class Thread - Ruby 2.2.4 Core API Reference API Reference

Ruby 2.2 Core API

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class Thread

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 Thread#[] 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.

In Files

thread.c
vm.c
vm_trace.c

Parent

Object

Public Class Methods

DEBUG → num click to toggle source

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

static VALUE
rb_thread_s_debug(void)
{
    return INT2NUM(rb_thread_debug_enabled);
}

DEBUG = num click to toggle source

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

static VALUE
rb_thread_s_debug_set(VALUE self, VALUE val)
{
    rb_thread_debug_enabled = RTEST(val) ? NUM2INT(val) : 0;
    return val;
}

abort_on_exception → true or false click to toggle source

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

The default is false.

When set to true, all threads will abort (the process will exit(0)) if an exception is raised in any thread.

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

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.
}

static VALUE
rb_thread_s_handle_interrupt(VALUE self, VALUE mask_arg)
{
    VALUE mask;
    rb_thread_t *th = GET_THREAD();
    VALUE r = Qnil;
    int state;

    if (!rb_block_given_p()) {
        rb_raise(rb_eArgError, "block is needed.");
    }

    mask = rb_convert_type(mask_arg, T_HASH, "Hash", "to_hash");
    rb_hash_foreach(mask, handle_interrupt_arg_check_i, 0);
    rb_ary_push(th->pending_interrupt_mask_stack, mask);
    if (!rb_threadptr_pending_interrupt_empty_p(th)) {
        th->pending_interrupt_queue_checked = 0;
        RUBY_VM_SET_INTERRUPT(th);
    }

    TH_PUSH_TAG(th);
    if ((state = EXEC_TAG()) == 0) {
        r = rb_yield(Qnil);
    }
    TH_POP_TAG();

    rb_ary_pop(th->pending_interrupt_mask_stack);
    if (!rb_threadptr_pending_interrupt_empty_p(th)) {
        th->pending_interrupt_queue_checked = 0;
        RUBY_VM_SET_INTERRUPT(th);
    }

    RUBY_VM_CHECK_INTS(th);

    if (state) {
        JUMP_TAG(state);
    }

    return r;
}

kill(thread) → thread click to toggle source

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

static VALUE
rb_thread_s_kill(VALUE obj, VALUE th)
{
    return rb_thread_kill(th);
}

list → array click to toggle source

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>

VALUE
rb_thread_list(void)
{
    VALUE ary = rb_ary_new();
    rb_vm_t *vm = GET_THREAD()->vm;
    rb_thread_t *th = 0;

    list_for_each(&vm->living_threads, th, vmlt_node) {
        switch (th->status) {
          case THREAD_RUNNABLE:
          case THREAD_STOPPED:
          case THREAD_STOPPED_FOREVER:
            rb_ary_push(ary, th->self);
          default:
            break;
        }
    }
    return ary;
}

main → thread click to toggle source

Returns the main thread.

static VALUE
rb_thread_s_main(VALUE klass)
{
    return rb_thread_main();
}

new { ... } → thread click to toggle source

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.

static VALUE
thread_s_new(int argc, VALUE *argv, VALUE klass)
{
    rb_thread_t *th;
    VALUE thread = rb_thread_alloc(klass);

    if (GET_VM()->main_thread->status == THREAD_KILLED)
        rb_raise(rb_eThreadError, "can't alloc thread");

    rb_obj_call_init(thread, argc, argv);
    GetThreadPtr(thread, th);
    if (!th->first_args) {
        rb_raise(rb_eThreadError, "uninitialized thread - check `%s#initialize'",
                 rb_class2name(klass));
    }
    return thread;
}

pass → nil click to toggle source

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.

static VALUE
thread_s_pass(VALUE klass)
{
    rb_thread_schedule();
    return Qnil;
}

pending_interrupt?(error = nil) → true/false click to toggle source

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

static VALUE
rb_thread_s_pending_interrupt_p(int argc, VALUE *argv, VALUE self)
{
    return rb_thread_pending_interrupt_p(argc, argv, GET_THREAD()->self);
}

start([args]*) {|args| block } → thread click to toggle source

fork([args]*) {|args| block } → thread

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.

static VALUE
thread_start(VALUE klass, VALUE args)
{
    return thread_create_core(rb_thread_alloc(klass), args, 0);
}

stop → nil click to toggle source

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"

VALUE
rb_thread_stop(void)
{
    if (rb_thread_alone()) {
        rb_raise(rb_eThreadError,
                 "stopping only thread\n\tnote: use sleep to stop forever");
    }
    rb_thread_sleep_deadly();
    return Qnil;
}

Public Instance Methods

thr[sym] → obj or nil click to toggle source

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

Thread#[] and Thread#[]= 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.

static VALUE
rb_thread_aref(VALUE thread, VALUE key)
{
    ID id = rb_check_id(&key);
    if (!id) return Qnil;
    return rb_thread_local_aref(thread, id);
}

thr[sym] = obj → obj click to toggle source

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

See also stop? and status.

static VALUE
rb_thread_alive_p(VALUE thread)
{
    rb_thread_t *th;
    GetThreadPtr(thread, th);

    if (rb_threadptr_dead(th))
        return Qfalse;
    return Qtrue;
}

backtrace → array click to toggle source

Returns the current backtrace of the target thread.

static VALUE
rb_thread_backtrace_m(int argc, VALUE *argv, VALUE thval)
{
    return rb_vm_thread_backtrace(argc, argv, thval);
}

backtrace_locations(*args) → array or nil click to toggle source

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.

static VALUE
rb_thread_backtrace_locations_m(int argc, VALUE *argv, VALUE thval)
{
    return rb_vm_thread_backtrace_locations(argc, argv, thval);
}

exit → thr or nil click to toggle source

kill → thr or nil

terminate → thr or nil

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.

