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Class: Thread

Relationships & Source Files
Namespace Children
Classes:
Backtrace
Extension / Inclusion / Inheritance Descendants
Subclasses:
Process::Waiter
Inherits: Object
Defined in: vm.c,
prelude.rb,
thread.c,
vm_trace.c

Overview

Threads are the Ruby implementation for a concurrent programming model.

Programs that require multiple threads of execution are a perfect candidate for Ruby's Thread class.

For example, we can create a new thread separate from the main thread's execution using .new.

thr = Thread.new { puts "Whats the big deal" }

Then we are able to pause the execution of the main thread and allow our new thread to finish, using #join:

thr.join #=> "Whats the big deal"

If we don't call thr.join before the main thread terminates, then all other threads including thr will be killed.

Alternatively, you can use an array for handling multiple threads at once, like in the following example:

threads = []
threads << Thread.new { puts "Whats the big deal" }
threads << Thread.new { 3.times { puts "Threads are fun!" } }

After creating a few threads we wait for them all to finish consecutively.

threads.each { |thr| thr.join }

Thread initialization

In order to create new threads, Ruby provides .new, .start, and .fork. A block must be provided with each of these methods, otherwise a ::ThreadError will be raised.

When subclassing the Thread class, the #initialize method of your subclass will be ignored by .start and .fork. Otherwise, be sure to call super in your #initialize method.

Thread termination

For terminating threads, Ruby provides a variety of ways to do this.

The class method .kill, is meant to exit a given thread:

thr = Thread.new { ... }
Thread.kill(thr) # sends exit() to thr

Alternatively, you can use the instance method #exit, or any of its aliases #kill or #terminate.

thr.exit

Thread status

Ruby provides a few instance methods for querying the state of a given thread. To get a string with the current thread's state use #status

thr = Thread.new { sleep }
thr.status # => "sleep"
thr.exit
thr.status # => false

You can also use #alive? to tell if the thread is running or sleeping, and #stop? if the thread is dead or sleeping.

Thread variables and scope

Since threads are created with blocks, the same rules apply to other Ruby blocks for variable scope. Any local variables created within this block are accessible to only this thread.

Fiber-local vs. Thread-local

Each fiber has its own bucket for #[] storage. When you set a new fiber-local it is only accessible within this ::Fiber. To illustrate:

Thread.new {
  Thread.current[:foo] = "bar"
  Fiber.new {
    p Thread.current[:foo] # => nil
  }.resume
}.join

This example uses #[] for getting and #[]= for setting fiber-locals, you can also use #keys to list the fiber-locals for a given thread and #key? to check if a fiber-local exists.

When it comes to thread-locals, they are accessible within the entire scope of the thread. Given the following example:

Thread.new{
  Thread.current.thread_variable_set(:foo, 1)
  p Thread.current.thread_variable_get(:foo) # => 1
  Fiber.new{
    Thread.current.thread_variable_set(:foo, 2)
    p Thread.current.thread_variable_get(:foo) # => 2
  }.resume
  p Thread.current.thread_variable_get(:foo)   # => 2
}.join

You can see that the thread-local :foo carried over into the fiber and was changed to 2 by the end of the thread.

This example makes use of #thread_variable_set to create new thread-locals, and #thread_variable_get to reference them.

There is also #thread_variables to list all thread-locals, and #thread_variable? to check if a given thread-local exists.

Exception handling

Any thread can raise an exception using the #raise instance method, which operates similarly to Kernel.raise.

However, it's important to note that an exception that occurs in any thread except the main thread depends on #abort_on_exception. This option is false by default, meaning that any unhandled exception will cause the thread to terminate silently when waited on by either #join or #value. You can change this default by either #abort_on_exception= true or setting $DEBUG to true.

With the addition of the class method .handle_interrupt, you can now handle exceptions asynchronously with threads.

Scheduling

Ruby provides a few ways to support scheduling threads in your program.

The first way is by using the class method .stop, to put the current running thread to sleep and schedule the execution of another thread.

Once a thread is asleep, you can use the instance method #wakeup to mark your thread as eligible for scheduling.

