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<a href="http://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
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<h1>Extensions</h1>
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<p>
LuaJIT is fully upwards-compatible with Lua 5.1. It supports all
<a href="http://www.lua.org/manual/5.1/manual.html#5"><span class="ext">»</span> standard Lua
library functions</a> and the full set of
<a href="http://www.lua.org/manual/5.1/manual.html#3"><span class="ext">»</span> Lua/C API
functions</a>.
</p>
<p>
LuaJIT is also fully ABI-compatible to Lua 5.1 at the linker/dynamic
loader level. This means you can compile a C module against the
standard Lua headers and load the same shared library from either Lua
or LuaJIT.
</p>
<p>
LuaJIT extends the standard Lua VM with new functionality and adds
several extension modules. Please note this page is only about
<em>functional</em> enhancements and not about performance enhancements,
such as the optimized VM, the faster interpreter or the JIT compiler.
</p>
<h2 id="modules">Extensions Modules</h2>
<p>
LuaJIT comes with several built-in extension modules:
</p>
<h3 id="bit"><tt>bit.*</tt> — Bitwise operations</h3>
<p>
LuaJIT supports all bitwise operations as defined by
<a href="http://bitop.luajit.org"><span class="ext">»</span> Lua BitOp</a>:
</p>
<pre class="code">
bit.tobit bit.tohex bit.bnot bit.band bit.bor bit.bxor
bit.lshift bit.rshift bit.arshift bit.rol bit.ror bit.bswap
</pre>
<p>
This module is a LuaJIT built-in — you don't need to download or
install Lua BitOp. The Lua BitOp site has full documentation for all
<a href="http://bitop.luajit.org/api.html"><span class="ext">»</span> Lua BitOp API functions</a>.
</p>
<p>
Please make sure to <tt>require</tt> the module before using any of
its functions:
</p>
<pre class="code">
local bit = require("bit")
</pre>
<p>
An already installed Lua BitOp module is ignored by LuaJIT.
This way you can use bit operations from both Lua and LuaJIT on a
shared installation.
</p>
<h3 id="ffi"><tt>ffi.*</tt> — FFI library</h3>
<p>
The <a href="ext_ffi.html">FFI library</a> allows calling external
C functions and the use of C data structures from pure Lua
code.
</p>
<h3 id="jit"><tt>jit.*</tt> — JIT compiler control</h3>
<p>
The functions in this module
<a href="ext_jit.html">control the behavior of the JIT compiler engine</a>.
</p>
<h3 id="c_api">C API extensions</h3>
<p>
LuaJIT adds some
<a href="ext_c_api.html">extra functions to the Lua/C API</a>.
</p>
<h2 id="library">Enhanced Standard Library Functions</h2>
<h3 id="xpcall"><tt>xpcall(f, err [,args...])</tt> passes arguments</h3>
<p>
Unlike the standard implementation in Lua 5.1, <tt>xpcall()</tt>
passes any arguments after the error function to the function
which is called in a protected context.
</p>
<h3 id="load"><tt>loadfile()</tt> etc. handle UTF-8 source code</h3>
<p>
Non-ASCII characters are handled transparently by the Lua source code parser.
This allows the use of UTF-8 characters in identifiers and strings.
A UTF-8 BOM is skipped at the start of the source code.
</p>
<h3 id="tostring"><tt>tostring()</tt> etc. canonicalize NaN and ±Inf</h3>
<p>
All number-to-string conversions consistently convert non-finite numbers
to the same strings on all platforms. NaN results in <tt>"nan"</tt>,
positive infinity results in <tt>"inf"</tt> and negative infinity results
in <tt>"-inf"</tt>.
</p>
<h3 id="tonumber"><tt>tonumber()</tt> etc. use builtin string to number conversion</h3>
<p>
All string-to-number conversions consistently convert integer and
floating-point inputs in decimal and hexadecimal on all platforms.
<tt>strtod()</tt> is <em>not</em> used anymore, which avoids numerous
problems with poor C library implementations. The builtin conversion
function provides full precision according to the IEEE-754 standard, it
works independently of the current locale and it supports hex floating-point
numbers (e.g. <tt>0x1.5p-3</tt>).
</p>
<h3 id="string_dump"><tt>string.dump(f [,strip])</tt> generates portable bytecode</h3>
<p>
An extra argument has been added to <tt>string.dump()</tt>. If set to
<tt>true</tt>, 'stripped' bytecode without debug information is
generated. This speeds up later bytecode loading and reduces memory
usage. See also the
<a href="running.html#opt_b"><tt>-b</tt> command line option</a>.
</p>
<p>
The generated bytecode is portable and can be loaded on any architecture
that LuaJIT supports, independent of word size or endianess. However the
bytecode compatibility versions must match. Bytecode stays compatible
for dot releases (x.y.0 → x.y.1), but may change with major or
minor releases (2.0 → 2.1) or between any beta release. Foreign
bytecode (e.g. from Lua 5.1) is incompatible and cannot be loaded.
