>__ptr64</tt>, +<tt>__declspec(align(n))</tt> and <tt>#pragma pack</tt>.</li> + +<li>All other GCC/MSVC-specific attributes are ignored.</li> + +</ul> +<p> +The following C types are predefined by the C parser (like +a <tt>typedef</tt>, except re-declarations will be ignored): +</p> +<ul> + +<li>Vararg handling: <tt>va_list</tt>, <tt>__builtin_va_list</tt>, +<tt>__gnuc_va_list</tt>.</li> + +<li>From <tt><stddef.h></tt>: <tt>ptrdiff_t</tt>, +<tt>size_t</tt>, <tt>wchar_t</tt>.</li> + +<li>From <tt><stdint.h></tt>: <tt>int8_t</tt>, <tt>int16_t</tt>, +<tt>int32_t</tt>, <tt>int64_t</tt>, <tt>uint8_t</tt>, +<tt>uint16_t</tt>, <tt>uint32_t</tt>, <tt>uint64_t</tt>, +<tt>intptr_t</tt>, <tt>uintptr_t</tt>.</li> + +<li>From <tt><unistd.h></tt> (POSIX): <tt>ssize_t</tt>.</li> + +</ul> +<p> +You're encouraged to use these types in preference to +compiler-specific extensions or target-dependent standard types. +E.g. <tt>char</tt> differs in signedness and <tt>long</tt> differs in +size, depending on the target architecture and platform ABI. +</p> +<p> +The following C features are <b>not</b> supported: +</p> +<ul> + +<li>A declaration must always have a type specifier; it doesn't +default to an <tt>int</tt> type.</li> + +<li>Old-style empty function declarations (K&R) are not allowed. +All C functions must have a proper prototype declaration. A +function declared without parameters (<tt>int foo();</tt>) is +treated as a function taking zero arguments, like in C++.</li> + +<li>The <tt>long double</tt> C type is parsed correctly, but +there's no support for the related conversions, accesses or arithmetic +operations.</li> + +<li>Wide character strings and character literals are not +supported.</li> + +<li><a href="#status">See below</a> for features that are currently +not implemented.</li> + +</ul> + +<h2 id="convert">C Type Conversion Rules</h2> + +<h3 id="convert_tolua">Conversions from C types to Lua objects</h3> +<p> +These conversion rules apply for <em>read accesses</em> to +C types: indexing pointers, arrays or +<tt>struct</tt>/<tt>union</tt> types; reading external variables or +constant values; retrieving return values from C 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">→<sup>sign-ext</sup> <tt>int32_t</tt> → <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">→<sup>zero-ext</sup> <tt>int32_t</tt> → <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">→ <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">→ <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 → <tt>false</tt>, otherwise <tt>true</tt></td><td class="convout">boolean</td></tr> +<tr class="odd separate"> +<td class="convin"><tt>enum</tt></td><td class="convop">boxed value</td><td class="convout">enum cdata</td></tr> +<tr class="even"> +<td class="convin">Complex number</td><td class="convop">boxed value</td><td class="convout">complex cdata</td></tr> +<tr class="odd"> +<td class="convin">Vector</td><td class="convop">boxed value</td><td class="convout">vector cdata</td></tr> +<tr class="even"> +<td class="convin">Pointer</td><td class="convop">boxed value</td><td class="convout">pointer cdata</td></tr> +<tr class="odd separate"> +<td class="convin">Array</td><td class="convop">boxed reference</td><td class="convout">reference cdata</td></tr> +<tr class="even"> +<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 are treated like their underlying type. +</p> +<p> +Reference types are dereferenced <em>before</em> a conversion can take +place — the conversion is applied to the C type pointed to +by the reference. +</p> + +<h3 id="convert_fromlua">Conversions from Lua objects to C types</h3> +<p> +These conversion rules apply for <em>write accesses</em> to +C types: indexing pointers, arrays or +<tt>struct</tt>/<tt>union</tt> types; initializing cdata objects; +casts to C types; writing to external variables; passing +arguments to C 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">→</td><td class="convout"><tt>double</tt></td></tr> +<tr class="even"> +<td class="convin">boolean</td><td class="convop"><tt>false</tt> → 0, <tt>true</tt> → 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> →</td><td class="convout"><tt>(void *)</tt></td></tr> +<tr class="even"> +<td class="convin">lightuserdata</td><td class="convop">lightuserdata address →</td><td class="convout"><tt>(void *)</tt></td></tr> +<tr class="odd"> +<td class="convin">userdata</td><td class="convop">userdata payload →</td><td class="convout"><tt>(void *)</tt></td></tr> +<tr class="even"> +<td class="convin">io.