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Node.js Foreign Function Interface

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Node.js Foreign Function Interface

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is a Node.js addon for loading and calling dynamic libraries using pure JavaScript. It can be used to create bindings to native libraries without writing any C++ code.

It also simplifies the augmentation of node.js with C code as it takes care of handling the translation of types across JavaScript and C, which can add reams of boilerplate code to your otherwise simple C. See the

for an example of this use case.

WARNING: node-ffi assumes you know what you're doing. You can pretty easily create situations where you will segfault the interpreter and unless you've got C debugger skills, you probably won't know what's going on.


var ffi = require('ffi');

var libm = ffi.Library('libm', { 'ceil': [ 'double', [ 'double' ] ] }); libm.ceil(1.5); // 2

// You can also access just functions in the current process by passing a null var current = ffi.Library(null, { 'atoi': [ 'int', [ 'string' ] ] }); current.atoi('1234'); // 1234

For a more detailed introduction, see the node-ffi tutorial page.


  • Linux, OS X, Windows, or Solaris.
  • libffi
    comes bundled with node-ffi; it does not need to be installed on your system.
  • The current version is tested to run on node v0.6, v0.8, v0.9 and v0.10.


Make sure you've installed all the necessary build tools for your platform, then invoke:

$ npm install ffi

Source Install / Manual Compilation

To compile from source it's easiest to use


$ npm install -g node-gyp

Now you can compile

$ git clone git://
$ cd node-ffi
$ node-gyp rebuild


The types that you specify in function declarations correspond to ref's types system. So see its docs for a reference if you are unfamiliar.

V8 and 64-bit Types

Internally, V8 stores integers that will fit into a 32-bit space in a 32-bit integer, and those that fall outside of this get put into double-precision floating point (FP) numbers. This is problematic because FP numbers are imprecise. To get around this, the methods in node-ffi that deal with 64-bit integers return strings and can accept strings as parameters.

Call Overhead

There is non-trivial overhead associated with FFI calls. Comparing a hard-coded binding version of

to an FFI version of
shows that the native hard-coded binding is orders of magnitude faster. So don't just use the C version of a function just because it's faster. There's a significant cost in FFI calls, so make them worth it.


MIT License. See the


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