Fiber/coroutine support for v8 and node.
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fibers(1) -- Fiber support for v8 and Node


via npm

  • npm install fibers
  • You're done!

from source

  • git clone git://
  • cd node-fibers
  • npm install

Note: node-fibers uses node-gyp for building. To manually invoke the build process, you can use node-gyp rebuild. This will put the compiled extension in build/Release/fibers.node. However, when you do require('fibers'), it will expect the module to be in, for example, bin/linux-x64-v8-3.11/fibers.node. You can manually put the module here every time you build, or you can use the included build script. Either npm install or node build -f will do this for you. If you are going to be hacking on node-fibers, it may be worthwhile to first do node-gyp configure and then for subsequent rebuilds you can just do node-gyp build which will be faster than a full npm install or node-gyp rebuild.


It's recommended that you use node 0.6.18 or higher with node-fibers. Using other versions may lead to instability during high loads.

using windows 8?

Windows 8 is a beta operating system and you may have issues with fibers. To use fibers in Windows 8 you may need to run node.exe in Windows 7 compatibility mode. Once Windows 8 is released this issue will be revisited. See gh-70 for more information.

other notes

Unlike most NodeJS projects, node-fibers is a C++ project. Some extra work is required to compile node-fibers, but pretty much every platform is supported in some way. Binary distributions in 32 and 64-bit forms are provided in npm for Linux, OS X, and Windows (special thanks to Jeroen Janssen for his work on fibers in Windows).

Support for Solaris, FreeBSD, and OpenBSD is provided by compiling the extension on your system during install time via node-gyp. If your operating system isn't listed here you may have luck copying the build process for one of the other OS's, assuming you are running a POSIX-like OS.

node 0.6.x is required to run this release of node-fibers. Older versions of node (0.4.x) are supported in older releases of node-fibers. See the 0.5.x branch of node-fibers for documentation.


The examples below describe basic use of Fiber, but note that it is not recommended to use Fiber without an abstraction in between your code and fibers. See "FUTURES" below for additional information.


This is a quick example of how you can write sleep() with fibers. Note that while the sleep() call is blocking inside the fiber, node is able to handle other events.

$ cat sleep.js
var Fiber = require('fibers');

function sleep(ms) {
	var fiber = Fiber.current;
	setTimeout(function() {;
	}, ms);

Fiber(function() {
	console.log('wait... ' + new Date);
	console.log('ok... ' + new Date);
console.log('back in main');
$ node sleep.js
wait... Fri Jan 21 2011 22:42:04 GMT+0900 (JST)
back in main
ok... Fri Jan 21 2011 22:42:05 GMT+0900 (JST)

Incremental Generator

Yielding execution will resume back in the fiber right where you left off. You can also pass values back and forth through yield() and run(). Again, the node event loop is never blocked while this script is running.

$ cat generator.js
var Fiber = require('fibers');

var inc = Fiber(function(start) {
	var total = start;
	while (true) {
		total += Fiber.yield(total);

for (var ii =; ii <= 10; ii = {
$ node generator.js

Fibonacci Generator

Expanding on the incremental generator above, we can create a generator which returns a new Fibonacci number with each invocation. You can compare this with the ECMAScript Harmony Generator Fibonacci example.

$ cat fibonacci.js
var Fiber = require('fibers');

// Generator function. Returns a function which returns incrementing
// Fibonacci numbers with each call.
function Fibonacci() {
	// Create a new fiber which yields sequential Fibonacci numbers
	var fiber = Fiber(function() {
		Fiber.yield(0); // F(0) -> 0
		var prev = 0, curr = 1;
		while (true) {
			var tmp = prev + curr;
			prev = curr;
			curr = tmp;
	// Return a bound handle to `run` on this fiber

// Initialize a new Fibonacci sequence and iterate up to 1597
var seq = Fibonacci();
for (var ii = seq(); ii <= 1597; ii = seq()) {
$ node fibonacci.js

Basic Exceptions

Fibers are exception-safe; exceptions will continue travelling through fiber boundaries:

$ cat error.js
var Fiber = require('fibers');

var fn = Fiber(function() {
	console.log('async work here...');
	console.log('still working...');
	console.log('just a little bit more...');
	throw new Error('oh crap!');

try {
	while (true) {;
} catch(e) {
	console.log('safely caught that error!');
$ node error.js
async work here...
still working...
just a little bit more...
safely caught that error!
Error: oh crap!
		at error.js:11:9


Using the Fiber class without an abstraction in between your code and the raw API is not recommended. Fiber is meant to implement the smallest amount of functionality in order make possible many different programming patterns. This makes the Fiber class relatively lousy to work with directly, but extremely powerful when coupled with a decent abstraction. There is no right answer for which abstraction is right for you and your project. Included with node-fibers is an implementation of "futures" which is fiber-aware. Usage of this library is documented below. There are several other externally-maintained options which can be found on the wiki. You should feel encouraged to be creative with fibers and build a solution which works well with your project. For instance, Future is not a good abstraction to use if you want to build a generator function (see Fibonacci example above).