You can also try .pass, which attempts to pass execution to another thread but is dependent on the OS whether a running thread will switch or not. The same goes for #priority, which lets you hint to the thread scheduler which threads you want to take precedence when passing execution. This method is also dependent on the OS and may be ignored on some platforms.

Class Attribute Summary

Class Method Summary

Instance Attribute Summary

Instance Method Summary

Constructor Details

.new ⇒ Thread .new(*args, &proc) ⇒ Thread .new(*args) {|args| ... } ⇒ Thread

Creates a new thread executing the given block.

Any args given to .new will be passed to the block:

arr = []
a, b, c = 1, 2, 3
Thread.new(a,b,c) { |d,e,f| arr << d << e << f }.join
arr #=> [1, 2, 3]

A ThreadError exception is raised if .new is called without a block.

If you're going to subclass Thread, be sure to call super in your #initialize method, otherwise a ::ThreadError will be raised.

Class Attribute Details

.abort_on_exception ⇒ Boolean (rw)

Returns the status of the global “abort on exception'' condition.

The default is false.

When set to true, if any thread is aborted by an exception, the raised exception will be re-raised in the main thread.

Can also be specified by the global $DEBUG flag or command line option -d.

See also .abort_on_exception=.

There is also an instance level method to set this for a specific thread, see #abort_on_exception.

.abort_on_exception=(boolean) ⇒ Boolean (rw)

When set to true, if any thread is aborted by an exception, the raised exception will be re-raised in the main thread. Returns the new state.

Thread.abort_on_exception = true
t1 = Thread.new do
  puts  "In new thread"
  raise "Exception from thread"
end
sleep(1)
puts "not reached"

This will produce:

In new thread
prog.rb:4: Exception from thread (RuntimeError)
 from prog.rb:2:in `initialize'
 from prog.rb:2:in `new'
 from prog.rb:2

See also .abort_on_exception.

There is also an instance level method to set this for a specific thread, see #abort_on_exception=.

DEBUG ⇒ Numeric (rw)

Returns the thread debug level. Available only if compiled with THREAD_DEBUG=-1.

DEBUG=(num) (rw)

Sets the thread debug level. Available only if compiled with THREAD_DEBUG=-1.

Class Method Details

.current ⇒ Thread

Returns the currently executing thread.

Thread.current   #=> #<Thread:0x401bdf4c run>

.exclusive ⇒ Object

Wraps the block in a single, VM-global Mutex#synchronize, returning the value of the block. A thread executing inside the exclusive section will only block other threads which also use the exclusive mechanism.

[ GitHub ] 

  


# File 'prelude.rb', line 10



def self.exclusive
  MUTEX_FOR_THREAD_EXCLUSIVE.synchronize{
    yield
  }
end


  








  

    .exit  ⇒ Thread   



  

    

Terminates the currently running thread and schedules another thread to be run.



If this thread is already marked to be killed, .exit returns the Thread.



If this is the main thread, or the last  thread, exit the process.


  









  

    

      .start([args]*) {|args| ... } ⇒ Thread 
      .fork([args]*) {|args| ... } ⇒ Thread 
    

    Also known as: .start
  



  

    

Basically the same as .new. However, if class Thread is subclassed, then calling .start in that subclass will not invoke the subclass's #initialize method.


  





  








  

    .handle_interrupt(hash)  ⇒ result of the block   



  

    

Changes asynchronous interrupt timing.



interrupt means asynchronous event and corresponding procedure by #raise, #kill, signal trap (not supported yet) and main thread termination (if main thread terminates, then all other thread will be killed).



The given hash has pairs like ExceptionClass => :TimingSymbol. Where the ExceptionClass is the interrupt handled by the given block. The TimingSymbol can be one of the following symbols:


:immediate



Invoke interrupts immediately.


:on_blocking



Invoke interrupts while BlockingOperation.


:never



Never invoke all interrupts.





BlockingOperation means that the operation will block the calling thread, such as read and write.  On CRuby implementation, BlockingOperation is any operation executed without GVL.



Masked asynchronous interrupts are delayed until they are enabled. This method is similar to sigprocmask(3).



NOTE



Asynchronous interrupts are difficult to use.



If you need to communicate between threads, please consider to use another way such as Queue.



Or use them with deep understanding about this method.



