This documentation is for LuaJIT 2.1. Please check the doc directory in each git branch for the version-specific documentation.

LuaJIT is fully upwards-compatible with Lua 5.1. It supports all standard Lua library functions and the full set of Lua/C API functions.

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.

LuaJIT extends the standard Lua VM with new functionality and adds several extension modules. Please note, this page is only about functional enhancements and not about performance enhancements, such as the optimized VM, the faster interpreter or the JIT compiler.

Extensions Modules

LuaJIT comes with several built-in extension modules:

bit.* — Bitwise operations

LuaJIT supports all bitwise operations as defined by Lua BitOp:

bit.tobit  bit.tohex  bit.bnot bit.bor  bit.bxor
bit.lshift bit.rshift bit.arshift bit.rol  bit.ror  bit.bswap

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 Lua BitOp API functions. The FFI adds support for 64 bit bitwise operations, using the same API functions.

Please make sure to require the module before using any of its functions:

local bit = require("bit")

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.

ffi.* — FFI library

The FFI library allows calling external C functions and the use of C data structures from pure Lua code.

jit.* — JIT compiler control

The functions in this module control the behavior of the JIT compiler engine.

C API extensions

LuaJIT adds some extra functions to the Lua/C API.


LuaJIT has an integrated profiler.

Enhanced Standard Library Functions

xpcall(f, err [,args...]) passes arguments

Unlike the standard implementation in Lua 5.1, xpcall() passes any arguments after the error function to the function which is called in a protected context.

load*() handle UTF-8 source code

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.

load*() add a mode parameter

As an extension from Lua 5.2, the functions loadstring(), loadfile() and (new) load() add an optional mode parameter.

The default mode string is "bt", which allows loading of both source code and bytecode. Use "t" to allow only source code or "b" to allow only bytecode to be loaded.

By default, the load* functions generate the native bytecode format. For cross-compilation purposes, add W to the mode string to force the 32 bit format and X to force the 64 bit format. Add both to force the opposite format. Note that non-native bytecode generated by load* cannot be run, but can still be passed to string.dump.

tostring() etc. canonicalize NaN and ±Inf

All number-to-string conversions consistently convert non-finite numbers to the same strings on all platforms. NaN results in "nan", positive infinity results in "inf" and negative infinity results in "-inf".

tonumber() etc. use builtin string to number conversion

All string-to-number conversions consistently convert integer and floating-point inputs in decimal, hexadecimal and binary on all platforms. strtod() is not 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. 0x1.5p-3).

string.dump(f [,mode]) generates portable bytecode

An extra argument has been added to string.dump(). If set to true or to a string which contains the character s, 'stripped' bytecode without debug information is generated. This speeds up later bytecode loading and reduces memory usage. See also the -b command line option.

The generated bytecode is portable and can be loaded on any architecture that LuaJIT supports. However, the bytecode compatibility versions must match. Bytecode only stays compatible within a major+minor version ( → x.y.bbb), except for development branches. Foreign bytecode (e.g. from Lua 5.1) is incompatible and cannot be loaded.

Note: LJ_GC64 mode requires a different frame layout, which implies a different, incompatible bytecode format between 32 bit and 64 bit ports. This may be rectified in the future. In the meantime, use the W and X modes of the load* functions for cross-compilation purposes.

Due to VM hardening, bytecode is not deterministic. Add d to the mode string to dump it in a deterministic manner: identical source code always gives a byte-for-byte identical bytecode dump. This feature is mainly useful for reproducible builds., nhash) allocates a pre-sized table

An extra library function can be made available via require(""). This creates a pre-sized table, just like the C API equivalent lua_createtable(). This is useful for big tables if the final table size is known and automatic table resizing is too expensive.

table.clear(tab) clears a table

An extra library function table.clear() can be made available via require("table.clear"). This clears all keys and values from a table, but preserves the allocated array/hash sizes. This is useful when a table, which is linked from multiple places, needs to be cleared and/or when recycling a table for use by the same context. This avoids managing backlinks, saves an allocation and the overhead of incremental array/hash part growth.

Please note, this function is meant for very specific situations. In most cases it's better to replace the (usually single) link with a new table and let the GC do its work.

Enhanced PRNG for math.random()

LuaJIT uses a Tausworthe PRNG with period 2^223 to implement math.random() and math.randomseed(). The quality of the PRNG results is much superior compared to the standard Lua implementation, which uses the platform-specific ANSI rand().

The PRNG generates the same sequences from the same seeds on all platforms and makes use of all bits in the seed argument. math.random() 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 math.random(n [,m]) to preserve uniformity.

Call math.randomseed() without any arguments to seed it from system entropy.

Important: Neither this nor any other PRNG based on the simplistic math.random() API is suitable for cryptographic use.

io.* functions handle 64 bit file offsets

The file I/O functions in the standard io.* 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 (fp:seek() method).

debug.* functions identify metamethods

debug.getinfo() and lua_getinfo() also return information about invoked metamethods. The namewhat field is set to "metamethod" and the name field has the name of the corresponding metamethod (e.g. "__index").

Fully Resumable VM

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 pcall() and xpcall(), across iterators and across metamethods.

Extensions from Lua 5.2

LuaJIT supports some language and library extensions from Lua 5.2. Features that are unlikely to break existing code are unconditionally enabled:

Other features are only enabled, if LuaJIT is built with -DLUAJIT_ENABLE_LUA52COMPAT:

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. _ENV).

Extensions from Lua 5.3

LuaJIT supports some extensions from Lua 5.3:

C++ Exception Interoperability

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:

Platform Compiler Interoperability
External frame unwinding GCC, Clang, MSVC Full
Internal frame unwinding + DWARF2 GCC, Clang Limited
Windows 64 bit non-MSVC Limited
Other platforms Other compilers No

Full interoperability means:

Limited interoperability means:

No interoperability means: