Module Lwt_io

module Lwt_io : sig..end

Buffered byte channels

A channel is a high-level object for performing input/output (IO). It allows to read/write from/to the outside world in an efficient way, by minimising the number of system calls.

An output channel is used to send data and an input channel is used to receive data.

If you are familiar with buffered channels you may be familiar too with the flush operation. Note that byte channels of this module are automatically flushed when there is nothing else to do (i.e. before the program becomes idle), so this means that you no longer have to write:

eprintf "log message\n";
      flush stderr;

to have your messages displayed.

Note about errors: input functions of this module raise End_of_file when the end-of-file is reached (i.e. when the read function returns 0). Other exceptions are ones caused by the backend read/write functions, such as Unix.Unix_error.

exception Channel_closed of string

Exception raised when a channel is closed. The parameter is a description of the channel.


type 'mode channel

Type of buffered byte channels

type input

Input mode

type output

Output mode

type 'a mode =  | Input | Output

Channel mode

val input : input mode

input input mode representation

val output : output mode

output output mode representation

type input_channel = input channel

Type of input channels

type output_channel = output channel

Type of output channels

val mode : 'a channel -> 'a mode

mode ch returns the mode of a channel

Well-known instances

val stdin : input_channel

The standard input, it reads data from Lwt_unix.​stdin

val stdout : output_channel

The standard output, it writes data to Lwt_unix.​stdout

val stderr : output_channel

The standard output for error messages, it writes data to Lwt_unix.​stderr

val zero : input_channel

Inputs which returns always '\x00'

val null : output_channel

Output which drops everything

Channels creation/manipulation

val pipe : 
  ?in_buffer:Lwt_bytes.t ->
  ?out_buffer:Lwt_bytes.t ->
  unit -> input_channel * output_channel

pipe ?in_buffer ?out_buffer () creates a pipe using Lwt_unix.​pipe and makes two channels from the two returned file descriptors

val make : 
  ?buffer:Lwt_bytes.t ->
  ?close:(unit -> unit Lwt.t) ->
  ?seek:(int64 -> Unix.seek_command -> int64 Lwt.t) ->
  mode:'mode mode ->
  (Lwt_bytes.t -> int -> int -> int Lwt.t) -> 'mode channel

make ?buffer ?close ~mode perform_io is the main function for creating new channels.

buffer : user-supplied buffer. When this argument is present, its value will be used as the buffer for the created channel. The size of buffer must conform to the limitations described in Lwt_io.​set_default_buffer_size. When this argument is not present, a new internal buffer of default size will be allocated for this channel.

Warning: do not use the same buffer for simultaneous work with more than one channel.

There are other functions in this module that take a buffer argument, sharing the same semantics.

close : close function of the channel. It defaults to Lwt.return

seek : same meaning as Unix.lseek

val of_bytes : mode:'mode mode -> Lwt_bytes.t -> 'mode channel

Create a channel from a byte array. Reading/writing is done directly on the provided array.

val of_fd : 
  ?buffer:Lwt_bytes.t ->
  ?close:(unit -> unit Lwt.t) ->
  mode:'mode mode -> Lwt_unix.file_descr -> 'mode channel

of_fd ?buffer ?close ~mode fd creates a channel from a file descriptor.

close : defaults to closing the file descriptor.

val of_unix_fd : 
  ?buffer:Lwt_bytes.t ->
  ?close:(unit -> unit Lwt.t) ->
  mode:'mode mode -> Unix.file_descr -> 'mode channel

of_unix_fd ?buffer ?close ~mode fd is a short-hand for:

of_fd ?buffer ?close (Lwt_unix.of_unix_file_descr fd)

val close : 'a channel -> unit Lwt.t

close ch closes the given channel. If ch is an output channel, it performs all pending actions, flushes it and closes it. If ch is an input channel, it just closes it immediately.

close returns the result of the close function of the channel. Multiple calls to close will return exactly the same value.

Note: you cannot use close on channels obtained with Lwt_io.​atomic.

val abort : 'a channel -> unit Lwt.t

abort ch abort current operations and close the channel immediately.

val atomic : ('a channel -> 'b Lwt.t) -> 'a channel -> 'b Lwt.t

atomic f transforms a sequence of io operations into one single atomic io operation.


  • the channel passed to f is invalid after f terminates
  • atomic can be called inside another atomic
val file_length : string -> int64 Lwt.t

Returns the length of a file

val buffered : 'a channel -> int

buffered oc returns the number of bytes in the buffer

val flush : output_channel -> unit Lwt.t

flush oc performs all pending writes on oc

val flush_all : unit -> unit Lwt.t

flush_all () flushes all open output channels

val buffer_size : 'a channel -> int

Returns the size of the internal buffer.

val resize_buffer : 'a channel -> int -> unit Lwt.t

Resize the internal buffer to the given size

val is_busy : 'a channel -> bool

is_busy channel returns whether the given channel is currently busy. A channel is busy when there is at least one job using it that has not yet terminated.

Random access

val position : 'a channel -> int64

position ch Returns the current position in the channel.

val set_position : 'a channel -> int64 -> unit Lwt.t

set_position ch pos Sets the position in the output channel. This does not work if the channel does not support random access.

val length : 'a channel -> int64 Lwt.t

Returns the length of the channel in bytes


Note: except for functions dealing with streams (Lwt_io.​read_chars and Lwt_io.​read_lines) all functions are atomic.

val read_char : input_channel -> char Lwt.t

read_char ic reads the next character of ic. Raises End_of_file if the end of the file is reached

val read_char_opt : input_channel -> char option Lwt.t

Same as read_byte but does not raise End_of_file on end of input

val read_chars : input_channel -> char Lwt_stream.t

read_chars ic returns a stream holding all characters of ic

val read_line : input_channel -> string Lwt.t

read_line ic reads one complete line from ic and returns it without the end of line. End of line is either "\n" or "\r\n".

