package uwt

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Stream handles provide an abstraction of a duplex communication channel. Stream.t is an abstract type, libuv provides 3 stream implementations in the form of Tcp.t, Pipe.t and Tty.t.

type t
include module type of Handle with type t := t
val close : t -> Int_result.unit

Handles are closed automatically, if they are not longer referenced from the OCaml heap. Nevertheless, you should nearly always close them with close, because:

  • if they wrap a file descriptor, you will sooner or later run out of file descriptors. The OCaml garbage collector doesn't give any guarantee, when orphaned memory blocks are removed.
  • you might have registered some repeatedly called action (e.g. timeout, read_start,...), that prevent that all references get removed from the OCaml heap.

However, it's safe to write code in this manner:

let s = Uwt.Tcp.init () in
let c = Uwt.Tcp.init () in
Uwt.Tcp.nodelay s false;
Uwt.Tcp.simultaneous_accepts true;
if foobar () then (* no file descriptor yet assigned, no need to worry
                     about exceptions inside foobar,... *)
  Lwt.return_unit (* no need to close *)

If you want - for whatever reason - keep a file descriptor open for the whole lifetime of your process, remember to keep a reference to its handle.

val close_noerr : t -> unit
val close_wait : t -> unit Lwt.t

Prefer close or close_noerr to close_wait. close or close_noerr return immediately (there are no useful error messages, beside perhaps a notice, that you've already closed that handle).

close_wait is only useful, if you intend to wait until all concurrent write and read threads related to this handle are canceled.

val is_active : t -> bool

Returns non-zero if the handle is active, zero if it's inactive. What "active" means depends on the type of handle:

  • A Async.t handle is always active and cannot be deactivated, except by closing it with uv_close().
  • A Pipe.t, Tcp.t, Udp.t, etc. handle - basically any handle that deals with i/o - is active when it is doing something that involves i/o, like reading, writing, connecting, accepting new connections, etc.

Rule of thumb: if a handle of type Uwt.Foo.t has a uv_foo_start() function, then it's active from the moment that function is called. Likewise, uv_foo_stop() deactivates the handle again.

val ref' : t -> unit

Reference the given handle. References are idempotent, that is, if a handle is already referenced calling this function again will have no effect.

val unref : t -> unit

Un-reference the given handle. References are idempotent, that is, if a handle is not referenced calling this function again will have no effect.

val has_ref : t -> bool

Returns non-zero if the handle is referenced, zero otherwise.

val to_handle : t -> Handle.t
val is_readable : t -> bool
val is_writable : t -> bool
val read_start : t -> cb:(Bytes.t uv_result -> unit) -> Int_result.unit

Read data from an incoming stream. The ~cb will be made several times until there is no more data to read or read_stop is called.

val read_start_exn : t -> cb:(Bytes.t uv_result -> unit) -> unit
val read_stop : t -> Int_result.unit

Stop reading data from the stream.

val read_stop_exn : t -> unit
val read : ?pos:int -> ?len:int -> t -> buf:bytes -> int Lwt.t

There is currently no uv_read function in libuv, just uv_read_start and uv_read_stop. This is a wrapper for your convenience. It calls read_stop internally, if you don't continue with reading immediately. Zero result indicates EOF.

In future libuv versions, there might be uv_read and uv_try_read functions (it was discussed several times). If these changes got merged, will wrap them - even if there will be small semantic differences.

It is currently not possible to start several read threads in parallel, you must serialize the requests manually. In the following example t2 will fail with EBUSY:

let t1 = t ~buf:buf1 in
let t2 = t ~buf:buf2 in
(* ... *)

Calling the function with ~len:0 has a dubious, system dependent semantic.

val read_ba : ?pos:int -> ?len:int -> t -> buf:buf -> int Lwt.t
val write_queue_size : t -> int

Returns the amount of queued bytes waiting to be sent

val try_write : ?pos:int -> ?len:int -> t -> buf:bytes ->

Write data to stream, but won't queue a write request if it can't be completed immediately.

val try_write_ba : ?pos:int -> ?len:int -> t -> buf:buf ->
val try_write_string : ?pos:int -> ?len:int -> t -> buf:string ->
val write : ?pos:int -> ?len:int -> t -> buf:bytes -> unit Lwt.t

Write data to stream

val write_string : ?pos:int -> ?len:int -> t -> buf:string -> unit Lwt.t
val write_ba : ?pos:int -> ?len:int -> t -> buf:buf -> unit Lwt.t
val write_raw : ?pos:int -> ?len:int -> t -> buf:bytes -> unit Lwt.t

write is eager - like the counterparts inside Lwt_unix. It first calls try_write internally to check if it can return immediately (without the overhead of creating a sleeping thread and waking it up later). If it can't write everything instantly, it will call write_raw internally. write_raw is exposed here mainly in order to write unit tests for it. But you can also use it, if you your ~buf is very large or you know for another reason, that try_write will fail.

val write_raw_string : ?pos:int -> ?len:int -> t -> buf:string -> unit Lwt.t
val write_raw_ba : ?pos:int -> ?len:int -> t -> buf:buf -> unit Lwt.t
val try_writev : t -> Iovec_write.t list ->

Windows doesn't support writing multiple buffers with a single syscall for some HANDLEs (e.g. it's supported for tcp handles, but not pipes). uwt then writes the buffers one by one

If the number of buffers is greater than IOV_MAX, libuv already contains the necessary workarounds

val writev : t -> Iovec_write.t list -> unit Lwt.t

See comment to



! This function will fail with Unix.EOPNOTSUPP on Windows for e.g. pipe handles

val writev_emul : t -> Iovec_write.t list -> unit Lwt.t

Similar to writev, but if passing several buffers at once is not supported by the OS, the buffers will be written one by one. Please note that as a consequence you should not start several writev_emul threads in parallel. The writing order would be surprising in this case. If you don't use windows, this function is identic to writev

val writev_raw : t -> Iovec_write.t list -> unit Lwt.t
val listen : t -> max:int -> cb:(t -> Int_result.unit -> unit) -> Int_result.unit

Start listening for incoming connections. ~max indicates the number of connections the kernel might queue, same as listen(2). When a new incoming connection is received ~cb is called.

val listen_exn : t -> max:int -> cb:(t -> Int_result.unit -> unit) -> unit
val shutdown : t -> unit Lwt.t

Shutdown the outgoing (write) side of a duplex stream. It waits for pending write requests to complete.

val set_blocking : t -> bool -> Int_result.unit

Just don't use this function. It will only cause trouble.


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