* file</td><td class="convop">get FILE * handle →</td><td class="convout"><tt>(void *)</tt></td></tr> +<tr class="odd 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="even"> +<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="odd"> +<td class="convin">string</td><td class="convop">string data →</td><td class="convout"><tt>const char[]</tt></td></tr> +<tr class="even separate"> +<td class="convin">function</td><td class="convop"><a href="#callback">create callback</a> →</td><td class="convout">C function type</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 →</td><td class="convout">C type</td></tr> +</table> +<p> +If the result type of this conversion doesn't match the +C type of the destination, the +<a href="#convert_between">conversion rules between C types</a> +are applied. +</p> +<p> +Reference types are immutable after initialization ("no re-seating 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 types</h3> +<p> +These conversion rules are more or less the same as the standard +C 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">→<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">→<sup>narrow or zero-extend</sup></td><td class="convout">Integer</td></tr> +<tr class="odd"> +<td class="convin">Integer</td><td class="convop">→<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">→<sup>trunc</sup> <tt>int32_t</tt> →<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">→<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">→<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 → 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> → 0, <tt>true</tt> → 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 function arguments</h3> +<p> +The following default conversion rules apply when passing Lua objects +to the variable argument part of vararg C 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">→</td><td class="convout"><tt>double</tt></td></tr> +<tr class="even"> +<td class="convin">boolean</td><td class="convop"><tt>false</tt> → 0, <tt>true</tt> → 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> →</td><td class="convout"><tt>(void *)</tt></td></tr> +<tr class="even"> +<td class="convin">userdata</td><td class="convop">userdata payload →</td><td class="convout"><tt>(void *)</tt></td></tr> +<tr class="odd"> +<td class="convin">lightuserdata</td><td class="convop">lightuserdata address →</td><td class="convout"><tt>(void *)</tt></td></tr> +<tr class="even separate"> +<td class="convin">string</td><td class="convop">string data →</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">→</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 → <tt>double</tt> +conversion rule applies. A vararg C function expecting an integer +will see a garbled or uninitialized value. +</p> + +<h2 id="init">Initializers</h2> +<p> +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 types involved: +</p> +<ul> +<li>If no initializers are given, the object is filled with zero bytes.</li> + +<li>Scalar types (numbers and pointers) accept a single initializer. +The Lua object is <a href="#convert_fromlua">converted to the scalar +C type</a>.</li> + +<li>Valarrays (complex numbers and vectors) are treated like scalars +when a single initializer is given. Otherwise they are treated like +regular arrays.</li> + +<li>Aggregate types (arrays and structs) accept either a single cdata +initializer of the same type (copy constructor), a single +<a href="#init_table">table initializer</a>, or a flat list of +initializers.</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 +remaining elements. This doesn't happen if two or more initializers +are given: all remaining uninitialized elements are filled with zero +bytes.</li> + +<li>Byte arrays may also be initialized with a Lua string. This copies +the whole string plus a terminating zero-byte. The copy stops early only +if the array has a known, fixed size.</li> + +<li>The fields of a <tt>struct</tt> are initialized in the order of +their declaration. Uninitialized fields are filled with zero +bytes.</li> + +<li>Only the first field of a <tt>union</tt> can be initialized with a +flat initializer.</li> + +<li>Elements or fields which are aggregates themselves are initialized +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="init_table">Table Initializers</h2> +<p> +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>The above logic only applies to arrays with a known fixed size. +A VLA is only initialized with the element(s) given in the table. +Depending on the use case, you may need to explicitly add a +<tt>NULL</tt> or <tt>0</tt> terminator to a VLA.