manual.mld

{0 Lwt manual }

{1 Introduction }

  When writing a program, a common developer's task is to handle I/O
  operations. Indeed, most software interacts with several different
  resources, such as:

{ul
  {- the kernel, by doing system calls,}
  {- the user, by reading the keyboard, the mouse, or any input device,}
  {- a graphical server, to build graphical user interface,}
  {- other computers, by using the network,}
  {- …and so on.}}

  When this list contains only one item, it is pretty easy to
  handle. However as this list grows it becomes harder and harder to
  make everything work together. Several choices have been proposed
  to solve this problem:

{ul
  {- using a main loop, and integrating all components we are
     interacting with into this main loop,}
  {- using preemptive system threads.}}

  Both solutions have their advantages and their drawbacks. For the
  first one, it may work, but it becomes very complicated to write
  a piece of asynchronous sequential code. The typical example is
  graphical user interfaces freezing and not redrawing themselves
  because they are waiting for some blocking part of the code to
  complete.

  If you already wrote code using preemptive threads, you should know
  that doing it right with threads is a difficult job. Moreover, system
  threads consume non-negligible resources, and so you can only launch
  a limited number of threads at the same time. Thus, this is not a
  general solution.

  [Lwt] offers a third alternative. It provides promises, which are
  very fast: a promise is just a reference that will be filled asynchronously,
  and calling a function that returns a promise does not require a new stack,
  new process, or anything else. It is just a normal, fast, function call.
  Promises compose nicely, allowing us to write highly asynchronous programs.

  In the first part, we will explain the concepts of [Lwt], then we will
  describe the main modules [Lwt] consists of.

{2 Finding examples }

  Additional sources of examples:

{ul
  {- {{: https://github.com/dkim/rwo-lwt#readme }Concurrent Programming with Lwt}}
  {- {{: https://mirage.io/docs/tutorial-lwt }Mirage Lwt Tutorial}}
  {- {{: https://baturin.org/code/lwt-counter-server/ }Simple Server with Lwt}}}

{1 The Lwt core library }

  In this section we describe the basics of [Lwt]. It is advised to
  start [utop] and try the given code examples.

{2 Lwt concepts }

  Let's take a classic function of the [Stdlib] module:

{[
# Stdlib.input_char;;
- : in_channel -> char = <fun>
]}

  This function will wait for a character to come on the given input
  channel, and then return it. The problem with this function is that it is
  blocking: while it is being executed, the whole program will be
  blocked, and other events will not be handled until it returns.

  Now, let's look at the lwt equivalent:

{[
# Lwt_io.read_char;;
- : Lwt_io.input_channel -> char Lwt.t = <fun>
]}

  As you can see, it does not return just a character, but something of
  type [char Lwt.t]. The type ['a Lwt.t] is the type
  of promises that can be fulfilled later with a value of type ['a].
  [Lwt_io.read_char] will try to read a character from the
  given input channel and {e immediately} return a promise, without
  blocking, whether a character is available or not. If a character is
  not available, the promise will just not be fulfilled {e yet}.

  Now, let's see what we can do with a [Lwt] promise. The following
  code creates a pipe, creates a promise that is fulfilled with the result of
  reading the input side:

{[
# let ic, oc = Lwt_io.pipe ();;
val ic : Lwt_io.input_channel = <abstr>
val oc : Lwt_io.output_channel = <abstr>
# let p = Lwt_io.read_char ic;;
val p : char Lwt.t = <abstr>
]}

  We can now look at the state of our newly created promise:

{[
# Lwt.state p;;
- : char Lwt.state = Lwt.Sleep
]}

  A promise may be in one of the following states:

{ul
  {- [Return x], which means that the promise has been fulfilled
     with the value [x]. This usually implies that the asynchronous
     operation, that you started by calling the function that returned the
     promise, has completed successfully.}
  {- [Fail exn], which means that the promise has been rejected
     with the exception [exn]. This usually means that the asynchronous
     operation associated with the promise has failed.}
  {- [Sleep], which means that the promise is has not yet been
     fulfilled or rejected, so it is {e pending}.}}

  The above promise [p] is pending because there is nothing yet
  to read from the pipe. Let's write something:

{[
# Lwt_io.write_char oc 'a';;
- : unit Lwt.t = <abstr>
# Lwt.state p;;
- : char Lwt.state = Lwt.Return 'a'
]}

  So, after we write something, the reading promise has been fulfilled
  with the value ['a'].