Usage



In this example, we can guard from #raise exceptions.



Using the :never TimingSymbol the ::RuntimeError exception will always be ignored in the first block of the main thread. In the second .handle_interrupt block we can purposefully handle ::RuntimeError exceptions.



th = Thread.new do
  Thread.handle_interrupt(RuntimeError => :never) {
    begin
      # You can write resource allocation code safely.
      Thread.handle_interrupt(RuntimeError => :immediate) {
        # ...
      }
    ensure
      # You can write resource deallocation code safely.
    end
  }
end
Thread.pass
# ...
th.raise "stop"



While we are ignoring the ::RuntimeError exception, it's safe to write our resource allocation code. Then, the ensure block is where we can safely deallocate your resources.



Guarding from Timeout::Error



In the next example, we will guard from the Timeout::Error exception. This will help prevent from leaking resources when Timeout::Error exceptions occur during normal ensure clause. For this example we use the help of the standard library Timeout, from lib/timeout.rb



require 'timeout'
Thread.handle_interrupt(Timeout::Error => :never) {
  timeout(10){
    # Timeout::Error doesn't occur here
    Thread.handle_interrupt(Timeout::Error => :on_blocking) {
      # possible to be killed by Timeout::Error
      # while blocking operation
    }
    # Timeout::Error doesn't occur here
  }
}



In the first part of the timeout block, we can rely on Timeout::Error being ignored. Then in the Timeout::Error => :on_blocking block, any operation that will block the calling thread is susceptible to a Timeout::Error exception being raised.



Stack control settings



It's possible to stack multiple levels of .handle_interrupt blocks in order to control more than one ExceptionClass and TimingSymbol at a time.



Thread.handle_interrupt(FooError => :never) {
  Thread.handle_interrupt(BarError => :never) {
     # FooError and BarError are prohibited.
  }
}



Inheritance with ExceptionClass



All exceptions inherited from the ExceptionClass parameter will be considered.



Thread.handle_interrupt(Exception => :never) {
  # all exceptions inherited from Exception are prohibited.
}


  









  

    .kill(thread)  ⇒ Thread   



  

    

Causes the given thread to exit, see also .exit.



count = 0
a = Thread.new { loop { count += 1 } }
sleep(0.1)       #=> 0
Thread.kill(a)   #=> #<Thread:0x401b3d30 dead>
count            #=> 93947
a.alive?         #=> false


  









  

    .list  ⇒ Array   



  

    

Returns an array of Thread objects for all threads that are either runnable or stopped.



Thread.new { sleep(200) }
Thread.new { 1000000.times {|i| i*i } }
Thread.new { Thread.stop }
Thread.list.each {|t| p t}



This will produce:



#<Thread:0x401b3e84 sleep>
#<Thread:0x401b3f38 run>
#<Thread:0x401b3fb0 sleep>
#<Thread:0x401bdf4c run>


  









  

    .main  ⇒ Thread   



  

    

Returns the main thread.


  









  

    .pass  ⇒ nil   



  

    

Give the thread scheduler a hint to pass execution to another thread. A running thread may or may not switch, it depends on OS and processor.


  









  

    .pending_interrupt?(error = nil)  ⇒ Boolean   



  

    

Returns whether or not the asynchronous queue is empty.



Since .handle_interrupt can be used to defer asynchronous events, this method can be used to determine if there are any deferred events.



If you find this method returns true, then you may finish :never blocks.



For example, the following method processes deferred asynchronous events immediately.



def Thread.kick_interrupt_immediately
  Thread.handle_interrupt(Object => :immediate) {
    Thread.pass
  }
end



If error is given, then check only for error type deferred events.



Usage



th = Thread.new{
  Thread.handle_interrupt(RuntimeError => :on_blocking){
    while true
      #...
      # reach safe point to invoke interrupt
      if Thread.pending_interrupt?
        Thread.handle_interrupt(Object => :immediate){}
      end
      #...
    end
  }
}
#...
th.raise # stop thread



This example can also be written as the following, which you should use to avoid asynchronous interrupts.



flag = true
th = Thread.new{
  Thread.handle_interrupt(RuntimeError => :on_blocking){
    while true
      #...
      # reach safe point to invoke interrupt
      break if flag == false
      #...
    end
  }
}
#...
flag = false # stop thread


  









  

    

      .start([args]*) {|args| ... } ⇒ Thread 
      .fork([args]*) {|args| ... } ⇒ Thread 
    

  



  

    

Alias for .fork.