</li> + +<li>A <tt>struct</tt>/<tt>union</tt> can be initialized in the +order of the declaration of its fields. Each field is initialized with +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>Otherwise, if neither index <tt>[0]</tt> nor <tt>[1]</tt> is present, +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>Uninitialized fields of a <tt>struct</tt> are filled with zero +bytes, except for the trailing VLA of a VLS.</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 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 predefined operations. +</p> +<p> +Reference types are dereferenced <em>before</em> performing each of +the operations below — the operation is applied to the +C type pointed to by the reference. +</p> +<p> +The predefined operations are always tried first before deferring to a +metamethod or index table (if any) for the corresponding ctype (except +for <tt>__new</tt>). An error is raised if the metamethod lookup or +index table lookup fails. +</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. +Scoped enum constants or static constants are treated like a constant +field.</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 — 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 — +assigning to an index of a vector raises an error.</li> + +</ul> +<p> +A ctype object can be indexed with a string key, too. The only +predefined operation is reading scoped constants of +<tt>struct</tt>/<tt>union</tt> types. All other accesses defer +to the corresponding metamethods or index tables (if any). +</p> +<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, 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> +<p> +The JIT compiler implements strict aliasing rules: accesses to different +types do <b>not</b> alias, except for differences in signedness (this +applies even to <tt>char</tt> pointers, unlike C99). Type punning +through unions is explicitly detected and allowed. +</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>. This is equivalent +to <tt>ffi.new(ct, ...)</tt>, unless a <tt>__new</tt> metamethod is +defined. The <tt>__new</tt> metamethod is called with the ctype object +plus any other arguments passed to the constructor. Note that you have to +use <tt>ffi.new</tt> inside the metamethod, since calling <tt>ct(...)</tt> +would cause infinite recursion.</li> + +<li><b>C 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 types</a> of the +parameters given by the function declaration. Arguments passed to the +variable argument part of vararg C function use +<a href="#convert_vararg">special conversion rules</a>. This +C function is called and the return value (if any) is +<a href="#convert_tolua">converted to a Lua object</a>.<br> +On Windows/x86 systems, <tt>__stdcall</tt> functions are automatically +detected, and a function declared as <tt>__cdecl</tt> (the default) is +silently fixed up after the first call.</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><b>64 bit integer arithmetic</b>: the standard arithmetic +operators (<tt>+ - * / % ^</tt> and unary +minus) 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 bit cdata object.<br> + +If one of the operands is an <tt>enum</tt> and the other operand is a +string, the string is converted to the value of a matching <tt>enum</tt> +constant before the above conversion.<br> + +These rules ensure that 64 bit integers are "sticky". Any +expression involving at least one 64 bit integer operand results +in another one. The undefined cases for the division, modulo and power +operators return <tt>2LL ^ 63</tt> or +<tt>2ULL ^ 63</tt>.<br> + +You'll have to explicitly convert a 64 bit integer to a Lua +number (e.g. for regular floating-point calculations) with +<tt>tonumber()</tt>. But note this may incur a precision loss.</li> + +<li><b>64 bit bitwise operations</b>: the rules for 64 bit +arithmetic operators apply analogously.<br> + +Unlike the other <tt>bit.*</tt> operations, <tt>bit.tobit()</tt> +converts a cdata number via <tt>int64_t</tt> to <tt>int32_t</tt> and +returns a Lua number.<br> + +For <tt>bit.band()</tt>, <tt>bit.bor()</tt> and <tt>bit.bxor()</tt>, the +conversion to <tt>int64_t</tt> or <tt>uint64_t</tt> applies to +<em>all</em> arguments, if <em>any</em> argument is a cdata number.<br> + +For all other operations, only the first argument is used to determine +the output type. This implies that a cdata number as a shift count for +shifts and rotates is accepted, but that alone does <em>not</em> cause +a cdata number output. + +</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><b>64 bit integer comparison</b>: 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.