Thread API

The Thread API provides a variation of coroutines for openos. A thread is superior to basic coroutines in many ways and, for many workflows, is easier to work with. An openos thread is an autonomous non-blocking detachable process.

• Autonomous: Threads asynchronously begin execution immediately after creation without needing to call resume. Unlike coroutines, threads self-resume after yielding and do not yield back to the line of code where they were created. Programming with coroutines builds on the cooperative yield and resume calls made explicitly by the user. Yielding or calling event.pull from a thread, however, only temporarily blocks just that thread and will continue on its own.
• Non-Blocking: Threads can call computer.pullSignal (or any higher level wrapper such as event.pull, io.pull, etc) without blocking the main kernel process nor any other thread. The thread itself is blocked until a signal or timeout occurs. The computational flow of the thread behaves just as if the same code were run in a script from the command line. Behind the scenes, the thread library is using pullSignal to swap between threads, and waking threads up when appropriate. This is very much unlike coroutines where computer.pullSignal blocks all other activity on the system until a signal or timeout occurs.
• Detachable: By default, threads are owned by the process in which they are created (note that threads themselves are a process). The owning process is known as the parent process. When a parent process is closing (i.e. it has reached the end of its execution) it will close(i.e. join) all of its running child threads. A process does not have to close a thread when:
  • The thread detaches from the parent process. see t:detach(). Note that a thread can also attach to another parent. see t:attach()
  • The parent process throws an exception or calls os.exit in which case all attached threads are killed. Note that rebooting also kills all threads.
  • The thread is killed. see t:kill()
  • The thread is manually suspended. see t:suspend()

A thread maintains an independent set of event registrations; it does not inherit any and it does not share any. Any event registration made (e.g. listeners or timers) inside a thread belongs to that thread.

• When a thread dies all of its event registrations die with it.
• A suspended thread ignores events (see t:status() suspended)
• A thread cannot access/remove/interfere with another thread's event registrations.
• A pushed event is observed by all running threads on the system.
• Two separate threads can event.pull for the same event, and each thread will observe the event independently.

There are two main use cases for using threads over other viable options

• You want to write a function that makes blocking calls without blocking the rest of your application.
• You want a long running background function without having to manage yielding and resuming it manually.

There are two sections of API defined here.

1. The thread api, or the static functions, provided by require("thread")
2. The thread handle api, or the api available the thread objects created by thread.create. In this documentation these thread handles will be denoted by just t.

• thread.create(thread_proc: function[, ...]): table

  Starts a new thread executing the function thread_proc and returns its thread handle, see Thread Handle API. This method takes an optional ... which is passed to thread_proc. The runtime of the thread continues autonomously.

snippet.lua

local thread = require("thread")
print("Main program start")
thread.create(function(a, b)
  print(a)
  os.sleep()
  print(b)
end, 'A', 'B')
print("Main program end")

Output:

Main program start
A
Main program end
B

• thread.waitForAll(threads: table[, timeout: number]): boolean, string

  Waits for the array of threads to complete. This blocking call can return in timeout seconds if provided. Returns success and an error message on failure. A thread is “completed” under multiple conditions, see t:join() for details.

snippet.lua

local thread = require("thread")
print("Main program start")
local t = thread.create(function(a, b)
  print(a)
  os.sleep()
  print(b)
end, 'A', 'B')
thread.waitForAll({t})
print("Main program end")

Output:

Main program start
A
B
Main program end

• thread.waitForAny(threads: table[, timeout: number): boolean, string

  Waits for any single thread to complete and is otherwise equivalent to thread.waitForAll()

snippet.lua

local thread = require("thread")
print("Main program start")
local t1 = thread.create(function(a, b)
  print(a)
  os.sleep()
  print(b)
end, 'A', 'B')
local t2 = thread.create(function(c, d)
  print(c)
  os.sleep()
  os.sleep()
  print(d)
end, 'C', 'D')
thread.waitForAny({t1, t2})
print("Main program end")

Output:

Main program start
A
C
B
Main program end
D

Please note that threads resume order is not specified and this example may print “D” before it prints “Main program end”

• thread.current(): table

  Returns the current thread t object. The init process does not represent a thread and nothing is returned from this method if called from the init process and not inside any thread.