{2 Primitives for promise creation }

  There are several primitives for creating [Lwt] promises. These
  functions are located in the module [Lwt].

  Here are the main primitives:

{ul
  {- [Lwt.return : 'a -> 'a Lwt.t] 
     creates a promise which is already fulfilled with the given value}
  {- [Lwt.fail : exn -> 'a Lwt.t]
     creates a promise which is already rejected with the given exception}
  {- [Lwt.wait : unit -> 'a Lwt.t * 'a Lwt.u]
     creates a pending promise, and returns it, paired with a resolver (of
     type ['a Lwt.u]), which must be used to resolve (fulfill or reject)
     the promise.}}

  To resolve a pending promise, use one of the following
  functions:

{ul
  {- [Lwt.wakeup : 'a Lwt.u -> 'a -> unit]
     fulfills the promise with a value.}
  {- [Lwt.wakeup_exn : 'a Lwt.u -> exn -> unit]
     rejects the promise with an exception.}}

  Note that it is an error to try to resolve the same promise twice. [Lwt]
  will raise [Invalid_argument] if you try to do so.

  With this information, try to guess the result of each of the
  following expressions:

{[
# Lwt.state (Lwt.return 42);;
# Lwt.state (Lwt.fail Exit);;
# let p, r = Lwt.wait ();;
# Lwt.state p;;
# Lwt.wakeup r 42;;
# Lwt.state p;;
# let p, r = Lwt.wait ();;
# Lwt.state p;;
# Lwt.wakeup_exn r Exit;;
# Lwt.state p;;
]}

{3 Primitives for promise composition }

  The most important operation you need to know is [bind]:

{[
val bind : 'a Lwt.t -> ('a -> 'b Lwt.t) -> 'b Lwt.t
]}

  [bind p f] creates a promise which waits for [p] to become
  become fulfilled, then passes the resulting value to [f]. If [p] is a
  pending promise, then [bind p f] will be a pending promise too,
  until [p] is resolved. If [p] is rejected, then the resulting
  promise will be rejected with the same exception. For example, consider the
  following expression:

{[
Lwt.bind
  (Lwt_io.read_line Lwt_io.stdin)
  (fun str -> Lwt_io.printlf "You typed %S" str)
]}

  This code will first wait for the user to enter a line of text, then
  print a message on the standard output.

  Similarly to [bind], there is a function to handle the case
  when [p] is rejected:

{[
val catch : (unit -> 'a Lwt.t) -> (exn -> 'a Lwt.t) -> 'a Lwt.t
]}

  [catch f g] will call [f ()], then wait for it to become
  resolved, and if it was rejected with an exception [exn], call
  [g exn] to handle it. Note that both exceptions raised with
  [Pervasives.raise] and [Lwt.fail] are caught by
  [catch].

{3 Cancelable promises }

  In some case, we may want to cancel a promise. For example, because it
  has not resolved after a timeout. This can be done with cancelable
  promises. To create a cancelable promise, you must use the
  [Lwt.task] function:

{[
val task : unit -> 'a Lwt.t * 'a Lwt.u
]}

  It has the same semantics as [Lwt.wait], except that the
  pending promise can be canceled with [Lwt.cancel]:

{[
val cancel : 'a Lwt.t -> unit
]}

  The promise will then be rejected with the exception
  [Lwt.Canceled]. To execute a function when the promise is
  canceled, you must use [Lwt.on_cancel]:

{[
val on_cancel : 'a Lwt.t -> (unit -> unit) -> unit
]}

  Note that canceling a promise does not automatically cancel the
  asynchronous operation that is going to resolve it. It does, however,
  prevent any further chained operations from running. The asynchronous
  operation associated with a promise can only be canceled if its implementation
  has taken care to set an [on_cancel] callback on the promise that
  it returned to you. In practice, most operations (such as system calls)
  can't be canceled once they are started anyway, so promise cancellation is
  useful mainly for interrupting future operations once you know that a chain of
  asynchronous operations will not be needed.