<br> + +If one of the operands is an <tt>enum</tt> and the other operand is a +string, the string is converted to the value of a matching <tt>enum</tt> +constant before the above conversion.<br> + +<li><b>Comparisons for equality/inequality</b> never raise an error. +Even incompatible pointers can be compared for equality by address. Any +other incompatible comparison (also with non-cdata objects) treats the +two sides as unequal.</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 — <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 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 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 types provided by the FFI library, just like you would in +C 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="param">Parameterized Types</h2> +<p> +To facilitate some abstractions, the two functions +<a href="ext_ffi_api.html#ffi_typeof"><tt>ffi.typeof</tt></a> and +<a href="ext_ffi_api.html#ffi_cdef"><tt>ffi.cdef</tt></a> support +parameterized types in C declarations. Note: none of the other API +functions taking a cdecl allow this. +</p> +<p> +Any place you can write a <b><tt>typedef</tt> name</b>, an +<b>identifier</b> or a <b>number</b> in a declaration, you can write +<tt>$</tt> (the dollar sign) instead. These placeholders are replaced in +order of appearance with the arguments following the cdecl string: +</p> +<pre class="code"> +-- Declare a struct with a parameterized field type and name: +ffi.cdef([[ +typedef struct { $ $; } foo_t; +]], type1, name1) + +-- Anonymous struct with dynamic names: +local bar_t = ffi.typeof("struct { int $, $; }", name1, name2) +-- Derived pointer type: +local bar_ptr_t = ffi.typeof("$ *", bar_t) + +-- Parameterized dimensions work even where a VLA won't work: +local matrix_t = ffi.typeof("uint8_t[$][$]", width, height) +</pre> +<p> +Caveat: this is <em>not</em> simple text substitution! A passed ctype or +cdata object is treated like the underlying type, a passed string is +considered an identifier and a number is considered a number. You must +not mix this up: e.g. passing <tt>"int"</tt> as a string doesn't work in +place of a type, you'd need to use <tt>ffi.typeof("int")</tt> instead. +</p> +<p> +The main use for parameterized types are libraries implementing abstract +data types +(<a href="https://www.freelists.org/post/luajit/ffi-type-of-pointer-to,8"><span class="ext">»</span> example</a>), +similar to what can be achieved with C++ template metaprogramming. +Another use case are derived types of anonymous structs, which avoids +pollution of the global struct namespace. +</p> +<p> +Please note that parameterized types are a nice tool and indispensable +for certain use cases. But you'll want to use them sparingly in regular +code, e.g. when all types are actually fixed. +</p> + +<h2 id="gc">Garbage Collection of cdata Objects</h2> +<p> +All explicitly (<tt>ffi.new()</tt>, <tt>ffi.cast()</tt> etc.) or +implicitly (accessors) created cdata objects are garbage collected. +You need to ensure to retain valid references to cdata objects +somewhere on a Lua stack, an upvalue or in a Lua table while they are +still in use. Once the last reference to a cdata object is gone, the +garbage collector will automatically free the memory used by it (at +the end of the next GC cycle). +</p> +<p> +Please note, that pointers themselves are cdata objects, however they +are <b>not</b> followed by the garbage collector. So e.g. if you +assign a cdata array to a pointer, you must keep the cdata object +holding the array alive as long as the pointer is still in use: +</p> +<pre class="code"> +ffi.cdef[[ +typedef struct { int *a; } foo_t; +]] + +local s = ffi.new("foo_t", ffi.new("int[10]")) -- <span style="color:#c00000;">WRONG!</span> + +local a = ffi.new("int[10]") -- <span style="color:#00a000;">OK</span> +local s = ffi.new("foo_t", a) +-- Now do something with 's', but keep 'a' alive until you're done. +</pre> +<p> +Similar rules apply for Lua strings which are implicitly converted to +<tt>"const char *"</tt>: the string object itself must be +referenced somewhere or it'll be garbage collected eventually. The +pointer will then point to stale data, which may have already been +overwritten. Note that <em>string literals</em> are automatically kept +alive as long as the function containing it (actually its prototype) +is not garbage collected. +</p> +<p> +Objects which are passed as an argument to an external C 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 <a href="#convert_vararg">passing specific C types to +vararg functions</a>. +</p> +<p> +Memory areas returned by C functions (e.g. from <tt>malloc()</tt>) +must be manually managed, of course (or