</p>
<h3 id="math_random">Enhanced PRNG for <tt>math.random()</tt></h3>
<p>
LuaJIT uses a Tausworthe PRNG with period 2^223 to implement
<tt>math.random()</tt> and <tt>math.randomseed()</tt>. The quality of
the PRNG results is much superior compared to the standard Lua
implementation which uses the platform-specific ANSI rand().
</p>
<p>
The PRNG generates the same sequences from the same seeds on all
platforms and makes use of all bits in the seed argument.
<tt>math.random()</tt> without arguments generates 52 pseudo-random bits
for every call. The result is uniformly distributed between 0.0 and 1.0.
It's correctly scaled up and rounded for <tt>math.random(n [,m])</tt> to
preserve uniformity.
</p>
<h3 id="io"><tt>io.*</tt> functions handle 64 bit file offsets</h3>
<p>
The file I/O functions in the standard <tt>io.*</tt> library handle
64 bit file offsets. In particular this means it's possible
to open files larger than 2 Gigabytes and to reposition or obtain
the current file position for offsets beyond 2 GB
(<tt>fp:seek()</tt> method).
</p>
<h3 id="debug_meta"><tt>debug.*</tt> functions identify metamethods</h3>
<p>
<tt>debug.getinfo()</tt> and <tt>lua_getinfo()</tt> also return information
about invoked metamethods. The <tt>namewhat</tt> field is set to
<tt>"metamethod"</tt> and the <tt>name</tt> field has the name of
the corresponding metamethod (e.g. <tt>"__index"</tt>).
</p>
<h2 id="resumable">Fully Resumable VM</h2>
<p>
The LuaJIT VM is fully resumable. This means you can yield from a
coroutine even across contexts, where this would not possible with
the standard Lua 5.1 VM: e.g. you can yield across <tt>pcall()</tt>
and <tt>xpcall()</tt>, across iterators and across metamethods.
</p>
<h2 id="lua52">Extensions from Lua 5.2</h2>
<p>
LuaJIT supports some language and library extensions from Lua 5.2.
Features that are unlikely to break existing code are unconditionally
enabled:
</p>
<ul>
<li><tt>goto</tt> and <tt>::labels::</tt>.</li>
<li>Hex escapes <tt>'\x3F'</tt> and <tt>'\*'</tt> escape in strings.</li>
<li><tt>load(string|reader [, chunkname [,mode [,env]]])</tt>.</li>
<li><tt>loadstring()</tt> is an alias for <tt>load()</tt>.</li>
<li><tt>loadfile(filename [,mode [,env]])</tt>.</li>
<li><tt>math.log(x [,base])</tt>.
<li><tt>string.rep(s, n [,sep])</tt>.
<li><tt>string.format()</tt>: <tt>%q</tt> reversible.
<tt>%s</tt> checks <tt>__tostring</tt>.
<tt>%a</tt> and <tt>"%A</tt> added.</li>
<li>String matching pattern <tt>%g</tt> added.</li>
<li><tt>io.read("*L")</tt>.</li>
<li><tt>io.lines()</tt> and <tt>file:lines()</tt> process
<tt>io.read()</tt> options.</li>
<li><tt>os.exit(status|true|false [,close])</tt>.</li>
<li><tt>package.searchpath(name, path [, sep [, rep]])</tt>.</li>
<li><tt>package.loadlib(name, "*")</tt>.</li>
<li><tt>debug.getinfo()</tt> returns <tt>nparams</tt> and <tt>isvararg</tt>
for option <tt>"u"</tt>.</li>
<li><tt>debug.getlocal()</tt> accepts function instead of level.</li>
<li><tt>debug.getlocal()</tt> and <tt>debug.setlocal()</tt> accept negative
indexes for varargs.</li>
<li><tt>debug.getupvalue()</tt> and <tt>debug.setupvalue()</tt> handle
C functions.</li>
<li><tt>debug.upvalueid()</tt> and <tt>debug.upvaluejoin()</tt>.</li>
<li>Command line option <tt>-E</tt>.</li>
<li>Command line checks <tt>__tostring</tt> for errors.</li>
</ul>
<p>
Other features are only enabled, if LuaJIT is built with
<tt>-DLUAJIT_ENABLE_LUA52COMPAT</tt>:
</p>
<ul>
<li><tt>goto</tt> is a keyword and not a valid variable name anymore.</li>
<li><tt>break</tt> can be placed anywhere. Empty statements (<tt>;;</tt>)
are allowed.</li>
<li><tt>__lt</tt>, <tt>__le</tt> are invoked for mixed types.</li>
<li><tt>__len</tt> for tables. <tt>rawlen()</tt> library function.</li>
<li><tt>pairs()</tt> and <tt>ipairs()</tt> check for <tt>__pairs</tt> and
<tt>__ipairs</tt>.</li>
<li><tt>coroutine.running()</tt> returns two results.</li>
<li><tt>table.pack()</tt> and <tt>table.unpack()</tt>
(same as <tt>unpack()</tt>).</li>
<li><tt>io.write()</tt> and <tt>file:write()</tt> return file handle
instead of <tt>true</tt>.</li>
<li><tt>os.execute()</tt> and <tt>pipe:close()</tt> return detailed
exit status.</li>
<li><tt>debug.setmetatable()</tt> returns object.</li>
<li><tt>debug.getuservalue()</tt> and <tt>debug.setuservalue()</tt>.</li>
<li>Remove <tt>math.mod()</tt>, <tt>string.gfind()</tt>.