  It is also possible to cancel a promise which has not been
  created directly by you with [Lwt.task]. In this case, the deepest
  cancelable promise that the given promise depends on will be canceled.

  For example, consider the following code:

{[
# let p, r = Lwt.task ();;
val p : '_a Lwt.t = <abstr>
val r : '_a Lwt.u = <abstr>
# let p' = Lwt.bind p (fun x -> Lwt.return (x + 1));;
val p' : int Lwt.t = <abstr>
]}

  Here, cancelling [p'] will in fact cancel [p], rejecting
  it with [Lwt.Canceled]. [Lwt.bind] will then propagate the
  exception forward to [p']:

{[
# Lwt.cancel p';;
- : unit = ()
# Lwt.state p;;
- : int Lwt.state = Lwt.Fail Lwt.Canceled
# Lwt.state p';;
- : int Lwt.state = Lwt.Fail Lwt.Canceled
]}

  It is possible to prevent a promise from being canceled
  by using the function [Lwt.protected]:

{[
val protected : 'a Lwt.t -> 'a Lwt.t
]}

  Canceling [(protected p)] will have no effect on [p].

{3 Primitives for concurrent composition }

  We now show how to compose several promises concurrently. The
  main functions for this are in the [Lwt] module: [join],
  [choose] and [pick].

  The first one, [join] takes a list of promises and returns a promise
  that is waiting for all of them to resolve:

{[
val join : unit Lwt.t list -> unit Lwt.t
]}

  Moreover, if at least one promise is rejected, [join l] will be rejected
  with the same exception as the first one, after all the promises are resolved.

  Conversely, [choose] waits for at least {e one} promise to become
  resolved, then resolves with the same value or exception:

{[
val choose : 'a Lwt.t list -> 'a Lwt.t
]}

  For example:

{[
# let p1, r1 = Lwt.wait ();;
val p1 : '_a Lwt.t = <abstr>
val r1 : '_a Lwt.u = <abstr>
# let p2, r2 = Lwt.wait ();;
val p2 : '_a Lwt.t = <abstr>
val r2 : '_a Lwt.u = <abstr>
# let p3 = Lwt.choose [p1; p2];;
val p3 : '_a Lwt.t = <abstr>
# Lwt.state p3;;
- : '_a Lwt.state = Lwt.Sleep
# Lwt.wakeup r2 42;;
- : unit = ()
# Lwt.state p3;;
- : int Lwt.state = Lwt.Return 42
]}

  The last one, [pick], is the same as [choose], except that it tries to cancel
  all other promises when one resolves. Promises created via [Lwt.wait()] are not cancellable
  and are thus not cancelled.

{3 Rules }

  A callback, like the [f] that you might pass to [Lwt.bind], is
  an ordinary OCaml function. [Lwt] just handles ordering calls to these
  functions.

  [Lwt] uses some preemptive threading internally, but all of your code
  runs in the main thread, except when you explicitly opt into additional
  threads with [Lwt_preemptive].

  This simplifies reasoning about critical sections: all the code in one
  callback cannot be interrupted by any of the code in another callback.
  However, it also carries the danger that if a single callback takes a very
  long time, it will not give [Lwt] a chance to run your other callbacks.
  In particular:

{ul
  {- do not write functions that may take time to complete, without splitting
     them up using [Lwt.pause] or performing some [Lwt] I/O,}
  {- do not do I/O that may block, otherwise the whole program will
     hang inside that callback. You must instead use the asynchronous I/O
     operations provided by [Lwt].}}

{2 The syntax extension }

  [Lwt] offers a PPX syntax extension which increases code readability and
  makes coding using [Lwt] easier. The syntax extension is documented
  in {!Ppx_lwt}.