use +<a href="ext_ffi_api.html#ffi_gc"><tt>ffi.gc()</tt></a>). Pointers to +cdata objects are indistinguishable from pointers returned by C +functions (which is one of the reasons why the GC cannot follow them). +</p> + +<h2 id="callback">Callbacks</h2> +<p> +The LuaJIT FFI automatically generates special callback functions +whenever a Lua function is converted to a C function pointer. This +associates the generated callback function pointer with the C type +of the function pointer and the Lua function object (closure). +</p> +<p> +This can happen implicitly due to the usual conversions, e.g. when +passing a Lua function to a function pointer argument. Or, you can use +<tt>ffi.cast()</tt> to explicitly cast a Lua function to a +C function pointer. +</p> +<p> +Currently, only certain C function types can be used as callback +functions. Neither C vararg functions nor functions with +pass-by-value aggregate argument or result types are supported. There +are no restrictions on the kind of Lua functions that can be called +from the callback — no checks for the proper number of arguments +are made. The return value of the Lua function will be converted to the +result type, and an error will be thrown for invalid conversions. +</p> +<p> +It's allowed to throw errors across a callback invocation, but it's not +advisable in general. Do this only if you know the C function, that +called the callback, copes with the forced stack unwinding and doesn't +leak resources. +</p> +<p> +One thing that's not allowed, is to let an FFI call into a C function +get JIT-compiled, which in turn calls a callback, calling into Lua again. +Usually this attempt is caught by the interpreter first and the +C function is blacklisted for compilation. +</p> +<p> +However, this heuristic may fail under specific circumstances: e.g. a +message polling function might not run Lua callbacks right away and the call +gets JIT-compiled. If it later happens to call back into Lua (e.g. a rarely +invoked error callback), you'll get a VM PANIC with the message +<tt>"bad callback"</tt>. Then you'll need to manually turn off +JIT-compilation with +<a href="ext_jit.html#jit_onoff_func"><tt>jit.off()</tt></a> for the +surrounding Lua function that invokes such a message polling function (or +similar). +</p> + +<h3 id="callback_resources">Callback resource handling</h3> +<p> +Callbacks take up resources — you can only have a limited number +of them at the same time (500 - 1000, depending on the +architecture). The associated Lua functions are anchored to prevent +garbage collection, too. +</p> +<p> +<b>Callbacks due to implicit conversions are permanent!</b> There is no +way to guess their lifetime, since the C side might store the +function pointer for later use (typical for GUI toolkits). The associated +resources cannot be reclaimed until termination: +</p> +<pre class="code"> +ffi.cdef[[ +typedef int (__stdcall *WNDENUMPROC)(void *hwnd, intptr_t l); +int EnumWindows(WNDENUMPROC func, intptr_t l); +]] + +-- Implicit conversion to a callback via function pointer argument. +local count = 0 +ffi.C.EnumWindows(function(hwnd, l) + count = count + 1 + return true +end, 0) +-- The callback is permanent and its resources cannot be reclaimed! +-- Ok, so this may not be a problem, if you do this only once. +</pre> +<p> +Note: this example shows that you <em>must</em> properly declare +<tt>__stdcall</tt> callbacks on Windows/x86 systems. The calling +convention cannot be automatically detected, unlike for +<tt>__stdcall</tt> calls <em>to</em> Windows functions. +</p> +<p> +For some use cases, it's necessary to free up the resources or to +dynamically redirect callbacks. Use an explicit cast to a +C function pointer and keep the resulting cdata object. Then use +the <a href="ext_ffi_api.html#callback_free"><tt>cb:free()</tt></a> +or <a href="ext_ffi_api.html#callback_set"><tt>cb:set()</tt></a> methods +on the cdata object: +</p> +<pre class="code"> +-- Explicitly convert to a callback via cast. +local count = 0 +local cb = ffi.cast("WNDENUMPROC", function(hwnd, l) + count = count + 1 + return true +end) + +-- Pass it to a C function. +ffi.C.EnumWindows(cb, 0) +-- EnumWindows doesn't need the callback after it returns, so free it. + +cb:free() +-- The callback function pointer is no longer valid and its resources +-- will be reclaimed. The created Lua closure will be garbage collected. +</pre> + +<h3 id="callback_performance">Callback performance</h3> +<p> +<b>Callbacks are slow!