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Fibers are primitives for implementing light weight cooperative concurrency in Ruby. Basically they are a means of creating code blocks that can be paused and resumed, much like threads. The main difference is that they are never preempted and that the scheduling must be done by the programmer and not the VM.

As opposed to other stackless light weight concurrency models, each fiber comes with a stack. This enables the fiber to be paused from deeply nested function calls within the fiber block. See the ruby(1) manpage to configure the size of the fiber stack(s).

When a fiber is created it will not run automatically. Rather it must be explicitly asked to run using the #resume method. The code running inside the fiber can give up control by calling ::yield in which case it yields control back to caller (the caller of the #resume).

Upon yielding or termination the Fiber returns the value of the last executed expression

For instance:

fiber = do
  Fiber.yield 1

puts fiber.resume
puts fiber.resume
puts fiber.resume


FiberError: dead fiber called

The #resume method accepts an arbitrary number of parameters, if it is the first call to resume then they will be passed as block arguments. Otherwise they will be the return value of the call to ::yield


fiber = do |first|
  second = Fiber.yield first + 2

puts fiber.resume 10
puts fiber.resume 14
puts fiber.resume 18


FiberError: dead fiber called

Public Class Methods

current() → fiber click to toggle source

Returns the current fiber. You need to require 'fiber' before using this method. If you are not running in the context of a fiber this method will return the root fiber.

               static VALUE
rb_fiber_s_current(VALUE klass)
    return rb_fiber_current();
yield(args, ...) → obj click to toggle source

Yields control back to the context that resumed the fiber, passing along any arguments that were passed to it. The fiber will resume processing at this point when resume is called next. Any arguments passed to the next resume will be the value that this ::yield expression evaluates to.

               static VALUE
rb_fiber_s_yield(int argc, VALUE *argv, VALUE klass)
    return rb_fiber_yield(argc, argv);

Public Instance Methods

alive? → true or false click to toggle source

Returns true if the fiber can still be resumed (or transferred to). After finishing execution of the fiber block this method will always return false. You need to require 'fiber' before using this method.

rb_fiber_alive_p(VALUE fibval)
    return FIBER_TERMINATED_P(fiber_ptr(fibval)) ? Qfalse : Qtrue;
inspect() click to toggle source
Alias for: to_s
raise → obj click to toggle source
raise(string) → obj
raise(exception [, string [, array]]) → obj

Raises an exception in the fiber at the point at which the last ::yield was called, or at the start if neither resume nor raise were called before.

With no arguments, raises a RuntimeError. With a single String argument, raises a RuntimeError with the string as a message. Otherwise, the first parameter should be the name of an Exception class (or an object that returns an Exception object when sent an exception message). The optional second parameter sets the message associated with the exception, and the third parameter is an array of callback information. Exceptions are caught by the rescue clause of begin...end blocks.

               static VALUE
rb_fiber_raise(int argc, VALUE *argv, VALUE fib)
    VALUE exc = rb_make_exception(argc, argv);
    return rb_fiber_resume(fib, -1, &exc);
resume(args, ...) → obj click to toggle source

Resumes the fiber from the point at which the last ::yield was called, or starts running it if it is the first call to resume. Arguments passed to resume will be the value of the ::yield expression or will be passed as block parameters to the fiber's block if this is the first resume.

Alternatively, when resume is called it evaluates to the arguments passed to the next ::yield statement inside the fiber's block or to the block value if it runs to completion without any ::yield

               static VALUE
rb_fiber_m_resume(int argc, VALUE *argv, VALUE fib)
    return rb_fiber_resume(fib, argc, argv);
to_s → string click to toggle source

Returns fiber information string.

               static VALUE
fiber_to_s(VALUE fibval)
    const rb_fiber_t *fib = fiber_ptr(fibval);
    const rb_proc_t *proc;
    char status_info[0x10];

    snprintf(status_info, 0x10, " (%s)", fiber_status_name(fib->status));
    if (!rb_obj_is_proc(fib->first_proc)) {
        VALUE str = rb_any_to_s(fibval);
        strlcat(status_info, ">", sizeof(status_info));
        rb_str_set_len(str, RSTRING_LEN(str)-1);
        rb_str_cat_cstr(str, status_info);
        return str;
    GetProcPtr(fib->first_proc, proc);
    return rb_block_to_s(fibval, &proc->block, status_info);
Also aliased as: inspect
transfer(args, ...) → obj click to toggle source

Transfer control to another fiber, resuming it from where it last stopped or starting it if it was not resumed before. The calling fiber will be suspended much like in a call to ::yield. You need to require 'fiber' before using this method.

The fiber which receives the transfer call is treats it much like a resume call. Arguments passed to transfer are treated like those passed to resume.

You cannot resume a fiber that transferred control to another one. This will cause a double resume error. You need to transfer control back to this fiber before it can yield and resume.


fiber1 = do
  puts "In Fiber 1"

fiber2 = do
  puts "In Fiber 2"
  puts "Never see this message"

fiber3 = do
  puts "In Fiber 3"



In fiber 2
In fiber 1
In fiber 3
               static VALUE
rb_fiber_m_transfer(int argc, VALUE *argv, VALUE fibval)
    rb_fiber_t *fib = fiber_ptr(fibval);
    fib->transferred = 1;
    return fiber_switch(fib, argc, argv, 0);