Alien - Pure Lua extensions

What is Alien?

Alien is a Foreign Function Interface (FFI) for Lua. An FFI lets you call functions in dynamic libraries (.so, .dylib, .dll, etc.) from Lua code without having to write, compile and link a C binding from the library to Lua. In other words, it lets you write extensions that call out to native code using just Lua.

Be careful when you use Alien: care has been taken to make it safe, but it is still very easy to crash Lua if you make a mistake. Alien itself is not as robust as a standard Lua extension, but you can use it to write extensions that won’t crash if you code them well.

Alien works on Unix-based systems and Windows. It has been tested on Linux x86, Linux x86_64, Linux ARM, FreeBSD x86, Windows x86, OS X x86, and OS X PPC. The Windows binary uses MSVCR80.DLL for compatibility with LuaBinaries.

Installing Alien

The easiest way to install Alien is through LuaRocks. Just do luarocks install alien.

Alien is based on libffi. On a GNU/Linux system you should be able to install it with your package manager; it is probably called something like libffi-dev (Debian, Ubuntu etc.) or libffi-devel (Fedora, CentOS etc.). If your system's package manager does not have libffi, or you don't have a package manager, you can get the source code from the libffi project.

Go to the Alien rock directory to see local copies of this documentation, as well as the test suite. If you are in the path of the test suite (tests) you can run the suite with:

lua -l luarocks.require test_alien.lua

Alien uses the GNU build system. For detailed instructions on installations that do not use LuaRocks, see INSTALL in the distribution.

For a quick start, if you checked out sources from Github (skip this step if you downloaded the .zip in the Downloads section):


Then run:

./configure && make [&& make install]

You may need to supply non-default paths (e.g. if you are using a system that supports more than one version of Lua). For example, on Debian or Ubuntu:

CPPFLAGS=‘-I/usr/include/lua5.1’ ./configure –libdir=/usr/local/lib/lua/5.1 –datadir=/usr/local/share/lua$

To run some tests:

make check

Alien installs one module, alien.

Basic Usage

The version of the alien library is available in alien.version.

Load a library with alien.load("foo"). By default, Alien assumes a naming scheme of libname.dylib for OSX and for other Unix systems. You can also specify the full name of the library by calling alien.load with a path or with the appropriate extension, such as alien.load("mylibs/") or alien.load(""). Either way you get back a reference to the library that you will use to access its functions.

You can also get a reference to the currently running module using alien.default, this lets you get references to any function exported by the module and its transitive dependencies on ELF and Mach-O systems.

Once you have a reference to a library you can get a reference to an exported function with libref.funcname. For example:

> def=alien.default
> =def.puts
alien function puts, library defaults

To use a function you first have to tell Alien the function prototype, using func:types(ret_type, arg_types…), where the types are one of the following strings: “void”, “byte”, “char”, “short”, “ushort”, “int”, “uint”, “long”, “ulong”, “ptrdiff_t”, “size_t”, “float”, “double”, “string”, “pointer”, “ref char”, “ref int”, “ref uint”, “ref double”, “longlong”, “ulonglong” and “callback”. Most correspond directly to C types; byte is a signed char, string is const char*, pointer is void*, callback is a generic function pointer, and ref char, ref int and ref double are by reference versions of the C types. Continuing the previous example:

> def.puts:types("int", "string")
> def.puts("foo")

As you can see, after defining the prototype you can call the function just as a Lua function. Alien converts Lua numbers to the C numeric types, converts nil to NULL and Lua strings to const char* for string, and converts nil to NULL and userdata to void* for pointer. The conversions work in reverse for the return value (with the pointer type converted to a light userdata).

By reference types are special; Alien allocates space on the stack for the argument, copies the Lua number you passed to it (converting appropriately), then calling the function with the address of this space. Then the value is converted back to a Lua number and returned after the function normal return value. An example, using scanf:

> scanf = alien.default.scanf
> scanf:types("int", "string", "ref int", "ref double")
> _, x, y = scanf("%i %lf", 0, 0)
23 42.5
> =x
> =y

You have to pass a value even if the function does not use it, as you can see above.

In most Lua implementations, “longlong” and “ulonglong” won’t fit in a Lua number, so automatic conversion will sometimes fail; on 64-bit machines the same is true of “ptrdiff_t” and “size_t”. However, as long as you perform no computation on such a value, its value will be preserved, so you can receive it from C and pass it back to C without worrying.

Another way to specify types is by passing a table to func:types. The array part of this table should have one item for each parameter, and you can also pass two hash keys, ret, the function’s return type (defaults to int as usual), and abi, the function’s calling convention (useful for Windows, where you can specify “stdcall” as the ABI for __stdcall functions. The default ABI is always “default”, and all systems also support “cdecl”, the usual C calling convention. On systems that don’t have the stdcall convention “stdcall” is the same as “default”.