</b> First, the C to Lua transition itself +has an unavoidable cost, similar to a <tt>lua_call()</tt> or +<tt>lua_pcall()</tt>. Argument and result marshalling add to that cost. +And finally, neither the C compiler nor LuaJIT can inline or +optimize across the language barrier and hoist repeated computations out +of a callback function. +</p> +<p> +Do not use callbacks for performance-sensitive work: e.g. consider a +numerical integration routine which takes a user-defined function to +integrate over. It's a bad idea to call a user-defined Lua function from +C code millions of times. The callback overhead will be absolutely +detrimental for performance. +</p> +<p> +It's considerably faster to write the numerical integration routine +itself in Lua — the JIT compiler will be able to inline the +user-defined function and optimize it together with its calling context, +with very competitive performance. +</p> +<p> +As a general guideline: <b>use callbacks only when you must</b>, because +of existing C APIs. E.g. callback performance is irrelevant for a +GUI application, which waits for user input most of the time, anyway. +</p> +<p> +For new designs <b>avoid push-style APIs</b>: a C function repeatedly +calling a callback for each result. Instead, <b>use pull-style APIs</b>: +call a C function repeatedly to get a new result. Calls from Lua +to C via the FFI are much faster than the other way round. Most well-designed +libraries already use pull-style APIs (read/write, get/put). +</p> + +<h2 id="clib">C Library Namespaces</h2> +<p> +A C library namespace is a special kind of object which allows +access to the symbols contained in shared libraries or the default +symbol namespace. The default +<a href="ext_ffi_api.html#ffi_C"><tt>ffi.C</tt></a> namespace is +automatically created when the FFI library is loaded. C library +namespaces for specific shared libraries may be created with the +<a href="ext_ffi_api.html#ffi_load"><tt>ffi.load()</tt></a> API +function. +</p> +<p> +Indexing a C library namespace object with a symbol name (a Lua +string) automatically binds it to the library. First, the symbol type +is resolved — it must have been declared with +<a href="ext_ffi_api.html#ffi_cdef"><tt>ffi.cdef</tt></a>. Then the +symbol address is resolved by searching for the symbol name in the +associated shared libraries or the default symbol namespace. Finally, +the resulting binding between the symbol name, the symbol type and its +address is cached. Missing symbol declarations or nonexistent symbol +names cause an error. +</p> +<p> +This is what happens on a <b>read access</b> for the different kinds of +symbols: +</p> +<ul> + +<li>External functions: a cdata object with the type of the function +and its address is returned.</li> + +<li>External variables: the symbol address is dereferenced and the +loaded value is <a href="#convert_tolua">converted to a Lua object</a> +and returned.</li> + +<li>Constant values (<tt>static const</tt> or <tt>enum</tt> +constants): the constant is <a href="#convert_tolua">converted to a +Lua object</a> and returned.</li> + +</ul> +<p> +This is what happens on a <b>write access</b>: +</p> +<ul> + +<li>External variables: the value to be written is +<a href="#convert_fromlua">converted to the C type</a> of the +variable and then stored at the symbol address.</li> + +<li>Writing to constant variables or to any other symbol type causes +an error, like any other attempted write to a constant location.</li> + +</ul> +<p> +C library namespaces themselves are garbage collected objects. If +the last reference to the namespace object is gone, the garbage +collector will eventually release the shared library reference and +remove all memory associated with the namespace. Since this may +trigger the removal of the shared library from the memory of the +running process, it's generally <em>not safe</em> to use function +cdata objects obtained from a library if the namespace object may be +unreferenced. +</p> +<p> +Performance notice: the JIT compiler specializes to the identity of +namespace objects and to the strings used to index it. This +effectively turns function cdata objects into constants. It's not +useful and actually counter-productive to explicitly cache these +function objects, e.g. <tt>local strlen = ffi.C.strlen</tt>. OTOH, it +<em>is</em> useful to cache the namespace itself, e.g. <tt>local C = +ffi.C</tt>. +</p> + +<h2 id="policy">No Hand-holding!