  To use the PPX syntax extension, add the [lwt_ppx] package when
  compiling:

{[
$ ocamlfind ocamlc -package lwt_ppx -linkpkg -o foo foo.ml
]}

  Or, in [utop]:

{[
# #require "lwt_ppx";;
]}

  [lwt_ppx] is distributed in a separate opam package of that same name.

  For a brief overview of the syntax, see the Correspondence table below.

{3 Correspondence table }

{table
  {tr
    {th Without Lwt}
    {th With Lwt}}
  {tr
    {td {[let pattern_1 = expr_1
and pattern_2 = expr2
…
and pattern_n = expr_n in
expr]}}
    {td {[let%lwt pattern_1 = expr_1
and pattern_2 = expr2
…
and pattern_n = expr_n in
expr]}}}
  {tr
    {td {[try expr with
| pattern_1 = expr_1
| pattern_2 = expr2
…
| pattern_n = expr_n]}}
    {td {[try%lwt expr with
| pattern_1 = expr_1
| pattern_2 = expr2
…
| pattern_n = expr_n]}}}
  {tr
    {td {[match expr with
| pattern_1 = expr_1
| pattern_2 = expr2
…
| pattern_n = expr_n]}}
    {td {[match%lwt expr with
| pattern_1 = expr_1
| pattern_2 = expr2
…
| pattern_n = expr_n]}}}
  {tr
    {td {[for ident = expr_init to expr_final do
  expr
done]}}
    {td {[for%lwt ident = expr_init to expr_final do
  expr
done]}}}
  {tr
    {td {[while expr do expr done]}}
    {td {[while%lwt expr do expr done]}}}
  {tr
    {td {[if expr then expr else expr]}}
    {td {[if%lwt expr then expr else expr]}}}
  {tr
    {td {[assert expr]}}
    {td {[assert%lwt expr]}}}
  {tr
    {td {[raise exn]}}
    {td {[[%lwt raise exn]]}}}}

{2 Backtrace support }

  If an exception is raised inside a callback called by Lwt, the backtrace
  provided by OCaml will not be very useful. It will end inside the Lwt
  scheduler instead of continuing into the code that started the operations that
  led to the callback call. To avoid this, and get good backtraces from Lwt, use
  the syntax extension. The [let%lwt] construct will properly propagate
  backtraces.

  As always, to get backtraces from an OCaml program, you need to either declare
  the environment variable [OCAMLRUNPARAM=b] or call
  [Printexc.record_backtrace true] at the start of your program, and be
  sure to compile it with [-g]. Most modern build systems add [-g] by
  default.

{2 [let*] syntax }

  To use Lwt with the [let*] syntax introduced in OCaml 4.08, you can open
  the [Syntax] module:

{[
open Syntax
]}

  Then, you can write

{[
let* () = Lwt_io.printl "Hello," in
let* () = Lwt_io.printl "world!" in
Lwt.return ()
]}

{2 Other modules of the core library }

  The core library contains several modules that only depend on
  [Lwt]. The following naming convention is used in [Lwt]: when a
  function takes as argument a function, returning a promise, that is going
  to be executed sequentially, it is suffixed with “[_s]”. And
  when it is going to be executed concurrently, it is suffixed with
  “[_p]”. For example, in the [Lwt_list] module we have:

{[
val map_s : ('a -> 'b Lwt.t) -> 'a list -> 'b list Lwt.t
val map_p : ('a -> 'b Lwt.t) -> 'a list -> 'b list Lwt.t
]}

{3 Mutexes }

  [Lwt_mutex] provides mutexes for [Lwt]. Its use is almost the
  same as the [Mutex] module of the thread library shipped with
  OCaml. In general, programs using [Lwt] do not need a lot of
  mutexes, because callbacks run without preempting each other. They are
  only useful for synchronising or sequencing complex operations spread over
  multiple callback calls.