  





  








  

    .stop  ⇒ nil   



  

    

Stops execution of the current thread, putting it into a “sleep'' state, and schedules execution of another thread.



a = Thread.new { print "a"; Thread.stop; print "c" }
sleep 0.1 while a.status!='sleep'
print "b"
a.run
a.join
#=> "abc"


  








Instance Attribute Details



  

    #abort_on_exception  ⇒ Boolean  (rw)  



  

    

Returns the status of the thread-local “abort on exception'' condition for this thr.



The default is false.



See also #abort_on_exception=.



There is also a class level method to set this for all threads, see .abort_on_exception.


  









  

    #abort_on_exception=(boolean)  ⇒ Boolean  (rw)  



  

    

When set to true, if this thr is aborted by an exception, the raised exception will be re-raised in the main thread.



See also #abort_on_exception.



There is also a class level method to set this for all threads, see .abort_on_exception=.


  









  

    #alive?  ⇒ Boolean  (readonly)  



  

    

Returns true if thr is running or sleeping.



thr = Thread.new { }
thr.join                #=> #<Thread:0x401b3fb0 dead>
Thread.current.alive?   #=> true
thr.alive?              #=> false



See also #stop? and #status.


  









  

    #priority  ⇒ Integer  (rw)  



  

    

Returns the priority of thr. Default is inherited from the current thread which creating the new thread, or zero for the initial main thread; higher-priority thread will run more frequently than lower-priority threads (but lower-priority threads can also run).



This is just hint for Ruby thread scheduler.  It may be ignored on some platform.



Thread.current.priority   #=> 0


  









  

    #priority=(integer)  ⇒ Thread  (rw)  



  

    

Sets the priority of thr to integer. Higher-priority threads will run more frequently than lower-priority threads (but lower-priority threads can also run).



This is just hint for Ruby thread scheduler.  It may be ignored on some platform.



count1 = count2 = 0
a = Thread.new do
      loop { count1 += 1 }
    end
a.priority = -1

b = Thread.new do
      loop { count2 += 1 }
    end
b.priority = -2
sleep 1   #=> 1
count1    #=> 622504
count2    #=> 5832


  









  

    #stop?  ⇒ Boolean  (readonly)  



  

    

Returns true if thr is dead or sleeping.



a = Thread.new { Thread.stop }
b = Thread.current
a.stop?   #=> true
b.stop?   #=> false



See also #alive? and #status.


  








Instance Method Details



  

    #[](sym)  ⇒ Object?   



  

    

Attribute Reference—Returns the value of a fiber-local variable (current thread's root fiber if not explicitly inside a ::Fiber), using either a symbol or a string name. If the specified variable does not exist, returns nil.



[
  Thread.new { Thread.current["name"] = "A" },
  Thread.new { Thread.current[:name]  = "B" },
  Thread.new { Thread.current["name"] = "C" }
].each do |th|
  th.join
  puts "#{th.inspect}: #{th[:name]}"
end



This will produce:



#<Thread:0x00000002a54220 dead>: A
#<Thread:0x00000002a541a8 dead>: B
#<Thread:0x00000002a54130 dead>: C



[] and #[]= are not thread-local but fiber-local. This confusion did not exist in Ruby 1.8 because fibers are only available since Ruby 1.9. Ruby 1.9 chooses that the methods behaves fiber-local to save following idiom for dynamic scope.



def meth(newvalue)
  begin
    oldvalue = Thread.current[:name]
    Thread.current[:name] = newvalue
    yield
  ensure
    Thread.current[:name] = oldvalue
  end
end



The idiom may not work as dynamic scope if the methods are thread-local and a given block switches fiber.



f = Fiber.new {
  meth(1) {
    Fiber.yield
  }
}
meth(2) {
  f.resume
}
f.resume
p Thread.current[:name]
#=> nil if fiber-local
#=> 2 if thread-local (The value 2 is leaked to outside of meth method.)