• t:resume(): boolean, string

  Resumes (or thaws) a suspended thread. Returns success and an error message on failure. A thread begins its life already in a running state and thus basic thread workflows will not ever need to call t:resume(). A “running” thread will autonomously continue until it completes. t:resume() is only necessary to resume a thread that has been suspended(t:suspend()). Note that because you are not directly resuming the thread any exceptions thrown from the thread are absorbed by the threading library and not exposed to your process.

  • At this time there is no way to hook in an exception handler for threads but for now event.onError is used to print the error message to “/tmp/event.log”. Please note that currently the hard interrupt exception is only thrown once, and the behavior of a process with threads when a hard interrupt is thrown is unspecified. At this time, any one of the threads or the parent process may take the exception. These details are not part of the specification for threads and any part of this implementation detail may change later.
• t:suspend(): boolean, string

  Suspends (or freezes) a running thread. Returns success and an error message on failure. A “suspended” thread never autonomously wakes up and dies as soon as its parent process (if attached) closes. A suspended thread ignores events. That means any event listeners or timers created inside the thread will not respond to event notifications. Note that threads do not buffer event signals and a suspended thread may miss event signals it was waiting for. For example, if a thread was last waiting on event.pull("modem_message") and is “suspended” and a “modem_message” is received by the computer then the thread will miss the event and never know it happened. Please note that if you suspend a thread that is blocked waiting for an event, it is unspecified which event the thread will receive when it is next resumed.
  Suspending the current thread causes the thread to immediately yield and does not resume until t:resume() is called explicitly elsewhere.

Special notes about t:resume, t:suspend

Do not think of these methods as coroutine.resume() nor coroutine.yield(). These methods are indirect and a thread will asynchronously start or stop running on its own. Contrast this to coroutine methods which directly and immediately invoke execution or leave execution of a coroutine. Consider these examples:

snippet.lua

local thread = require("thread")
local t -- this example needs an upvalue to t
t = thread.create(function()
  print("start")
  thread.current():suspend()
  print("after suspend")
  os.sleep()
  print("after sleep")
end)

Output:

start

snippet.lua

local thread = require("thread")
local t -- this example needs an upvalue to t
t = thread.create(function()
  print("start")
  thread.current():suspend()
  print("after suspend")
  os.sleep()
  print("after sleep")
end)
print("outside thread create")
t:resume()
print("after resume")

Output:

start
outside thread create
after suspend
after resume
after sleep

• t:kill()

  Stabby stab! Kills the thread dead. The thread is terminated and will not continue its thread function. Any event registrations it made will die with it. Keep in mind that the core underlying Lua type is a coroutine which is not a preemptive thread. Thus, the thread's stopping points are deterministic, meaning that you can predict exactly where the thread will stop. Consider this example:

snippet.lua

local thread = require("thread")
local t = thread.create(function()
  while true do
    print("running")
    print("still running")
    os.sleep()
    print("after sleep")
  end
  print("unreachable code")
end)
print("before kill")
t:kill()
print("after kill")

Output:

running
still running
before kill
after kill

• t:status(): string

  Returns the thread status as a string.

  • “running”

    A running thread will continue (autonomously reactivating) after yields and blocking calls until its thread function exits. This is the default and initial state of a created thread. A thread remains in the “running” state even when blocked or not active. A running thread can be suspended(t:suspend()) or killed (t:kill()) but not resumed(t:resume()). A running thread will block calls to t:join() and block its parent from closing. Unlike a coroutine which appears “suspended” when not executing in this very moment, a thread state remains “running” even when waiting for an event.

