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<a href="https://luajit.org"><span>Lua<span id="logo">JIT</span></span></a>
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<h1>Frequently Asked Questions (FAQ)</h1>
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<dl id="info">
<dt>Q: Where can I learn more about LuaJIT and Lua?</dt>
<dd>
<ul style="padding: 0;">
<li>The <a href="https://luajit.org/list.html"><span class="ext">»</span> LuaJIT mailing list</a> focuses on topics
related to LuaJIT.</li>
<li>The <a href="http://wiki.luajit.org/"><span class="ext">»</span> LuaJIT wiki</a> gathers community
resources about LuaJIT.</li>
<li>News about Lua itself can be found at the
<a href="https://www.lua.org/lua-l.html"><span class="ext">»</span> Lua mailing list</a>.
The mailing list archives are worth checking out for older postings
about LuaJIT.</li>
<li>The <a href="https://lua.org"><span class="ext">»</span> main Lua.org site</a> has complete
<a href="https://www.lua.org/docs.html"><span class="ext">»</span> documentation</a> of the language
and links to books and papers about Lua.</li>
<li>The community-managed <a href="http://lua-users.org/wiki/"><span class="ext">»</span> Lua Wiki</a>
has information about diverse topics.</li>
</ul></dd>
</dl>
<dl id="tech">
<dt>Q: Where can I learn more about the compiler technology used by LuaJIT?</dt>
<dd>
Please use the following Google Scholar searches to find relevant papers:<br>
Search for: <a href="https://scholar.google.com/scholar?q=Trace+Compiler"><span class="ext">»</span> Trace Compiler</a><br>
Search for: <a href="https://scholar.google.com/scholar?q=JIT+Compiler"><span class="ext">»</span> JIT Compiler</a><br>
Search for: <a href="https://scholar.google.com/scholar?q=Dynamic+Language+Optimizations"><span class="ext">»</span> Dynamic Language Optimizations</a><br>
Search for: <a href="https://scholar.google.com/scholar?q=SSA+Form"><span class="ext">»</span> SSA Form</a><br>
Search for: <a href="https://scholar.google.com/scholar?q=Linear+Scan+Register+Allocation"><span class="ext">»</span> Linear Scan Register Allocation</a><br>
Here is a list of the <a href="http://lua-users.org/lists/lua-l/2009-11/msg00089.html"><span class="ext">»</span> innovative features in LuaJIT</a>.<br>
And, you know, reading the source is of course the only way to enlightenment.
</dd>
</dl>
<dl id="arg">
<dt>Q: Why do I get this error: "attempt to index global 'arg' (a nil value)"?<br>
Q: My vararg functions fail after switching to LuaJIT!</dt>
<dd>LuaJIT is compatible to the Lua 5.1 language standard. It doesn't
support the implicit <tt>arg</tt> parameter for old-style vararg
functions from Lua 5.0.<br>Please convert your code to the
<a href="https://www.lua.org/manual/5.1/manual.html#2.5.9"><span class="ext">»</span> Lua 5.1
vararg syntax</a>.</dd>
</dl>
<dl id="x87">
<dt>Q: Why do I get this error: "bad FPU precision"?<br>
<dt>Q: I get weird behavior after initializing Direct3D.<br>
<dt>Q: Some FPU operations crash after I load a Delphi DLL.<br>
</dt>
<dd>
DirectX/Direct3D (up to version 9) sets the x87 FPU to single-precision
mode by default. This violates the Windows ABI and interferes with the
operation of many programs — LuaJIT is affected, too. Please make
sure you always use the <tt>D3DCREATE_FPU_PRESERVE</tt> flag when
initializing Direct3D.<br>
Direct3D version 10 or higher do not show this behavior anymore.
Consider testing your application with older versions, too.<br>
Similarly, the Borland/Delphi runtime modifies the FPU control word and
enables FP exceptions. Of course this violates the Windows ABI, too.
Please check the Delphi docs for the Set8087CW method.</dd>
</dl>
<dl id="ctrlc">
<dt>Q: Sometimes Ctrl-C fails to stop my Lua program. Why?</dt>
<dd>The interrupt signal handler sets a Lua debug hook. But this is
ignored by compiled code. If your program is running in a tight loop
and never falls back to the interpreter, the debug hook never runs and
can't throw the "interrupted!" error.<br>
You have to press Ctrl-C twice to get stop your program. That's similar
to when it's stuck running inside a C function under the Lua interpreter.</dd>
</dl>
<dl id="order">
<dt>Q: Table iteration with <tt>pairs()</tt> does not result in the same order?</dt>
<dd>The order of table iteration is explicitly <b>undefined</b> by
the Lua language standard.<br>
Different Lua implementations or versions may use different orders for
otherwise identical tables. Different ways of constructing a table may
result in different orders, too.<br>
Due to improved VM security, LuaJIT 2.1 may even use a different order
on separate VM invocations or when string keys are newly interned.<br><br>
If your program relies on a deterministic order, it has a bug. Rewrite it,
so it doesn't rely on the key order. Or sort the table keys, if you must.</dd>
</dl>
<dl id="sandbox">
<dt>Q: Can Lua code be safely sandboxed?</dt>
<dd>
Maybe for an extremly restricted subset of Lua and if you relentlessly
scrutinize every single interface function you offer to the untrusted code.<br>
Although Lua provides some sandboxing functionality (<tt>setfenv()</tt>, hooks),
it's very hard to get this right even for the Lua core libraries. Of course,
you'll need to inspect any extension library, too. And there are libraries
that are inherently unsafe, e.g. the <a href="ext_ffi.html">FFI library</a>.<br>
More reading material at the <a href="http://lua-users.org/wiki/SandBoxes"><span class="ext">»</span> Lua Wiki</a> and <a href="https://en.wikipedia.org/wiki/Sandbox_(computer_security)"><span class="ext">»</span> Wikipedia</a>.<br><br>
Relatedly, <b>loading untrusted bytecode is not safe!</b><br>
It's trivial to crash the Lua or LuaJIT VM with maliciously crafted bytecode.
This is well known and there's no bytecode verification on purpose, so please
don't report a bug about it. Check the <tt>mode</tt> parameter for the
<tt>load*()</tt> functions to disable loading of bytecode.<br><br>
<b>In general, the only promising approach is to sandbox Lua code at the
process level and not the VM level.</b>
</dd>
</dl>
<dl id="arch">
<dt>Q: Lua runs everywhere. Why doesn't LuaJIT support my CPU?</dt>
<dd>Because it's a compiler — it needs to generate native
machine code. This means the code generator must be ported to each
architecture. And the fast interpreter is written in assembler and
must be ported, too. This is quite an undertaking.<br>
The <a href="install.html">install documentation</a> shows the supported
architectures.<br>
Other architectures may follow based on sufficient user demand and
market-relevance of the architecture. Sponsoring is required to develop
the port itself, to integrate it and to continuously maintain it in the
actively developed branches.</dd>
</dl>
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