</ul>
<p>
Note: this provides only partial compatibility with Lua 5.2 at the
language and Lua library level. LuaJIT is API+ABI-compatible with
Lua 5.1, which prevents implementing features that would otherwise
break the Lua/C API and ABI (e.g. <tt>_ENV</tt>).
</p>
<h2 id="exceptions">C++ Exception Interoperability</h2>
<p>
LuaJIT has built-in support for interoperating with C++ exceptions.
The available range of features depends on the target platform and
the toolchain used to compile LuaJIT:
</p>
<table class="exc">
<tr class="exchead">
<td class="excplatform">Platform</td>
<td class="exccompiler">Compiler</td>
<td class="excinterop">Interoperability</td>
</tr>
<tr class="odd separate">
<td class="excplatform">POSIX/x64, DWARF2 unwinding</td>
<td class="exccompiler">GCC 4.3+</td>
<td class="excinterop"><b style="color: #00a000;">Full</b></td>
</tr>
<tr class="even">
<td class="excplatform">Other platforms, DWARF2 unwinding</td>
<td class="exccompiler">GCC</td>
<td class="excinterop"><b style="color: #c06000;">Limited</b></td>
</tr>
<tr class="odd">
<td class="excplatform">Windows/x64</td>
<td class="exccompiler">MSVC or WinSDK</td>
<td class="excinterop"><b style="color: #00a000;">Full</b></td>
</tr>
<tr class="even">
<td class="excplatform">Windows/x86</td>
<td class="exccompiler">Any</td>
<td class="excinterop"><b style="color: #a00000;">No</b></td>
</tr>
<tr class="odd">
<td class="excplatform">Other platforms</td>
<td class="exccompiler">Other compilers</td>
<td class="excinterop"><b style="color: #a00000;">No</b></td>
</tr>
</table>
<p>
<b style="color: #00a000;">Full interoperability</b> means:
</p>
<ul>
<li>C++ exceptions can be caught on the Lua side with <tt>pcall()</tt>,
<tt>lua_pcall()</tt> etc.</li>
<li>C++ exceptions will be converted to the generic Lua error
<tt>"C++ exception"</tt>, unless you use the
<a href="ext_c_api.html#mode_wrapcfunc">C call wrapper</a> feature.</li>
<li>It's safe to throw C++ exceptions across non-protected Lua frames
on the C stack. The contents of the C++ exception object
pass through unmodified.</li>
<li>Lua errors can be caught on the C++ side with <tt>catch(...)</tt>.
The corresponding Lua error message can be retrieved from the Lua stack.</li>
<li>Throwing Lua errors across C++ frames is safe. C++ destructors
will be called.</li>
</ul>
<p>
<b style="color: #c06000;">Limited interoperability</b> means:
</p>
<ul>
<li>C++ exceptions can be caught on the Lua side with <tt>pcall()</tt>,
<tt>lua_pcall()</tt> etc.</li>
<li>C++ exceptions will be converted to the generic Lua error
<tt>"C++ exception"</tt>, unless you use the
<a href="ext_c_api.html#mode_wrapcfunc">C call wrapper</a> feature.</li>
<li>C++ exceptions will be caught by non-protected Lua frames and
are rethrown as a generic Lua error. The C++ exception object will
be destroyed.</li>
<li>Lua errors <b>cannot</b> be caught on the C++ side.</li>
<li>Throwing Lua errors across C++ frames will <b>not</b> call
C++ destructors.</li>
</ul>
<p>
<b style="color: #a00000;">No interoperability</b> means:
</p>
<ul>
<li>It's <b>not</b> safe to throw C++ exceptions across Lua frames.</li>
<li>C++ exceptions <b>cannot</b> be caught on the Lua side.</li>
<li>Lua errors <b>cannot</b> be caught on the C++ side.</li>
<li>Throwing Lua errors across C++ frames will <b>not</b> call
C++ destructors.</li>
<li>Additionally, on Windows/x86 with SEH-based C++ exceptions:
it's <b>not</b> safe to throw a Lua error across any frames containing
a C++ function with any try/catch construct or using variables with
(implicit) destructors. This also applies to any functions which may be
inlined in such a function. It doesn't matter whether <tt>lua_error()</tt>
is called inside or outside of a try/catch or whether any object actually
needs to be destroyed: the SEH chain is corrupted and this will eventually
lead to the termination of the process.</li>
</ul>
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