This is the previous example using this alternate definition:

> scanf = alien.default.scanf
> scanf:types{ ret = "int", "string", "ref int", "ref double" }
> _, x, y = scanf("%i %lf", 0, 0)
23 42.5
> =x
> =y

If you get raw function pointer (returned from a function, for example, or passed to a callback), you can turn it into an Alien function with alien.funcptr(fptr). This returns an Alien function object that you can type and call function normally.


To work with C APIs that require you to allocate memory that is mutated by the library, Alien provides a buffer abstraction. The function alien.buffer allocates a new buffer. If you call it with no arguments it will allocate a buffer of size BUFSIZ (the default C buffer size on your platform). If call it with a number it will allocate a buffer with this number of bytes. If you pass it a string it will allocate a buffer that is a copy of the string. If you pass a light userdata it will use this userdata as the buffer (be careful with that).

After making a buffer you can pass it in place of any argument of string or pointer type.

#buf gives its size in bytes, while buf:strlen() returns the result of calling strlen on the buffer.

You can access the i-th character of a buffer with buf[i], and you can set its value with buf[i] = v. Notice that these are C characters (bytes), not Lua 1-character strings, so you need to use string.char and string.byte to convert between Lua characters and C characters. Access to Alien buffers from Lua is 1-based, not 0-based.

You can also get and set other values by using buf:get(offset, type), and set it by buf:set(offset, val, type). The offset is in bytes, not in elements, so if buf has three int values: their offsets are 1, 5 and 9, respectively, assuming each int is four bytes long.

The get and set operations do no bounds-checking, so where possible use the safer alien.array abstraction that is built on top of buffers (see below).

To retrieve part of the buffer as a string, use buf:tostring(len, offset). Both arguments are optional: the first gives the number of characters to return; if omitted, the buffer is treated as a C string, and the contents up to the first NUL is returned. The second argument gives the offset to start at within the buffer, and defaults to the start of the buffer (1). tostring(buf) is the same as buf:tostring().

To get a pointer to a buffer, use buf:topointer(offset); the argument is optional, defaulting to 1.

To turn a pointer into a buffer offset, use buf:tooffset(pointer).

You can reallocate a buffer using buf:realloc(newsize). This uses the current Lua state’s allocation function.

An example of how to use a buffer:

> gets = alien.default.gets
> gets:types("pointer", "string")
> buf = alien.buffer()
> gets(buf)
Foo bar
> =tostring(buf)
Foo bar

Alien also provides alien.memmove and alien.memset, which work exactly like the C functions of the same name, and can be used on buffers or other memory. alien.memmove can take a string as its second (source) argument.


Arrays are buffers with an extra layer of safety and sugar on top. You create an array with alien.array(type, length), where type is the Alien type of the array’s elements, and length is how many elements the array has. After creating an array arr you can get the type of its elements with arr.type, how many elements it has with arr.length, and the size (in bytes) of each element with arr.size. The underlying buffer is arr.buffer.

You can access the i-th element with arr[i], and set it with arr[i] = val. Type conversions are the same as with buffers, or function calls. Storing a string or userdata in an array pins it so it won’t be collected while it is in the array. You can resize an array too: arr:realloc(newlen) changes the length to newlen.

For convenience alien.array also accepts two other forms: alien.array(type, tab) creates an array with the same length as tab and initializes it with its values; alien.array(type, length, buf) creates an array with buf as the underlying buffer. You can also iterate over the array’s contents with arr:ipairs().

The following example shows an use of arrays:

local function sort(a, b)
  return a - b
local compare = alien.callback(sort, "int", "ref int", "ref int")

local qsort = alien.default.qsort
qsort:types("void", "pointer", "int", "int", "callback")

local nums = alien.array(t, { 4, 5, 3, 2, 6, 1 })
qsort(nums.buffer, nums.length, nums.size, compare)
for i, v in nums:ipairs() do print(v) end

This prints numbers one to six on the console.


Alien also has basic support for declarative structs that is also implemented as a layer of sugar on the basic buffers. It uses a slightly modified version of Roberto Ierusalimschy’s struct library that can unpack binary blobs (userdata) instead of just strings. The alien.defstruct(description) function creates a struct type with the given description, which is a list of pairs with the name and type of each field, where the type is any basic alien type (no structs inside structs yet). For example:

rect = alien.defstruct{
  { "left", "long" },
  { "top", "long" },
  { "right", "long" },
  { "bottom", "long" }

This creates a new struct type with four fields of type “long”, and stores it in rect. To create an instance of this structure (backed by a buffer) call rect:new(). You can then set the fields of the struct just like you do on a Lua table, like r.left = 3. To get the underlying buffer (to pass it to a C function, for example) you have to call the instance, r(). Continuing the example:

r = rect:new()
r.left = 2
doubleleft = alien.rectdll.double_left
doubleleft:types("void", "pointer")
assert(r.left == 4)

You can also pass a buffer or other userdata to the new method of your struct type, and in this case this will be the backing store of the struct instance you are creating. This is useful for unpacking a foreign struct that a C function returned.