  • “suspended”

    A suspended thread will remain suspended and never self resume execution of its thread function. A suspended thread is automatically killed when its attached parent closes or when you attempt to t:join() it. A suspended thread ignores event signals, and any event registrations made from the context of the thread, or any child threads created therein, also ignore any event signals. A suspended thread's children behave as if suspended even if their status is “running”. A suspended thread can be resumed(t:resume()) or killed (t:kill()) but not suspended(t:suspend()).

  • “dead”

    A dead thread has completed or aborted its execution or has been terminated. It cannot be resumed(t:resume()) nor suspended(t:suspend()). A dead thread does not block a parent process from closing. Killing a dead thread is not an error but does nothing.

Status Examples

snippet.lua

local thread = require("thread")
local t = thread.create(function()
  print("before sleep")
  os.sleep()
  print("after sleep")
end)
print(t:status())

Output:

before sleep
running
after sleep

snippet.lua

local thread = require("thread")
local t = thread.create(function()
  print("before sleep")
  os.sleep()
  print("after sleep")
end)
t:suspend()
print(t:status())
os.sleep(10)
print(t:status())
t:resume()
print("after resume")
print(t:status())

Output:

before sleep
suspended
suspended
after resume
dead

• t:attach([level: number]): boolean, string

  Attaches a thread to a process, conventionally known as a child thread or attached thread. level is an optional used to get parent processes, 0 or nil uses the currently running process. When initially created a thread is already attached to the current process. This method returns nil and an error message if level refers to a nonexistent process, otherwise it returns truthy. An attached thread blocks its parent process from closing until the thread dies (or is killed, or the parent process aborts).

• t:detach(): table, string

  Detaches a thread from its parent if it has one. Returns nil and an error message if no action was taken, otherwise returns self (handy if you want to create and detach a thread in one line). A detached thread will continue to run until the computer is shutdown or rebooted, or the thread dies.

snippet.lua

local detached_thread = thread.create(function() end):detach()

• t:join([timeout: number]): boolean, string

  Blocks the caller until t is no longer running or (optionally) returns false if timeout seconds is reached. After a call to t:join() the thread state is “dead”. Any of the following circumstances allow join to finish and unblock the caller

  • The thread continues running until it returns from its thread function
  • The thread aborts, or throws an uncaught exception
  • The thread is suspended
  • The thread is killed

    Calling thread.waitForAll({t}) is functionally equivalent to calling t:join(). When a processs is closing it will call thread.waitForAll on the group of its child threads if it has any. A child thread blocks its parent thread by the same machanism.

This example demonstrates what happens when a thread throws an exception. A thread stops executing and becomes “dead” when it throws an uncaught exception. The exception message is not printed to stdout nor stderr (see t:resume()) for details.

snippet.lua

local thread = require("thread")
local tty = require("tty")
 
print("p start")
local reader = thread.create(function()
  print("reader start")
  error("thread abort") -- throws an exception
  print("reader done")
end)
print("p end", reader:status())

Output

p start
reader start
p end	dead

This example demonstrates how you would register a function that handles soft interrupts (^c) to close file handles, release resources, etc, and then exit the whole program.

snippet.lua

local thread = require("thread")
local event = require("event")
 
local cleanup_thread = thread.create(function()
  event.pull("interrupted")
  print("cleaning up resources")
end)
 
local main_thread = thread.create(function()
  print("main program")
  while true do
    io.write("input: ")
    io.read()
  end
end)
 
thread.waitForAny({cleanup_thread, main_thread})
os.exit(0) -- closes all remaining threads

Assuming the user presses ^c to send an interrupt Output

main program
input: ^c
cleaning up resources

This example demonstrates that now OpenOS supports non blocking threads.

snippet.lua

local event = require("event")
local thread = require("thread")
thread.create(function()
  a,b,c,d,e,f,g = coroutine.yield()
  print(a,b,c,d,e,f,g)
  print(event.pull())
end)
 
event.push("custom_event_a")
print("done")
event.push("custom_event_b", 2)

Output

custom_event_a
done
custom_event_b	2