For thread-local variables, please see #thread_variable_get and #thread_variable_set.


  









  

    #[]=(sym, obj)  ⇒ Object   



  

    

Attribute Assignment—Sets or creates the value of a fiber-local variable, using either a symbol or a string.



See also #[].



For thread-local variables, please see #thread_variable_set and #thread_variable_get.


  









  

    #add_trace_func(proc)  ⇒ Proc   



  

    

Adds proc as a handler for tracing.



See #set_trace_func and Kernel.set_trace_func.


  









  

    #backtrace  ⇒ Array   



  

    

Returns the current backtrace of the target thread.


  









  

    #backtrace_locations(*args)  ⇒ Array?   



  

    

Returns the execution stack for the target thread—an array containing backtrace location objects.



See ::Thread::Backtrace::Location for more information.



This method behaves similarly to Kernel.caller_locations except it applies to a specific thread.


  









  

    

      #exit  ⇒ Thread? 
      #kill  ⇒ Thread? 
      #terminate  ⇒ Thread? 
    

    Also known as: #kill, #terminate
  



  

    

Terminates thr and schedules another thread to be run.



If this thread is already marked to be killed, #exit returns the Thread.



If this is the main thread, or the last thread, exits the process.


  





  








  

    #group  ⇒ thgrp?   



  

    

Returns the ::ThreadGroup which contains the given thread, or returns nil if thr is not a member of any group.



Thread.main.group   #=> #<ThreadGroup:0x4029d914>


  









  

    #inspect  ⇒ String   



  

    

Dump the name, id, and status of thr to a string.


  









  

    

      #join  ⇒ Thread 
      #join(limit)  ⇒ Thread 
    

  



  

    

The calling thread will suspend execution and run this thr.



Does not return until thr exits or until the given limit seconds have passed.



If the time limit expires, nil will be returned, otherwise thr is returned.



Any threads not joined will be killed when the main program exits.



If thr had previously raised an exception and the .abort_on_exception or $DEBUG flags are not set, (so the exception has not yet been processed), it will be processed at this time.



a = Thread.new { print "a"; sleep(10); print "b"; print "c" }
x = Thread.new { print "x"; Thread.pass; print "y"; print "z" }
x.join # Let thread x finish, thread a will be killed on exit.
#=> "axyz"



The following example illustrates the limit parameter.



y = Thread.new { 4.times { sleep 0.1; puts 'tick... ' }}
puts "Waiting" until y.join(0.15)



This will produce:



tick...
Waiting
tick...
Waiting
tick...
tick...


  





  








  

    #key?(sym)  ⇒ Boolean   



  

    

Returns true if the given string (or symbol) exists as a fiber-local variable.



me = Thread.current
me[:oliver] = "a"
me.key?(:oliver)    #=> true
me.key?(:stanley)   #=> false


  









  

    #keys  ⇒ Array   



  

    

Returns an array of the names of the fiber-local variables (as Symbols).



thr = Thread.new do
  Thread.current[:cat] = 'meow'
  Thread.current["dog"] = 'woof'
end
thr.join   #=> #<Thread:0x401b3f10 dead>
thr.keys   #=> [:dog, :cat]


  









  

    

      #exit  ⇒ Thread? 
      #kill  ⇒ Thread? 
      #terminate  ⇒ Thread? 
    

  



  

    

Alias for #exit.


  





  








  

    #pending_interrupt?(error = nil)  ⇒ Boolean   



  

    

Returns whether or not the asynchronous queue is empty for the target thread.



If error is given, then check only for error type deferred events.



See .pending_interrupt? for more information.


  









  

    

      #raise  
      #raise(string)  
      #raise(exception [, string [, array]])  
    

  



  

    

Raises an exception from the given thread. The caller does not have to be thr. See Kernel.raise for more information.



Thread.abort_on_exception = true
a = Thread.new { sleep(200) }
a.raise("Gotcha")



This will produce:



prog.rb:3: Gotcha (RuntimeError)
 from prog.rb:2:in `initialize'
 from prog.rb:2:in `new'
 from prog.rb:2


  





  








  

    #run  ⇒ Thread   



  

    

Wakes up thr, making it eligible for scheduling.



a = Thread.new { puts "a"; Thread.stop; puts "c" }
sleep 0.1 while a.status!='sleep'
puts "Got here"
a.run
a.join



This will produce:



a
Got here
c



See also the instance method #wakeup.