{3 Lists }

  The [Lwt_list] module defines iteration and scanning functions
  over lists, similar to the ones of the [List] module, but using
  functions that return a promise. For example:

{[
val iter_s : ('a -> unit Lwt.t) -> 'a list -> unit Lwt.t
val iter_p : ('a -> unit Lwt.t) -> 'a list -> unit Lwt.t
]}

  In [iter_s f l], [iter_s] will call f on each elements
  of [l], waiting for resolution between each element. On the
  contrary, in [iter_p f l], [iter_p] will call f on all
  elements of [l], only then wait for all the promises to resolve.

{3 Data streams }

  [Lwt] streams are used in a lot of places in [Lwt] and its
  submodules. They offer a high-level interface to manipulate data flows.

  A stream is an object which returns elements sequentially and
  lazily. Lazily means that the source of the stream is touched only for new
  elements when needed. This module contains a lot of stream
  transformation, iteration, and scanning functions.

  The common way of creating a stream is by using
  [Lwt_stream.from] or by using [Lwt_stream.create]:

{[
val from : (unit -> 'a option Lwt.t) -> 'a Lwt_stream.t
val create : unit -> 'a Lwt_stream.t * ('a option -> unit)
]}

  As for streams of the standard library, [from] takes as
  argument a function which is used to create new elements.

  [create] returns a function used to push new elements
  into the stream and the stream which will receive them.

  For example:

{[
# let stream, push = Lwt_stream.create ();;
val stream : '_a Lwt_stream.t = <abstr>
val push : '_a option -> unit = <fun>
# push (Some 1);;
- : unit = ()
# push (Some 2);;
- : unit = ()
# push (Some 3);;
- : unit = ()
# Lwt.state (Lwt_stream.next stream);;
- : int Lwt.state = Lwt.Return 1
# Lwt.state (Lwt_stream.next stream);;
- : int Lwt.state = Lwt.Return 2
# Lwt.state (Lwt_stream.next stream);;
- : int Lwt.state = Lwt.Return 3
# Lwt.state (Lwt_stream.next stream);;
- : int Lwt.state = Lwt.Sleep
]}

  Note that streams are consumable. Once you take an element from a
  stream, it is removed from the stream. So, if you want to iterate two times
  over a stream, you may consider “cloning” it, with
  [Lwt_stream.clone]. Cloned stream will return the same
  elements in the same order. Consuming one will not consume the other.
  For example:

{[
# let s = Lwt_stream.of_list [1; 2];;
val s : int Lwt_stream.t = <abstr>
# let s' = Lwt_stream.clone s;;
val s' : int Lwt_stream.t = <abstr>
# Lwt.state (Lwt_stream.next s);;
- : int Lwt.state = Lwt.Return 1
# Lwt.state (Lwt_stream.next s);;
- : int Lwt.state = Lwt.Return 2
# Lwt.state (Lwt_stream.next s');;
- : int Lwt.state = Lwt.Return 1
# Lwt.state (Lwt_stream.next s');;
- : int Lwt.state = Lwt.Return 2
]}

{3 Mailbox variables }

  The [Lwt_mvar] module provides mailbox variables. A mailbox
  variable, also called a “mvar”, is a cell which may contain 0 or 1
  element. If it contains no elements, we say that the mvar is empty,
  if it contains one, we say that it is full. Adding an element to a
  full mvar will block until one is taken. Taking an element from an
  empty mvar will block until one is added.

  Mailbox variables are commonly used to pass messages between chains of
  callbacks being executed concurrently.

  Note that a mailbox variable can be seen as a pushable stream with a
  limited memory.

{1 Running an Lwt program }

  An [Lwt] computation you have created will give you something of type
  [Lwt.t], a promise. However, even though you have the promise, the
  computation may not have run yet, and the promise might still be pending.

