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Use the 'ffi' package instead. See


0.5.7 • Public • Published


Node.js Foreign Function Interface Build Status

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, 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 0.6.x.
  • If you need node 0.4.x support, use the 0.4 branch of node-ffi.


$ npm install node-ffi

Most popular operating systems have a pre-compiled binary that comes along with node-ffi, so most of the time you will not need to compile anything! (Unless of course you want to, then see below).


To compile from source it's easiest to use node-gyp:

$ npm install -g node-gyp

Now you can compile node-ffi:

$ git clone git://
$ cd node-ffi
$ node-gyp configure build


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
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.


MIT License. See the LICENSE file.




npm i node-ffi

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