  









  

    #safe_level  ⇒ Integer   



  

    

Returns the safe level in effect for thr. Setting thread-local safe levels can help when implementing sandboxes which run insecure code.



thr = Thread.new { $SAFE = 3; sleep }
Thread.current.safe_level   #=> 0
thr.safe_level              #=> 3


  









  

    

      #set_trace_func(proc)  ⇒ Proc 
      #set_trace_func(nil)  ⇒ nil 
    

  



  

    

Establishes proc on thr as the handler for tracing, or disables tracing if the parameter is nil.



See Kernel.set_trace_func.


  





  








  

    #status  ⇒ String, ...   



  

    

Returns the status of thr.


"sleep"



Returned if this thread is sleeping or waiting on I/O


"run"



When this thread is executing


"aborting"



If this thread is aborting


false



When this thread is terminated normally


nil



If terminated with an exception.





a = Thread.new { raise("die now") }
b = Thread.new { Thread.stop }
c = Thread.new { Thread.exit }
d = Thread.new { sleep }
d.kill                  #=> #<Thread:0x401b3678 aborting>
a.status                #=> nil
b.status                #=> "sleep"
c.status                #=> false
d.status                #=> "aborting"
Thread.current.status   #=> "run"



See also the instance methods #alive? and #stop?


  









  

    

      #exit  ⇒ Thread? 
      #kill  ⇒ Thread? 
      #terminate  ⇒ Thread? 
    

  



  

    

Alias for #exit.


  





  








  

    #thread_variable?(key)  ⇒ Boolean   



  

    

Returns true if the given string (or symbol) exists as a thread-local variable.



me = Thread.current
me.thread_variable_set(:oliver, "a")
me.thread_variable?(:oliver)    #=> true
me.thread_variable?(:stanley)   #=> false



Note that these are not fiber local variables.  Please see #[] and #thread_variable_get for more details.


  









  

    #thread_variable_get(key)  ⇒ Object?   



  

    

Returns the value of a thread local variable that has been set.  Note that these are different than fiber local values.  For fiber local values, please see #[] and #[]=.



Thread local values are carried along with threads, and do not respect fibers.  For example:



Thread.new {
  Thread.current.thread_variable_set("foo", "bar") # set a thread local
  Thread.current["foo"] = "bar"                    # set a fiber local

  Fiber.new {
    Fiber.yield [
      Thread.current.thread_variable_get("foo"), # get the thread local
      Thread.current["foo"],                     # get the fiber local
    ]
  }.resume
}.join.value # => ['bar', nil]



The value “bar” is returned for the thread local, where nil is returned for the fiber local.  The fiber is executed in the same thread, so the thread local values are available.


  









  

    #thread_variable_set(key, value)    



  

    

Sets a thread local with key to #value.  Note that these are local to threads, and not to fibers.  Please see #thread_variable_get and #[] for more information.


  









  

    #thread_variables  ⇒ Array   



  

    

Returns an array of the names of the thread-local variables (as Symbols).



thr = Thread.new do
  Thread.current.thread_variable_set(:cat, 'meow')
  Thread.current.thread_variable_set("dog", 'woof')
end
thr.join               #=> #<Thread:0x401b3f10 dead>
thr.thread_variables   #=> [:dog, :cat]



Note that these are not fiber local variables.  Please see #[] and #thread_variable_get for more details.


  









  

    #value  ⇒ Object   



  

    

Waits for thr to complete, using #join, and returns its value or raises the exception which terminated the thread.



a = Thread.new { 2 + 2 }
a.value   #=> 4

b = Thread.new { raise 'something went wrong' }
b.value   #=> RuntimeError: something went wrong


  









  

    #wakeup  ⇒ Thread   



  

    

Marks a given thread as eligible for scheduling, however it may still remain blocked on I/O.



Note: This does not invoke the scheduler, see #run for more information.



c = Thread.new { Thread.stop; puts "hey!" }
sleep 0.1 while c.status!='sleep'
c.wakeup
c.join
#=> "hey!"