Pointer Unpacking

Alien also provides three convenience functions that let you dereference a pointer and convert the value to a Lua type:

The numeric*type* functions take an optional second argument that tells how many items to unpack from the userdata. For example, if ptr is a pointer to an array of four floats, the following code unpacks this array:

> fs = alien.tofloat(ptr, 4)
> =#fs

Use these functions with extra care, as they don’t make any safety checks. For more advanced unmarshalling use the alien.unpack function.


A common pattern when wrapping objects from C libraries is to put a pointer to this object inside a full userdata, then associate this userdata with a metatable that is associated with a string tag. This tag is used to check if the userdata is a valid userdata in each function that uses it. As the userdata is a full userdata it also can have a __gc metamethod for resource reclamation.

Alien has three functions that let you replicate this pattern in your extensions:

For example, suppose libfoo has a create_foo function that returns a Foo* object. These objects have to be properly disposed by calling destroy_foo when they are not used anymore. This is easy to implement:

local tag_foo = alien.tag("libfoo_foo")"pointer")"void", "pointer")

function new_foo()
  local foo =
  return alien.wrap("libfoo_foo", foo)

tag_foo = {
  __gc = function (obj)
           local foo = alien.unwrap("libfoo_foo", obj)

Then on any function that operates on Foo* types you first unwrap to get the pointer, then pass it to the function in libfoo.

Error Codes

Several operating system functions return errors on a special variable called errno. To get the value of errno with Alien call alien.errno().


Some libraries have functions that take callbacks, functions that get called by the library. Most GUI libraries use callbacks, but even the C library has qsort. Alien lets you create a callback from a Lua function with alien.callback. You pass the function and the callback prototype that the library expects. Alien will return a callback object that you can pass in any argument of callback type. A simple example, using qsort:

local function cmp(a, b)
  return a - b
local cmp_cb = alien.callback(sort, "int", "ref char", "ref char")

local qsort = alien.default.qsort
qsort:types("void", "pointer", "int", "int", "callback")

local chars = alien.buffer("spam, spam, and spam")
qsort(chars, chars:strlen(), alien.sizeof("char"), cmp_cb)
assert(chars:tostring() == "   ,,aaaadmmmnpppsss")

The qsort function sorts an array in-place, so we have to use a buffer.

Callbacks are callable from Lua just like any other Alien function, and you can freely change their types with their “types” method.

Magic Numbers

C libraries are full of symbolic constants that are in truth magic numbers, as they are replaced by the preprocessor before even the C compiler has a chance to see them. This means that all these constants are on header files. This also includes things such as the layout and size of structures the library depends on. All this information can change from version to version of the library, or from platform to platform.

Alien provides a utility script called constants that makes it easier to work with these numbers. This utility takes three arguments on the command line: a definitions file, the name of the C file you want it to generate, and the name of a Lua file that the C file will generate when compiled and run. The definitions file can contain preprocessor directives, blank lines, and lines with definitions either of the form identifier or lua_identifier = c_identifier. The first form is equivalent to identifier = identifier. It is best to explain by example (from a libevent binding):

#include <sys/types.h>
#include <event.h>

EV_SIZE = sizeof(struct event)

Lines with preprocessor directives are copied verbatim to the C file constants generates. The above definitions file generates this C file:

/* Generated by Alien constants */

#include <stdio.h>

#include <sys/types.h>
#include <event.h>
#define LUA_FILE "event_constants.lua"
int main() {
  FILE *f = fopen(LUA_FILE, "w+");
  fprintf(f, "-- Generated by Alien constants\n\n");
  fprintf(f, "%s = %i\n", "EV_SIZE ",  sizeof(struct event));
  fprintf(f, "%s = %i\n", "EV_READ", EV_READ);
  fprintf(f, "%s = %i\n", "EV_WRITE", EV_WRITE);
  fprintf(f, "%s = %i\n", "EV_TIMEOUT", EV_TIMEOUT);
  fprintf(f, "%s = %i\n", "EVLOOP_NONBLOCK", EVLOOP_NONBLOCK);
  fprintf(f, "%s = %i\n", "EVLOOP_ONCE", EVLOOP_ONCE);

When compiled and run, this generates this file on a Linux/Intel system:

-- Generated by Alien constants

EV_SIZE  = 84

These steps (generating the C file, compiling, generating the Lua file) are best done in the build step of your extension.


You can query what platform your extension is running on with alien.platform. Currently this can be one of “linux”, “bsd”, “darwin” or “windows”. Other platforms will be added as they are tested. You can use this information for conditional execution in your extensions.

You can get the sizes of the types Alien supports using alien.sizeof("type"), as the qsort example in the Callbacks section shows. You can also get structure aligment information with alien.align("type").

Several extensions may need to create Lua tables with preallocated array and/or hash parts, for performance reasons (implementing a circular queue, for example). Alien exposes the lua_createtable function as alien.table(narray, nhash).


Alien is designed and implemented by Fabio Mascarenhas. It uses the great libffi library by Anthony Green (and others) to do the heavy lifting of calling to and from C. The name is stolen from Common Lisp FFIs.



Alien’s uses the MIT license, reproduced below:

Copyright © 2008-2012 Fabio Mascarenhas

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.