  For example if your program is just:

{[
let _ = Lwt_io.printl "Hello, world!"
]}

  you have no guarantee that the promise for writing ["Hello, world!"]
  on the terminal will be resolved before the program exits. In order
  to wait for the promise to resolve, you have to call the function
  [Lwt_main.run]:

{[
val Lwt_main.run : 'a Lwt.t -> 'a
]}

  This function waits for the given promise to resolve and returns
  its result. In fact it does more than that; it also runs the
  scheduler which is responsible for making asynchronous computations progress
  when events are received from the outside world.

  So basically, when you write a [Lwt] program, you must call
  [Lwt_main.run] on your top-level, outer-most promise. For instance:

{[
let () = Lwt_main.run (Lwt_io.printl "Hello, world!")
]}

  Note that you must not make nested calls to [Lwt_main.run]. It
  cannot be used anywhere else to get the result of a promise.

{1 The [lwt.unix] library }

  The package [lwt.unix] contains all [Unix]-dependent
  modules of [Lwt]. Among all its features, it implements Lwt-friendly,
  non-blocking versions of functions of the OCaml standard and Unix libraries.

{2 Unix primitives }

  Module [Lwt_unix] provides non-blocking system calls. For example,
  the [Lwt] counterpart of [Unix.read] is:

{[
val read : file_descr -> string -> int -> int -> int Lwt.t
]}

  [Lwt_io] provides features similar to buffered channels of
  the standard library (of type [in_channel] or
  [out_channel]), but with non-blocking semantics.

  [Lwt_gc] allows you to register a finalizer that returns a
  promise. At the end of the program, [Lwt] will wait for all these
  finalizers to resolve.

{2 The Lwt scheduler }

  Operations doing I/O have to be resumed when some events are received by
  the process, so they can resolve their associated pending promises.
  For example, when you read from a file descriptor, you
  may have to wait for the file descriptor to become readable if no
  data are immediately available on it.

  [Lwt] contains a scheduler which is responsible for managing
  multiple operations waiting for events, and restarting them when needed.
  This scheduler is implemented by the two modules [Lwt_engine]
  and [Lwt_main]. [Lwt_engine] is a low-level module, it
  provides a signature for custom I/O multiplexers as well as two built-in
  implementations, [libev] and [select]. The signature is given by the
  class [Lwt_engine.t].

  [libev] is used by default on Linux, because it supports any
  number of file descriptors, while [select] supports only 1024. [libev]
  is also much more efficient. On Windows, [Unix.select] is used because
  [libev] does not work properly. The user may change the backend in use at
  any time.

  If you see an [Invalid_argument] error on [Unix.select], it
  may be because the 1024 file descriptor limit was exceeded. Try
  switching to [libev], if possible.

  The engine can also be used directly in order to integrate other
  libraries with [Lwt]. For example, [GTK] needs to be notified
  when some events are received. If you use [Lwt] with [GTK]
  you need to use the [Lwt] scheduler to monitor [GTK]
  sources. This is what is done by the [Lwt_glib] library.

  The [Lwt_main] module contains the {e main loop} of
  [Lwt]. It is run by calling the function [Lwt_main.run]:

{[
val Lwt_main.run : 'a Lwt.t -> 'a
]}

  This function continuously runs the scheduler until the promise passed
  as argument is resolved.

  To make sure [Lwt] is compiled with [libev] support,
  tell opam that the library is available on the system by installing the
  {{: https://opam.ocaml.org/packages/conf-libev/conf-libev.4-11/ }conf-libev}
  package. You may get the actual library with your system package manager:

{ul
  {- [brew install libev] on MacOSX,}
  {- [apt-get install libev-dev] on Debian/Ubuntu, or}
  {- [yum install libev-devel] on CentOS, which requires to set
     [export C_INCLUDE_PATH=/usr/include/libev/] and
     [export LIBRARY_PATH=/usr/lib64/] before calling
     [opam install conf-libev].}}


{2 Logging }

  For logging, we recommend the [logs] package from opam, which includes an
  Lwt-aware module [Logs_lwt].

