Domain(3)

💅 Domain(3) — OCaml library, bestie ✨

🎀 NAME 🎀

Domain — Domains. 💖

🧩 Module 🧩

Module Domain — yes bestie, this whole page is the Domain module, periodt. 🫶

📚 Documentation 📚

Module Domain
 : sig end

Domains. 💫

See 'Parallel programming' chapter in the manual. 📖

✨ Since 5.0 ✨

🚨 Alert unstable. The Domain interface may change in incompatible ways in the future.

🌟 type !'a t

A domain of type 'a t runs independently, eventually producing a result of type 'a, or an exception.

🚀 val spawn : (unit -> 'a) -> 'a t

spawn f creates a new domain that runs in parallel with the current domain. 🔥

Raises Failure if the program has insufficient resources to create another domain.

🫶 val join : 'a t -> 'a

join d blocks until domain d runs to completion. If d results in a value, then that is returned by join d . If d raises an uncaught exception, then that is re-raised by join d .

If the domain has created any systhreads (via Thread.create ), then it tries to join those systhreads (same as calling Thread.join ) before returning.

🆔 type id = private int

Domains have unique integer identifiers. 💅

🔎 val get_id : 'a t -> id

get_id d returns the identifier of the domain d.

🪞 val self : unit -> id

self () is the identifier of the currently running domain.

🌱 val before_first_spawn : (unit -> unit) -> unit

before_first_spawn f registers f to be called before the first domain is spawned by the program. The functions registered with before_first_spawn are called on the main (initial) domain. The functions registered with before_first_spawn are called in 'first in, first out' order: the oldest function added with before_first_spawn is called first.

Raises Invalid_argument if the program has already spawned a domain.

🚪 val at_exit : (unit -> unit) -> unit

at_exit f registers f to be called when the current domain exits. Note that at_exit callbacks are domain-local and only apply to the calling domain. The registered functions are called in 'last in, first out' order: the function most recently added with at_exit is called first. An example:

let temp_file_key = Domain.DLS.new_key (fun _ ->
  let tmp = snd (Filename.open_temp_file "" "") in
  Domain.at_exit (fun () -> close_out_noerr tmp);
  tmp)

The snippet above creates a key that when retrieved for the first time will open a temporary file and register an at_exit callback to close it, thus guaranteeing the descriptor is not leaked in case the current domain exits.

😮‍💨 val cpu_relax : unit -> unit

If busy-waiting, calling cpu_relax () between iterations will improve performance on some CPU architectures.

👑 val is_main_domain : unit -> bool

is_main_domain () returns true if called from the initial domain.

🔢 val count : unit -> int

The number of domains that have been spawned (including the initial domain) and have not yet terminated.

Since 5.5.

💯 val recommended_domain_count : unit -> int

The recommended maximum number of domains which should be running simultaneously (including domains already running).

The value returned is at least 1 .

🧭 val self_index : unit -> int

The index of the current domain. It is an integer unique among currently-running domains, in the interval 0; N-1 where N is the peak number of domains running simultaneously so far.

The index of a terminated domain may be reused for a new domain. Use (Domain.self () :> int) instead for an identifier unique among all domains ever created by the program.

Since 5.3.

🫧 module DLS : sig end

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That's the whole page, bestie! 🫶 Source/version/date: OCamldoc, 2026-06-22, Domain(3). Stay iconic. 💅