Javascript callbacks#

Changed in Koffi 2.4


The function koffi.proto() was introduced in Koffi 2.4, it was called koffi.callback() in earlier versions.

Callback types#

Changed in Koffi 2.7

In order to pass a JS function to a C function expecting a callback, you must first create a callback type with the expected return type and parameters. The syntax is similar to the one used to load functions from a shared library.

1// ES6 syntax: import koffi from 'koffi';
2const koffi = require('koffi');
4// With the classic syntax, this callback expects an integer and returns nothing
5const ExampleCallback = koffi.proto('ExampleCallback', 'void', ['int']);
7// With the prototype parser, this callback expects a double and float, and returns the sum as a double
8const AddDoubleFloat = koffi.proto('double AddDoubleFloat(double d, float f)');

For alternative calling conventions (such as stdcall on Windows x86 32-bit), you can specify as the first argument with the classic syntax, or after the return type in prototype strings, like this:

1const HANDLE = koffi.pointer('HANDLE', koffi.opaque());
2const HWND = koffi.alias('HWND', HANDLE);
4// These two declarations work the same, and use the __stdcall convention on Windows x86
5const EnumWindowsProc = koffi.proto('bool __stdcall EnumWindowsProc (HWND hwnd, long lParam)');
6const EnumWindowsProc = koffi.proto('__stdcall', 'EnumWindowsProc', 'bool', ['HWND', 'long']);


You have to make sure you get the calling convention right (such as specifying __stdcall for a Windows API callback), or your code will crash on Windows 32-bit.

Before Koffi 2.7, it was impossible to use an alternative callback calling convention with the classic syntax. Use a prototype string or upgrade to Koffi 2.7 to solve this limitation.

Once your callback type is declared, you can use a pointer to it in struct definitions, as function parameters and/or return types, or to call/decode function pointers.


Callbacks have changed in version 2.0.

In Koffi 1.x, callbacks were defined in a way that made them usable directly as parameter and return types, obscuring the underlying pointer.

Now, you must use them through a pointer: void CallIt(CallbackType func) in Koffi 1.x becomes void CallIt(CallbackType *func) in version 2.0 and newer.

Consult the migration guide for more information.

Transient and registered callbacks#

Koffi only uses predefined static trampolines, and does not need to generate code at runtime, which makes it compatible with platforms with hardened W^X migitations (such as PaX mprotect). However, this imposes some restrictions on the maximum number of callbacks, and their duration.

Thus, Koffi distinguishes two callback modes:

  • Transient callbacks can only be called while the C function they are passed to is running, and are invalidated when it returns. If the C function calls the callback later, the behavior is undefined, though Koffi tries to detect such cases. If it does, an exception will be thrown, but this is no guaranteed. However, they are simple to use, and don’t require any special handling.

  • Registered callbacks can be called at any time, but they must be manually registered and unregistered. A limited number of registered callbacks can exist at the same time.

You need to specify the correct calling convention on x86 platforms, or the behavior is undefined (Node will probably crash). Only cdecl and stdcall callbacks are supported.

Transient callbacks#

Use transient callbacks when the native C function only needs to call them while it runs (e.g. qsort, progress callback, sqlite3_exec). Here is a small example with the C part and the JS part.

1#include <string.h>
3int TransferToJS(const char *name, int age, int (*cb)(const char *str, int age))
5    char buf[64];
6    snprintf(buf, sizeof(buf), "Hello %s!", str);
7    return cb(buf, age);
 1// ES6 syntax: import koffi from 'koffi';
 2const koffi = require('koffi');
 4const lib = koffi.load('./'); // Fake path
 6const TransferCallback = koffi.proto('int TransferCallback(const char *str, int age)');
 8const TransferToJS = lib.func('TransferToJS', 'int', ['str', 'int', koffi.pointer(TransferCallback)]);
10let ret = TransferToJS('Niels', 27, (str, age) => {
11    console.log(str);
12    console.log('Your age is:', age);
13    return 42;
17// This example prints:
18//   Hello Niels!
19//   Your age is: 27
20//   42

Registered callbacks#

New in Koffi 2.0 (explicit this binding in Koffi 2.2)

Use registered callbacks when the function needs to be called at a later time (e.g. log handler, event handler, fopencookie/funopen). Call koffi.register(func, type) to register a callback function, with two arguments: the JS function, and the callback type.

