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FFI package for Lua 5.1 and Lua 5.2
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soumith Merge pull request #35 from afonso360/master
Support newer versions of lua
Latest commit 532c757 Jul 21, 2017
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PATENTS Fix LICENSE and PATENTS file Sep 16, 2015
call.c Changed cdata to native Lua 5.3 numbers Jun 17, 2016
call_arm.dasc Add ARM header files Jan 19, 2016
call_arm.h Add ARM header files Jan 19, 2016
call_x86.h Fix invalid >2GB relative jumps to shared global functions. Oct 27, 2016
ctype.c Fix LICENSE and PATENTS file Sep 16, 2015
generate_call_h.bat Add ARM CE support. Jul 25, 2011
luaffi-scm-1.rockspec Add Travis CI build for Lua 5.1 and 5.2 Nov 15, 2015
pretty.lua Add table pretty-printer to help with debugging Apr 25, 2012
test.lua Make default target when running Apr 14, 2016

Build Status


This is a library for calling C function and manipulating C types from lua. It is designed to be interface compatible with the FFI library in LuaJIT (see It can parse C function declarations and struct definitions that have been directly copied out of C header files and into lua source as a string.

This is a fork of



Currently supported:

  • Linux x86/x64
  • OS X x86/x64

Runs with Lua 5.1, 5.2, and 5.3


In a terminal:

git clone
cd luaffifb
luarocks make


This library is designed to be source compatible with LuaJIT's FFI extension. The documentation at describes the API and semantics.

Pointer Comparison

Use ffi.NULL instead of nil when checking for NULL pointers.'void *', 0) == ffi.NULL -- true

Known Issues

  • Comparing a ctype pointer to nil doesn't work the same as in LuaJIT (see above). This is unfixable with the current metamethod semantics.
  • Constant expressions can't handle non integer intermediate values (eg offsetof won't work because it manipulates pointers)
  • Not all metamethods work with Lua 5.1 (eg char* + number). This is due to the way metamethods are looked up with mixed types in Lua 5.1. If you need this upgrade to Lua 5.2 or use boxed numbers (uint64_t and uintptr_t).
  • All bitfields are treated as unsigned (does anyone even use signed bitfields?). Note that "int s:8" is unsigned on unix x86/x64, but signed on windows.

How it works

Types are represented by a struct ctype structure and an associated user value table. The table is shared between all related types for structs, unions, and functions. It's members have the types of struct members, function argument types, etc. The struct ctype structure then contains the modifications from the base type (eg number of pointers, array size, etc).

Types are pushed into lua as a userdata containing the struct ctype with a user value (or fenv in 5.1) set to the shared type table.

Boxed cdata types are pushed into lua as a userdata containing the struct cdata structure (which contains the struct ctype of the data as its header) followed by the boxed data.

The functions in ffi.C provide the cdata and ctype metatables and ffi.* functions which manipulate these two types.

C functions (and function pointers) are pushed into lua as a lua c function with the function pointer cdata as the first upvalue. The actual code is JITed using dynasm (see call_x86.dasc). The JITed code does the following in order:

  1. Calls the needed unpack functions in ffi.C placing each argument on the HW stack
  2. Updates errno
  3. Performs the C call
  4. Retrieves errno
  5. Pushes the result back into lua from the HW register or stack
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