If the end of line is reached before reading any character, End_of_file is raised. If it is reached before reading an end of line but characters have already been read, they are returned.

val read_line_opt : input_channel -> string option Lwt.t

Same as Lwt_io.​read_line but do not raise End_of_file on end of input.

val read_lines : input_channel -> string Lwt_stream.t

read_lines ic returns a stream holding all lines of ic

val read : ?count:int -> input_channel -> string Lwt.t

read ?count ic reads at most count characters from ic. It returns "" if the end of input is reached. If count is not specified, it reads all bytes until the end of input.

val read_into : input_channel -> bytes -> int -> int -> int Lwt.t

read_into ic buffer offset length reads up to length bytes, stores them in buffer at offset offset, and returns the number of bytes read.

Note: read_into does not raise End_of_file, it returns a length of 0 instead.

val read_into_exactly : input_channel -> bytes -> int -> int -> unit Lwt.t

read_into_exactly ic buffer offset length reads exactly length bytes and stores them in buffer at offset offset. Raises End_of_file on end of input

val read_value : input_channel -> 'a Lwt.t

read_value channel reads a marshaled value from channel; it corresponds to the standard library's Marshal.from_channel. The corresponding writing function is Lwt_io.​write_value.

Note that reading marshaled values is not, in general, type-safe. See the warning in the description of module Marshal for details. The short version is: if you read a value of one type, such as string, when a value of another type, such as int has actually been marshaled to channel, you may get arbitrary behavior, including segmentation faults, access violations, security bugs, etc.


Note: as for reading functions, all functions except Lwt_io.​write_chars and Lwt_io.​write_lines are atomic.

For example if you use Lwt_io.​write_line in two different threads, the two operations will be serialized, and lines cannot be mixed.

val write_char : output_channel -> char -> unit Lwt.t

write_char oc char writes char on oc

val write_chars : output_channel -> char Lwt_stream.t -> unit Lwt.t

write_chars oc chars writes all characters of chars on oc

val write : output_channel -> string -> unit Lwt.t

write oc str writes all characters of str on oc

val write_line : output_channel -> string -> unit Lwt.t

write_line oc str writes str on oc followed by a new-line.

val write_lines : output_channel -> string Lwt_stream.t -> unit Lwt.t

write_lines oc lines writes all lines of lines to oc

val write_from : output_channel -> bytes -> int -> int -> int Lwt.t

write_from oc buffer offset length writes up to length bytes to oc, from buffer at offset offset and returns the number of bytes actually written

val write_from_string : output_channel -> string -> int -> int -> int Lwt.t
val write_from_exactly : 
  output_channel -> bytes -> int -> int -> unit Lwt.t

write_from_exactly oc buffer offset length writes all length bytes from buffer at offset offset to oc

val write_from_string_exactly : 
  output_channel -> string -> int -> int -> unit Lwt.t
val write_value : 
  output_channel ->
  ?flags:Marshal.extern_flags list -> 'a -> unit Lwt.t

write_value channel ?flags v writes v to channel using the Marshal module of the standard library. See Marshal.to_channel for an explanation of ?flags.

The corresponding reading function is Lwt_io.​read_value. See warnings about type safety in the description of Lwt_io.​read_value.


These functions are basically helpers. Also you may prefer using the name Lwt_io.​printl rather than Lwt_io.​write_line because it is shorter.

The general name of a printing function is <prefix>print<suffixes>,

where <prefix> is one of:

  • 'f', which means that the function takes as argument a channel
  • nothing, which means that the function prints on Lwt_io.​stdout
  • 'e', which means that the function prints on Lwt_io.​stderr

and <suffixes> is a combination of:

  • 'l' which means that a new-line character is printed after the message
  • 'f' which means that the function takes as argument a format instead of a string
val fprint : output_channel -> string -> unit Lwt.t
val fprintl : output_channel -> string -> unit Lwt.t
val fprintf : 
  output_channel ->
  ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a
val fprintlf : 
  output_channel ->
  ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a
val print : string -> unit Lwt.t
val printl : string -> unit Lwt.t
val printf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a
val printlf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a
val eprint : string -> unit Lwt.t
val eprintl : string -> unit Lwt.t
val eprintf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a
val eprintlf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a


val hexdump_stream : output_channel -> char Lwt_stream.t -> unit Lwt.t

hexdump_stream oc byte_stream produces the same output as the command hexdump -C.

val hexdump : output_channel -> string -> unit Lwt.t

hexdump oc str = hexdump_stream oc (Lwt_stream.of_string str)

File utilities

type file_name = string

Type of file names

val open_file : 
  ?buffer:Lwt_bytes.t ->
  ?flags:Unix.open_flag list ->
  ?perm:Unix.file_perm ->
  mode:'a mode -> file_name -> 'a channel Lwt.t

open_file ?buffer ?flags ?perm ~mode filename opens the file with name filename and returns a channel for reading/writing it. Raises Unix.Unix_error on error.

val with_file : 
  ?buffer:Lwt_bytes.t ->
  ?flags:Unix.open_flag list ->
  ?perm:Unix.file_perm ->
  mode:'a mode ->
  file_name -> ('a channel -> 'b Lwt.t) -> 'b Lwt.t

with_file ?buffer ?flags ?perm ~