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Class: Fiber

Relationships & Source Files
Inherits: Object
Defined in: cont.c,
cont.c

Overview

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 small 4KB stack. This enables the fiber to be paused from deeply nested function calls within the fiber block.

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 = Fiber.new do
  Fiber.yield 1
  2
end

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

produces

1
2
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

Example:

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

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

produces

12
14
FiberError: dead fiber called

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Class Method Details

.current ⇒ Fiber

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.

.yield(args, ...) ⇒ Object

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.

Instance Attribute Details

#alive? ⇒ Boolean (readonly)

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.

Instance Method Details

#resume(args, ...) ⇒ Object

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

#transfer(args, ...) ⇒ Object

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.

Example:

fiber1 = Fiber.new do
  puts "In Fiber 1"
  Fiber.yield
end

fiber2 = Fiber.new do
  puts "In Fiber 2"
  fiber1.transfer
  puts "Never see this message"
end

fiber3 = Fiber.new do
  puts "In Fiber 3"
end

fiber2.resume
fiber3.resume

produces

In fiber 2
In fiber 1
In fiber 3