Node.js Foreign Function Interface
JavaScript C++ C
Switch branches/tags
Nothing to show
Clone or download
Pull request Compare This branch is 1 commit ahead of substack:master.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Rick Branson rick [at] diodeware [dot] com


node-ffi 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 example/factorial 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("node-ffi");

var libm = new 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 = new FFI.Library(null, { "atoi": [ "int32", [ "string" ] ] });
current.atoi("1234"); // 1234


  • Linux, OS X, or Solaris.
  • You will need node.js 0.2.3+
  • libffi installed -- with development headers (comes with OS X and most Linux distros)


$ npm install node-ffi


$ git clone git://
$ cd node-ffi
$ node-waf configure build
$ node test.js
$ node-waf install


int8        Signed 8-bit Integer
uint8       Unsigned 8-bit Integer
int16       Signed 16-bit Integer
uint16      Unsigned 16-bit Integer
int32       Signed 32-bit Integer
uint32      Unsigned 32-bit Integer
int64       Signed 64-bit Integer 
uint64      Unsigned 64-bit Integer 
float       Single Precision Floating Point Number (float)
double      Double Precision Floating Point Number (double)
pointer     Pointer Type
string      Null-Terminated String (char *)

In addition to the basic types, there are type aliases for common C types.

byte        unsigned char
char        char
uchar       unsigned char
short       short
ushort		unsigned short
int			int
uint		unsigned int
long		long
ulong		unsigned long
longlong	long long
ulonglong	unsigned long long
size_t      platform-dependent, usually pointer size

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 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 strtoul() to an FFI version of strtoul() shows that the native hard-coded binding is 5x 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.


See LICENSE file.