Frequently Asked Questions (FAQ)
- Q: Where can I learn more about LuaJIT and Lua?
- The LuaJIT mailing list focuses on topics
related to LuaJIT.
- The LuaJIT wiki gathers community
resources about LuaJIT.
- News about Lua itself can be found at the
Lua mailing list.
The mailing list archives are worth checking out for older postings
- The main Lua.org site has complete
documentation of the language
and links to books and papers about Lua.
- The community-managed Lua Wiki
has information about diverse topics.
- Q: Where can I learn more about the compiler technology used by LuaJIT?
I'm planning to write more documentation about the internals of LuaJIT.
In the meantime, please use the following Google Scholar searches
to find relevant papers:
Search for: Trace Compiler
Search for: JIT Compiler
Search for: Dynamic Language Optimizations
Search for: SSA Form
Search for: Linear Scan Register Allocation
Here is a list of the innovative features in LuaJIT.
And, you know, reading the source is of course the only way to enlightenment. :-)
- Q: Why do I get this error: "attempt to index global 'arg' (a nil value)"?
Q: My vararg functions fail after switching to LuaJIT!
- LuaJIT is compatible to the Lua 5.1 language standard. It doesn't
support the implicit arg parameter for old-style vararg
functions from Lua 5.0.
Please convert your code to the
- Q: Why do I get this error: "bad FPU precision"?
- Q: I get weird behavior after initializing Direct3D.
- Q: Some FPU operations crash after I load a Delphi DLL.
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 D3DCREATE_FPU_PRESERVE flag when
Direct3D version 10 or higher do not show this behavior anymore.
Consider testing your application with older versions, too.
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.
- Q: Sometimes Ctrl-C fails to stop my Lua program. Why?
- The interrupt signal handler sets a Lua debug hook. But this is
currently ignored by compiled code (this will eventually be fixed). 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
In the meantime 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.
- Q: Why doesn't my favorite power-patch for Lua apply against LuaJIT?
- Because it's a completely redesigned VM and has very little code
in common with Lua anymore. Also, if the patch introduces changes to
the Lua semantics, these would need to be reflected everywhere in the
VM, from the interpreter up to all stages of the compiler.
use only standard Lua language constructs. For many common needs you
can use source transformations or use wrapper or proxy functions.
The compiler will happily optimize away such indirections.
- Q: Lua runs everywhere. Why doesn't LuaJIT support my CPU?
- 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.
The install documentation shows the supported
architectures. Other architectures will follow based on sufficient user
demand and/or sponsoring.
- Q: When will feature X be added? When will the next version be released?
- When it's ready.
C'mon, it's open source — I'm doing it on my own time and you're
getting it for free. You can either contribute a patch or sponsor
the development of certain features, if they are important to you.