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Using js-ctypes

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Before you can use js-ctypes, you need to import the ctypes.jsm code module. This is as simple as including the following line of code in the desired JavaScript scope:


Loading a native library

Once you've imported the code module, you can call the ctypes.open() method to load each native library you wish to use. On Windows, for example, you might load the system user32 library like this:

var lib = ctypes.open("user32.dll");

On Mac OS X, you can load the Core Foundation library from the Core Foundation framework like this:

var coreFoundation = ctypes.open("/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation");

The returned object is a Library object that you use to declare functions and data types for use with the loaded library.

Note: js-ctypes only works with C libraries; you can't use C++ methods directly. Instead, you'll need to create a shim library that uses C functions that then call into the C++ library for you.

Library search paths

If you specify a full path, that path is used to load the library. Otherwise, the system looks for the library in its standard locations.


On Windows, the following locations are searched for the library, in this order:

  1. The application's directory.
  2. The system directory.
  3. The 16-bit system directory.
  4. The Windows directory.
  5. The current working directory
  6. The directories listed in the PATH environment variable.
Note: This information comes from this article on MSDN.

After you're done

When you're finished using a library, you should close it by calling the Library object's close() method:


If you fail to close the library, it will be automatically closed when it is garbage collected.

Using the library

You may need to declare new types. These can be simple types or more complex types such as structures. See Declaring types for details. You will almost certainly need to declare one or more functions, so that you can call them.

Once you've declared the types and functions, you can write your code to make use of them. Instantiating C data objects and referencing them is covered in the article Working with data.

Memory management

If JS code creates a structure or array, that memory will be valid as long as the JS object stays alive. Pointers to that memory must be carefully managed to make sure the underlying memory is still referenced.

When binary code hands back a pointer/handle to allocated memory, the JS code must make sure to free that memory with the correct allocator. It is usually best to expose a freeing function from the binary.

Keeping objects alive

The following js-ctypes objects will hold references to objects, keeping them alive. This is not an exhaustive list, but will help you understand memory management and how it affects your use of js-ctypes:

  • A function or static data declared using the declare() method will hold that library alive.
  • A CType will hold referent CType objects alive.
  • A CData will hold referent CData objects alive, in specific circumstances. For example, a CData object produced by accessing a field of a structure or the internals of an array will hold the referent objects alive.

What won't keep objects alive

It's important to note that getting direct access to the contents of a CData object using address(), addressOfElement(), or contents, will result in a CData object that does not hold its referent alive. Be sure to hold an explicit reference to be sure the referent object doesn't get garbage collected before you're done using it.


You also need to be sure to retain references to any JavaScript code that native code may call back into. This should be obvious, but is important enough to be worth stating explicitly.

When in doubt, malloc()

When you absolutely, positively need to keep data around, you can use malloc() to allocate it directly. This bypasses JavaScript's memory management and lets you handle memory management yourself.


These examples offer a quick look at how js-ctypes is used. See js-ctypes examples for more intricate examples.

Calling Windows routines

This example demonstrates how to use ctypes to call a Win32 API.


var lib = ctypes.open("C:\\WINDOWS\\system32\\user32.dll");

/* Declare the signature of the function we are going to call */
var msgBox = lib.declare("MessageBoxW",
var MB_OK = 0;

var ret = msgBox(0, "Hello world", "title", MB_OK);


In line 3, the user32.dll system library is loaded. Line 6 declares msgBox() to be a method that calls the Windows function MessageBoxW. Line 15 calls the msgBox() routine, which displays the alert.

The last thing we do is call lib.close() to close the library when we're done using it.

Instead of defining the whole path, you may also just give the file name.


How we knew how to declare the function was by going to MSDN site and looking at the MessageBox (MessageBoxW is just a unicode version of same function) function. Also learn about the lib.declare function used here: lib.declare. Learn about the data types used here: Data Types. We see that it needs to be defined like this:

int WINAPI MessageBox(
  _In_opt_  HWND hWnd,
  _In_opt_  LPCTSTR lpText,
  _In_opt_  LPCTSTR lpCaption,
  _In_      UINT uType

So we read this article here on defining types and replicate it: Declaring Types

var lib = ctypes.open("user32.dll");

Or even without the extension.

var lib = ctypes.open("user32");

Calling Carbon routines on Mac OS X

This example demonstrates how to use ctypes to call a Carbon function on Mac OS X.

Note: This example will not work on 64bit OS X, you will likely need to change to the Cocoa API.
/* build a Str255 ("Pascal style") string from the passed-in string */

function makeStr(str) {
  return String.fromCharCode(str.length) + str;


var carbon = ctypes.open("/System/Library/Frameworks/Carbon.framework/Carbon");

stdAlert = carbon.declare("StandardAlert",       /* function name */
                          ctypes.default_abi,    /* ABI type */
                          ctypes.int16_t,        /* return type */
                          ctypes.int16_t,        /* alert type */
                          ctypes.char.ptr,       /* primary text */
                          ctypes.char.ptr,       /* secondary text */
                          ctypes.uint32_t,       /* alert param */
                          ctypes.int16_t);       /* item hit */

var hit = 0;
var msgErr = makeStr("Carbon Says...");
var msgExp = makeStr("We just called the StandardAlert Carbon function from JavaScript!");

var err = stdAlert(1, msgErr, msgExp, 0, hit);


The makeStr() function is a utility routine that takes as input a standard JavaScript string and returns a Carbon-style "Pascal" string, which is a length byte followed by the characters of the string itself. Note that this only works correctly if the string is in fact under 256 characters; if it's longer, this will fail spectacularly.

In line 9, the Carbon library is loaded from the system's Carbon framework.

Line 11 declares the stdAlert() function, which will call the Carbon StandardAlert routine. It uses the default ABI, returns a 16-bit integer (which is a Carbon OSErr value), and accepts an integer (the alert type), two strings, a pointer to a parameter block, which we aren't using, and another integer, which is used to return the hit item. See Apple's documentation for StandardAlert for details.

After that, we simply set up our parameters by using makeStr() to generate the two Str255 strings we need, then call stdAlert(), which produces the following alert window:


The last thing we do is call carbon.close() to close the library when we're done using it.

Calling LibC routines on Linux/POSIX

This example demonstrates how to use ctypes to call a libc function on Linux.

/* import js-ctypes */

/* Open the library */
try {
  /* Linux */
  var libc = ctypes.open("libc.so.6");
} catch (e) {
  /* Most other Unixes */
  libc = ctypes.open("libc.so");

/* Import a function */
var puts = libc.declare("puts",             /* function name */
                        ctypes.default_abi, /* call ABI */
                        ctypes.int,         /* return type */
                        ctypes.char.ptr);   /* argument type */

var ret = puts("Hello World from js-ctypes!");

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 Contributors to this page: jswisher
 Last updated by: jswisher,