VALUE
rb_thread_kill(VALUE thread)
{
    rb_thread_t *th;

    GetThreadPtr(thread, th);

    if (th->to_kill || th->status == THREAD_KILLED) {
        return thread;
    }
    if (th == th->vm->main_thread) {
        rb_exit(EXIT_SUCCESS);
    }

    thread_debug("rb_thread_kill: %p (%"PRI_THREAD_ID")\n", (void *)th, thread_id_str(th));

    if (th == GET_THREAD()) {
        /* kill myself immediately */
        rb_threadptr_to_kill(th);
    }
    else {
        rb_threadptr_pending_interrupt_enque(th, eKillSignal);
        rb_threadptr_interrupt(th);
    }
    return thread;
}

group → thgrp or nil click to toggle source

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>

VALUE
rb_thread_group(VALUE thread)
{
    rb_thread_t *th;
    VALUE group;
    GetThreadPtr(thread, th);
    group = th->thgroup;

    if (!group) {
        group = Qnil;
    }
    return group;
}

inspect → string click to toggle source

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

static VALUE
rb_thread_inspect(VALUE thread)
{
    return rb_thread_inspect_msg(thread, 1, 1, 1);
}

join → thr click to toggle source

join(limit) → thr

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...

static VALUE
thread_join_m(int argc, VALUE *argv, VALUE self)
{
    rb_thread_t *target_th;
    double delay = DELAY_INFTY;
    VALUE limit;

    GetThreadPtr(self, target_th);

    rb_scan_args(argc, argv, "01", &limit);
    if (!NIL_P(limit)) {
        delay = rb_num2dbl(limit);
    }

    return thread_join(target_th, delay);
}

key?(sym) → true or false click to toggle source

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

static VALUE
rb_thread_key_p(VALUE self, VALUE key)
{
    rb_thread_t *th;
    ID id = rb_check_id(&key);

    GetThreadPtr(self, th);

    if (!id || !th->local_storage) {
        return Qfalse;
    }
    if (st_lookup(th->local_storage, id, 0)) {
        return Qtrue;
    }
    return Qfalse;
}

keys → array click to toggle source

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]

static VALUE
rb_thread_keys(VALUE self)
{
    rb_thread_t *th;
    VALUE ary = rb_ary_new();
    GetThreadPtr(self, th);

    if (th->local_storage) {
        st_foreach(th->local_storage, thread_keys_i, ary);
    }
    return ary;
}

exit → thr or nil click to toggle source

kill → thr or nil

terminate → thr or nil

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.

VALUE
rb_thread_kill(VALUE thread)
{
    rb_thread_t *th;

    GetThreadPtr(thread, th);

    if (th->to_kill || th->status == THREAD_KILLED) {
        return thread;
    }
    if (th == th->vm->main_thread) {
        rb_exit(EXIT_SUCCESS);
    }

    thread_debug("rb_thread_kill: %p (%"PRI_THREAD_ID")\n", (void *)th, thread_id_str(th));

    if (th == GET_THREAD()) {
        /* kill myself immediately */
        rb_threadptr_to_kill(th);
    }
    else {
        rb_threadptr_pending_interrupt_enque(th, eKillSignal);
        rb_threadptr_interrupt(th);
    }
    return thread;
}

pending_interrupt?(error = nil) → true/false click to toggle source

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.

static VALUE
rb_thread_pending_interrupt_p(int argc, VALUE *argv, VALUE target_thread)
{
    rb_thread_t *target_th;

    GetThreadPtr(target_thread, target_th);

    if (rb_threadptr_pending_interrupt_empty_p(target_th)) {
        return Qfalse;
    }
    else {
        if (argc == 1) {
            VALUE err;
            rb_scan_args(argc, argv, "01", &err);
            if (!rb_obj_is_kind_of(err, rb_cModule)) {
                rb_raise(rb_eTypeError, "class or module required for rescue clause");
            }
            if (rb_threadptr_pending_interrupt_include_p(target_th, err)) {
                return Qtrue;
            }
            else {
                return Qfalse;
            }
        }
        return Qtrue;
    }
}

priority → integer click to toggle source

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

static VALUE
rb_thread_priority(VALUE thread)
{
    rb_thread_t *th;
    GetThreadPtr(thread, th);
    return INT2NUM(th->priority);
}

priority= integer → thr click to toggle source

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

static VALUE
rb_thread_priority_set(VALUE thread, VALUE prio)
{
    rb_thread_t *th;
    int priority;
    GetThreadPtr(thread, th);


#if USE_NATIVE_THREAD_PRIORITY
    th->priority = NUM2INT(prio);
    native_thread_apply_priority(th);
#else
    priority = NUM2INT(prio);
    if (priority > RUBY_THREAD_PRIORITY_MAX) {
        priority = RUBY_THREAD_PRIORITY_MAX;
    }
    else if (priority < RUBY_THREAD_PRIORITY_MIN) {
        priority = RUBY_THREAD_PRIORITY_MIN;
    }
    th->priority = priority;
#endif
    return INT2NUM(th->priority);
}

raise click to toggle source

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

static VALUE
thread_raise_m(int argc, VALUE *argv, VALUE self)
{
    rb_thread_t *target_th;
    rb_thread_t *th = GET_THREAD();
    GetThreadPtr(self, target_th);
    rb_threadptr_raise(target_th, argc, argv);

    /* To perform Thread.current.raise as Kernel.raise */
    if (th == target_th) {
        RUBY_VM_CHECK_INTS(th);
    }
    return Qnil;
}

run → thr click to toggle source

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