</h2> +<p> +The FFI library has been designed as <b>a low-level library</b>. The +goal is to interface with C code and C data types with a +minimum of overhead. This means <b>you can do anything you can do +from C</b>: access all memory, overwrite anything in memory, call +machine code at any memory address and so on. +</p> +<p> +The FFI library provides <b>no memory safety</b>, unlike regular Lua +code. It will happily allow you to dereference a <tt>NULL</tt> +pointer, to access arrays out of bounds or to misdeclare +C functions. If you make a mistake, your application might crash, +just like equivalent C code would. +</p> +<p> +This behavior is inevitable, since the goal is to provide full +interoperability with C code. Adding extra safety measures, like +bounds checks, would be futile. There's no way to detect +misdeclarations of C functions, since shared libraries only +provide symbol names, but no type information. Likewise, there's no way +to infer the valid range of indexes for a returned pointer. +</p> +<p> +Again: the FFI library is a low-level library. This implies it needs +to be used with care, but it's flexibility and performance often +outweigh this concern. If you're a C or C++ developer, it'll be easy +to apply your existing knowledge. OTOH, writing code for the FFI +library is not for the faint of heart and probably shouldn't be the +first exercise for someone with little experience in Lua, C or C++. +</p> +<p> +As a corollary of the above, the FFI library is <b>not safe for use by +untrusted Lua code</b>. If you're sandboxing untrusted Lua code, you +definitely don't want to give this code access to the FFI library or +to <em>any</em> cdata object (except 64 bit integers or complex +numbers). Any properly engineered Lua sandbox needs to provide safety +wrappers for many of the standard Lua library functions — +similar wrappers need to be written for high-level operations on FFI +data types, too. +</p> + +<h2 id="status">Current Status</h2> +<p> +The initial release of the FFI library has some limitations and is +missing some features. Most of these will be fixed in future releases. +</p> +<p> +<a href="#clang">C language support</a> is +currently incomplete: +</p> +<ul> +<li>C declarations are not passed through a C pre-processor, +yet.</li> +<li>The C parser is able to evaluate most constant expressions +commonly found in C header files. However, it doesn't handle the +full range of C expression semantics and may fail for some +obscure constructs.</li> +<li><tt>static const</tt> declarations only work for integer types +up to 32 bits. Neither declaring string constants nor +floating-point constants is supported.</li> +<li>Packed <tt>struct</tt> bitfields that cross container boundaries +are not implemented.</li> +<li>Native vector types may be defined with the GCC <tt>mode</tt> or +<tt>vector_size</tt> attribute. But no operations other than loading, +storing and initializing them are supported, yet.</li> +<li>The <tt>volatile</tt> type qualifier is currently ignored by +compiled code.</li> +<li><a href="ext_ffi_api.html#ffi_cdef"><tt>ffi.cdef</tt></a> silently +ignores most re-declarations. Note: avoid re-declarations which do not +conform to C99. The implementation will eventually be changed to +perform strict checks.</li> +</ul> +<p> +The JIT compiler already handles a large subset of all FFI operations. +It automatically falls back to the interpreter for unimplemented +operations (you can check for this with the +<a href="running.html#opt_j"><tt>-jv</tt></a> command line option). +The following operations are currently not compiled and may exhibit +suboptimal performance, especially when used in inner loops: +</p> +<ul> +<li>Vector operations.</li> +<li>Table initializers.</li> +<li>Initialization of nested <tt>struct</tt>/<tt>union</tt> types.</li> +<li>Non-default initialization of VLA/VLS or large C types +(> 128 bytes or > 16 array elements).</li> +<li>Bitfield initializations.</li> +<li>Pointer differences for element sizes that are not a power of +two.</li> +<li>Calls to C functions with aggregates passed or returned by +value.</li> +<li>Calls to ctype metamethods which are not plain functions.</li> +<li>ctype <tt>__newindex</tt> tables and non-string lookups in ctype +<tt>__index</tt> tables.</li> +<li><tt>tostring()</tt> for cdata types.</li> +<li>Calls to <tt>ffi.cdef()</tt>, <tt>ffi.load()</tt> and +<tt>ffi.metatype()</tt>.</li> +</ul> +<p> +Other missing features: +</p> +<ul> +<li>Arithmetic for <tt>complex</tt> numbers.</li> +<li>Passing structs by value to vararg C functions.</li> +<li><a href="extensions.html#exceptions">C++ exception interoperability</a> +does not extend to C functions called via the FFI, if the call is +compiled.</li> +</ul> +<br class="flush"> +</div> +<div id="foot"> +<hr class="hide"> +Copyright © 2005-2022 +<span class="noprint"> +· +<a href="contact.html">Contact</a> +</span> +</div> +</body> +</html> |