{1 The Lwt.react library }

  The [Lwt_react] module provides helpers for using the [react]
  library with [Lwt]. It extends the [React] module by adding
  [Lwt]-specific functions. It can be used as a replacement of
  [React]. For example you can add at the beginning of your
  program:

{[
open Lwt_react
]}

  instead of:

{[
open React
]}

  or:

{[
module React = Lwt_react
]}

  Among the added functionalities we have [Lwt_react.E.next], which
  takes an event and returns a promise which will be pending until the next
  occurrence of this event. For example:

{[
# open Lwt_react;;
# let event, push = E.create ();;
val event : '_a React.event = <abstr>
val push : '_a -> unit = <fun>
# let p = E.next event;;
val p : '_a Lwt.t = <abstr>
# Lwt.state p;;
- : '_a Lwt.state = Lwt.Sleep
# push 42;;
- : unit = ()
# Lwt.state p;;
- : int Lwt.state = Lwt.Return 42
]}

  Another interesting feature is the ability to limit events
  (resp. signals) from occurring (resp. changing) too often. For example,
  suppose you are doing a program which displays something on the screen
  each time a signal changes. If at some point the signal changes 1000
  times per second, you probably don't want to render it 1000 times per
  second. For that you use [Lwt_react.S.limit]:

{[
val limit : (unit -> unit Lwt.t) -> 'a React.signal -> 'a React.signal
]}

  [Lwt_react.S.limit f signal] returns a signal which varies as
  [signal] except that two consecutive updates are separated by a
  call to [f]. For example if [f] returns a promise which is pending
  for 0.1 seconds, then there will be no more than 10 changes per
  second:

{[
open Lwt_react

let draw x =
  (* Draw the screen *)
  …

let () =
  (* The signal we are interested in: *)
  let signal = … in

  (* The limited signal: *)
  let signal' = S.limit (fun () -> Lwt_unix.sleep 0.1) signal in

  (* Redraw the screen each time the limited signal change: *)
  S.notify_p draw signal'
]}

{1 Other libraries }

{2 Parallelise computations to other cores }

  If you have some compute-intensive steps within your program, you can execute
  them on a separate core. You can get performance benefits from the
  parallelisation. In addition, whilst your compute-intensive function is running
  on a different core, your normal I/O-bound tasks continue running on the
  original core.

  The module {!Lwt_domain} from the [lwt_domain] package provides all the
  necessary helpers to achieve this. It is based on the [Domainslib] library
  and uses similar concepts (such as tasks and pools).

  First, you need to create a task pool:

{[
val setup_pool : ?name:string -> int -> pool
]}

  Then you simple detach the function calls to the created pool:

{[
val detach : pool -> ('a -> 'b) -> 'a -> 'b Lwt.t
]}

  The returned promise resolves as soon as the function returns.

{2 Detaching computation to preemptive threads }

  It may happen that you want to run a function which will take time to
  compute or that you want to use a blocking function that cannot be
  used in a non-blocking way. For these situations, [Lwt] allows you to
  {e detach} the computation to a preemptive thread.

  This is done by the module [Lwt_preemptive] of the
  [lwt.unix] package which maintains a pool of system
  threads. The main function is:

{[
val detach : ('a -> 'b) -> 'a -> 'b Lwt.t
]}

  [detach f x] will execute [f x] in another thread and
  return a pending promise, usable from the main thread, which will be fulfilled
  with the result of the preemptive thread.

  If you want to trigger some [Lwt] operations from your detached thread,
  you have to call back into the main thread using
  [Lwt_preemptive.run_in_main]:

{[
val run_in_main : (unit -> 'a Lwt.t) -> 'a
]}

  This is roughly the equivalent of [Lwt.main_run], but for detached
  threads, rather than for the whole process. Note that you must not call
  [Lwt_main.run] in a detached thread.

{2 SSL support }

  The library [Lwt_ssl] allows use of SSL asynchronously.