When you are done, call koffi.unregister() (with the value returned by koffi.register()) to release the slot. A maximum of 8192 callbacks can exist at the same time. Failure to do so will leak the slot, and subsequent registrations may fail (with an exception) once all slots are used.

The example below shows how to register and unregister delayed callbacks.

 1static const char *(*g_cb1)(const char *name);
 2static void (*g_cb2)(const char *str);
 4void RegisterFunctions(const char *(*cb1)(const char *name), void (*cb2)(const char *str))
 6    g_cb1 = cb1;
 7    g_cb2 = cb2;
10void SayIt(const char *name)
12    const char *str = g_cb1(name);
13    g_cb2(str);
 1// ES6 syntax: import koffi from 'koffi';
 2const koffi = require('koffi');
 4const lib = koffi.load('./'); // Fake path
 6const GetCallback = koffi.proto('const char *GetCallback(const char *name)');
 7const PrintCallback = koffi.proto('void PrintCallback(const char *str)');
 9const RegisterFunctions = lib.func('void RegisterFunctions(GetCallback *cb1, PrintCallback *cb2)');
10const SayIt = lib.func('void SayIt(const char *name)');
12let cb1 = koffi.register(name => 'Hello ' + name + '!', koffi.pointer(GetCallback));
13let cb2 = koffi.register(console.log, 'PrintCallback *');
15RegisterFunctions(cb1, cb2);
16SayIt('Kyoto'); // Prints Hello Kyoto!

Starting with Koffi 2.2, you can optionally specify the this value for the function as the first argument.

1class ValueStore {
2    constructor(value) { this.value = value; }
3    get() { return this.value; }
6let store = new ValueStore(42);
8let cb1 = koffi.register(store.get, 'IntCallback *'); // If a C function calls cb1 it will fail because this will be undefined
9let cb2 = koffi.register(store, store.get, 'IntCallback *'); // However in this case, this will match the store object

Special considerations#

Decoding pointer arguments#

New in Koffi 2.2, changed in Koffi 2.3

Koffi does not have enough information to convert callback pointer arguments to an appropriate JS value. In this case, your JS function will receive an opaque External object.

You can pass this value through to another C function that expects a pointer of the same type, or you can use koffi.decode() function to decode pointer arguments.

The following examples uses it to sort an array of strings in-place with the standard C function qsort():

 1// ES6 syntax: import koffi from 'koffi';
 2const koffi = require('koffi');
 4const lib = koffi.load('');
 6const SortCallback = koffi.proto('int SortCallback(const void *first, const void *second)');
 7const qsort = lib.func('void qsort(_Inout_ void *array, size_t count, size_t size, SortCallback *cb)');
 9let array = ['foo', 'bar', '123', 'foobar'];
11qsort(, 'char **'), array.length, koffi.sizeof('void *'), (ptr1, ptr2) => {
12    let str1 = koffi.decode(ptr1, 'char *');
13    let str2 = koffi.decode(ptr2, 'char *');
15    return str1.localeCompare(str2);
18console.log(array); // Prints ['123', 'bar', 'foo', 'foobar']

Asynchronous callbacks#

New in Koffi 2.2.2

JS execution is inherently single-threaded, so JS callbacks must run on the main thread. There are two ways you may want to call a callback function from another thread:

  • Call the callback from an asynchronous FFI call (e.g. waitpid.async)

  • Inside a synchronous FFI call, pass the callback to another thread

In both cases, Koffi will queue the call back to JS to run on the main thread, as soon as the JS event loop has a chance to run (for example when you await a promise).


Be careful, you can easily get into a deadlock situation if you call a callback from a secondary thread and your main thread never lets the JS event loop run (for example, if the main thread waits for the secondary thread to finish something itself).

Handling of exceptions#

If an exception happens inside the JS callback, the C API will receive 0 or NULL (depending on the return value type).

Handle the exception yourself (with try/catch) if you need to handle exceptions differently.