Using Future to wrap existing node functions. At no point is the node event loop blocked:

$ cat ls.js
var Future = require('fibers/future'), wait = Future.wait;
var fs = require('fs');

// This wraps existing functions assuming the last argument of the passed
// function is a callback. The new functions created immediately return a
// future and the future will resolve when the callback is called (which
// happens behind the scenes).
var readdir = Future.wrap(fs.readdir);
var stat = Future.wrap(fs.stat);

Fiber(function() {
	// Get a list of files in the directory
	var fileNames = readdir('.').wait();
	console.log('Found '+ fileNames.length+ ' files');

	// Stat each file
	var stats = [];
	for (var ii = 0; ii < fileNames.length; ++ii) {

	// Print file size
	for (var ii = 0; ii < fileNames.length; ++ii) {
		console.log(fileNames[ii]+ ': '+ stats[ii].get().size);
$ node ls.js 
Found 11 files
bin: 4096
fibers.js: 1708
.gitignore: 37 8664
future.js: 5833
.git: 4096
src: 4096
ls.js: 860
Makefile: 436
package.json: 684

The future API is designed to make it easy to move between classic callback-style code and fiber-aware waiting code:

$ cat sleep.js
var Future = require('fibers/future'), wait = Future.wait;

// This function returns a future which resolves after a timeout. This
// demonstrates manually resolving futures.
function sleep(ms) {
	var future = new Future;
	setTimeout(function() {
	}, ms);
	return future;

// You can create functions which automatically run in their own fiber and
// return futures that resolve when the fiber returns (this probably sounds
// confusing.. just play with it to understand).
var calcTimerDelta = function(ms) {
	var start = new Date;
	return new Date - start;
}.future(); // <-- important!

// And futures also include node-friendly callbacks if you don't want to use
// wait()
calcTimerDelta(2000).resolve(function(err, val) {
	console.log('Set timer for 2000ms, waited '+ val+ 'ms');
$ node sleep.js
Set timer for 2000ms, waited 2009ms


Fiber's definition looks something like this:

 * Instantiate a new Fiber. You may invoke this either as a function or as
 * a constructor; the behavior is the same.
 * When run() is called on this fiber for the first time, `fn` will be
 * invoked as the first frame on a new stack. Execution will continue on
 * this new stack until `fn` returns, or Fiber.yield() is called.
 * After the function returns the fiber is reset to original state and
 * may be restarted with another call to run().
function Fiber(fn) {
	[native code]

 * `Fiber.current` will contain the currently-running Fiber. It will be
 * `undefined` if there is no fiber (i.e. the main stack of execution).
 * See "Garbage Collection" for more information on responsible use of
 * `Fiber.current`.
Fiber.current = undefined;

 * `Fiber.yield()` will halt execution of the current fiber and return control
 * back to original caller of run(). If an argument is supplied to yield(),
 * run() will return that value.
 * When run() is called again, yield() will return.
 * Note that this function is a global to allow for correct garbage
 * collection. This results in no loss of functionality because it is only
 * valid to yield from the currently running fiber anyway.
 * Note also that `yield` is a reserved word in Javascript. This is normally
 * not an issue, however some code linters may complain. Rest assured that it
 * will run fine now and in future versions of Javascript.
Fiber.yield = function(param) {
	[native code]

 * run() will start execution of this Fiber, or if it is currently yielding,
 * it will resume execution. If an argument is supplied, this argument will
 * be passed to the fiber, either as the first parameter to the main
 * function [if the fiber has not been started] or as the return value of
 * yield() [if the fiber is currently yielding].
 * This function will return either the parameter passed to yield(), or the
 * returned value from the fiber's main function.
 */ = function(param) {
	[native code]

 * reset() will terminate a running Fiber and restore it to its original
 * state, as if it had returned execution.
 * This is accomplished by causing yield() to throw an exception, and any
 * futher calls to yield() will also throw an exception. This continues
 * until the fiber has completely unwound and returns.
 * If the fiber returns a value it will be returned by reset().
 * If the fiber is not running, reset() will have no effect.
Fiber.prototype.reset = function() {
	[native code]

 * throwInto() will cause a currently yielding fiber's yield() call to
 * throw instead of return gracefully. This can be useful for notifying a
 * fiber that you are no longer interested in its task, and that it should
 * give up.
 * Note that if the fiber does not handle the exception it will continue to
 * bubble up and throwInto() will throw the exception right back at you.
Fiber.prototype.throwInto = function(exception) {
	[native code]


If you intend to build generators, iterators, or "lazy lists", you should be aware that all fibers must eventually unwind. This is implemented by causing yield() to throw unconditionally when the library is trying to unwind your fiber-- either because reset() was called, or all handles to the fiber were lost and v8 wants to delete it.

Something like this will, at some point, cause an infinite loop in your application:

var fiber = Fiber(function() {
	while (true) {
		try {
		} catch(e) {}

If you either call reset() on this fiber, or the v8 garbage collector decides it is no longer in use, the fiber library will attempt to unwind the fiber by causing all calls to yield() to throw. However, if you catch these exceptions and continue anyway, an infinite loop will occur.

There are other garbage collection issues that occur with misuse of fiber handles. If you grab a handle to a fiber from within itself, you should make sure that the fiber eventually unwinds. This application will leak memory:

var fiber = Fiber(function() {
	var that = Fiber.current;
fiber = undefined;

There is no way to get back into the fiber that was started, however it's impossible for v8's garbage collector to detect this. With a handle to the fiber still outstanding, v8 will never garbage collect it and the stack will remain in memory until the application exits.

Thus, you should take care when grabbing references to Fiber.current.