{1 Writing stubs using [Lwt] }

{2 Thread-safe notifications }

  If you want to notify the main thread from another thread, you can use the [Lwt]
  thread safe notification system. First you need to create a notification identifier
  (which is just an integer) from the OCaml side using the
  [Lwt_unix.make_notification] function, then you can send it from either the
  OCaml code with [Lwt_unix.send_notification] function, or from the C code using
  the function [lwt_unix_send_notification] (defined in [lwt_unix_.h]).

  Notifications are received and processed asynchronously by the main thread.

{2 Jobs }

  For operations that cannot be executed asynchronously, [Lwt]
  uses a system of jobs that can be executed in a different threads. A
  job is composed of three functions:

{ul
  {- A stub function to create the job. It must allocate a new job
     structure and fill its [worker] and [result] fields. This
     function is executed in the main thread.
     The return type for the OCaml external must be of the form ['a job].}
  {- A function which executes the job. This one may be executed asynchronously
     in another thread. This function must not:
     {ul
       {- access or allocate OCaml block values (tuples, strings, …),}
       {- call OCaml code.}}}
  {- A function which reads the result of the job, frees resources and
     returns the result as an OCaml value. This function is executed in
     the main thread.}}

  With [Lwt < 2.3.3], 4 functions (including 3 stubs) were
  required. It is still possible to use this mode but it is
  deprecated.

  We show as example the implementation of [Lwt_unix.mkdir]. On the C
  side we have:

{@c[/**/
/* Structure holding informations for calling [mkdir]. */
struct job_mkdir {
  /* Informations used by lwt.
     It must be the first field of the structure. */
  struct lwt_unix_job job;
  /* This field store the result of the call. */
  int result;
  /* This field store the value of [errno] after the call. */
  int errno_copy;
  /* Pointer to a copy of the path parameter. */
  char* path;
  /* Copy of the mode parameter. */
  int mode;
  /* Buffer for storing the path. */
  char data[];
};

/* The function calling [mkdir]. */
static void worker_mkdir(struct job_mkdir* job)
{
  /* Perform the blocking call. */
  job->result = mkdir(job->path, job->mode);
  /* Save the value of errno. */
  job->errno_copy = errno;
}

/* The function building the caml result. */
static value result_mkdir(struct job_mkdir* job)
{
  /* Check for errors. */
  if (job->result < 0) {
    /* Save the value of errno so we can use it
       once the job has been freed. */
    int error = job->errno_copy;
    /* Copy the contents of job->path into a caml string. */
    value string_argument = caml_copy_string(job->path);
    /* Free the job structure. */
    lwt_unix_free_job(&job->job);
    /* Raise the error. */
    unix_error(error, "mkdir", string_argument);
  }
  /* Free the job structure. */
  lwt_unix_free_job(&job->job);
  /* Return the result. */
  return Val_unit;
}

/* The stub creating the job structure. */
CAMLprim value lwt_unix_mkdir_job(value path, value mode)
{
  /* Get the length of the path parameter. */
  mlsize_t len_path = caml_string_length(path) + 1;
  /* Allocate a new job. */
  struct job_mkdir* job =
    (struct job_mkdir*)lwt_unix_new_plus(struct job_mkdir, len_path);
  /* Set the offset of the path parameter inside the job structure. */
  job->path = job->data;
  /* Copy the path parameter inside the job structure. */
  memcpy(job->path, String_val(path), len_path);
  /* Initialize function fields. */
  job->job.worker = (lwt_unix_job_worker)worker_mkdir;
  job->job.result = (lwt_unix_job_result)result_mkdir;
  /* Copy the mode parameter. */
  job->mode = Int_val(mode);
  /* Wrap the structure into a caml value. */
  return lwt_unix_alloc_job(&job->job);
}
]}

  and on the ocaml side:

{[
(* The stub for creating the job. *)
external mkdir_job : string -> int -> unit job = "lwt_unix_mkdir_job"

(* The ocaml function. *)
let mkdir name perms = Lwt_unix.run_job (mkdir_job name perms)
]}