package lwt

  1. Overview
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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 =
  1. | Input : input mode
  2. | Output : output mode

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.

  • parameter 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 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.

  • parameter close

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

  • parameter seek

    same meaning as Unix.lseek

  • parameter perform_io

    is the read or write function. It is called when more input is needed or when the buffer need to be flushed.

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.

  • parameter 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 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

Retrieves the length of the file at the given path. If the path refers to a directory, the returned promise is rejected with Unix.(Unix_error (EISDIR, _, _)).

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.

val is_closed : 'a channel -> bool

is_closed channel returns whether the given channel is currently closed.

  • since 4.2.0

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 (read_chars and 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 Lwt_io.read_char, 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 input 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 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

If ~count is specified, read ~count ic reads at most ~count bytes from ic in one read operation. Note that fewer than ~count bytes can be read. This can happen for multiple reasons, including end of input, or no more data currently available. Check the size of the resulting string. read resolves with "" if the input channel is already at the end of input.

If ~count is not specified, read ic 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_into_bigstring : input_channel -> Lwt_bytes.t -> int -> int -> int Lwt.t
val read_into_exactly_bigstring : input_channel -> Lwt_bytes.t -> int -> int -> unit Lwt.t
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 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 write_chars and write_lines are atomic.

For example if you use 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_bigstring : output_channel -> Lwt_bytes.t -> int -> int -> int Lwt.t
val write_from_string : output_channel -> string -> int -> int -> int Lwt.t

See write.

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_exactly_bigstring : output_channel -> Lwt_bytes.t -> int -> int -> unit Lwt.t
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 read_value. See warnings about type safety in the description of read_value.


These functions are basically helpers. Also you may prefer using the name printl rather than 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 stdout
  • 'e', which means that the function prints on 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

%! does nothing here. To flush the channel, use Lwt_io.flush channel.

val fprintlf : output_channel -> ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a

%! does nothing here. To flush the channel, use Lwt_io.flush channel.

val print : string -> unit Lwt.t
val printl : string -> unit Lwt.t
val printf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a

%! does nothing here. To flush the channel, use Lwt_io.(flush stdout).

val printlf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a

%! does nothing here. To flush the channel, use Lwt_io.(flush stdout).

val eprint : string -> unit Lwt.t
val eprintl : string -> unit Lwt.t
val eprintf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a

%! does nothing here. To flush the channel, use Lwt_io.(flush stderr).

val eprintlf : ('a, unit, string, unit Lwt.t) Pervasives.format4 -> 'a

%! does nothing here. To flush the channel, use Lwt_io.(flush stderr).


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

Lwt_io.open_file ~mode file opens the given file, either for reading (with ~mode:Input) or for writing (with ~mode:Output). The returned channel provides buffered I/O on the file.

If ~buffer is supplied, it is used as the I/O buffer.

If ~flags is supplied, the file is opened with the given flags (see Unix.open_flag). Note that ~flags is used exactly as given. For example, opening a file with ~flags and ~mode:Input does not implicitly add O_RDONLY. So, you should include O_RDONLY when opening for reading (~mode:Input), and O_WRONLY when opening for writing (~mode:Input). It is also recommended to include O_NONBLOCK, unless you are sure that the file cannot be a socket or a named pipe.

The default permissions used for creating new files are 0o666, i.e. reading and writing are allowed for the file owner, group, and everyone. These default permissions can be overridden by supplying ~perm.

Note: if opening for writing (~mode:Output), and the file already exists, open_file truncates (clears) the file by default. If you would like to keep the pre-existing contents of the file, use the ~flags parameter to pass a custom flags list that does not include Unix.O_TRUNC.

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

Lwt_io.with_file ~mode filename f opens the given using Lwt_io.open_file, and passes the resulting channel to f. Lwt_io.with_file ensures that the channel is closed when the promise returned by f resolves, or if f raises an exception.

See Lwt_io.open_file for a description of the arguments, warnings, and other notes.

val open_temp_file : ?buffer:Lwt_bytes.t -> ?flags:Unix.open_flag list -> ?perm:Unix.file_perm -> ?temp_dir:string -> ?prefix:string -> ?suffix:string -> unit -> (string * output_channel) Lwt.t

open_temp_file () starts creating a new temporary file, and evaluates to a promise for the pair of the file's name, and an output channel for writing to the file.

The caller should take care to delete the file later. Alternatively, see Lwt_io.with_temp_file.

The ?buffer and ?perm arguments are passed directly to an internal call to Lwt_io.open_file.

If not specified, ?flags defaults to [O_CREATE; O_EXCL; O_WRONLY; O_CLOEXEC]. If specified, the specified flags are used exactly. Note that these should typically contain at least O_CREAT and O_EXCL, otherwise open_temp_file may open an existing file.

?temp_dir can be used to choose the directory in which the file is created. For the current directory, use Filename.current_dir_name. If not specified, the directory is taken from Filename.get_temp_dir_name, which is typically set to your system temporary file directory.

?prefix helps determine the name of the file. It will be the prefix concatenated with a random sequence of characters. If not specified, open_temp_file uses some default prefix.

?suffix is like prefix, but it is appended at the end of the filename. In particular, it can be used to set the extension. This argument is supported since Lwt 4.4.0.

  • since 3.2.0
val with_temp_file : ?buffer:Lwt_bytes.t -> ?flags:Unix.open_flag list -> ?perm:Unix.file_perm -> ?temp_dir:string -> ?prefix:string -> ?suffix:string -> ((string * output_channel) -> 'b Lwt.t) -> 'b Lwt.t

with_temp_file f calls open_temp_file (), passing all optional arguments directly to it. It then attaches f to run after the file is created, passing the filename and output channel to f. When the promise returned by f is resolved, with_temp_file closes the channel and deletes the temporary file by calling Lwt_unix.unlink.

  • since 3.2.0
val create_temp_dir : ?perm:Unix.file_perm -> ?parent:string -> ?prefix:string -> ?suffix:string -> unit -> string Lwt.t

Creates a temporary directory, and returns a promise that resolves to its path. The caller must take care to remove the directory. Alternatively, see Lwt_io.with_temp_dir.

If ~perm is specified, the directory is created with the given permissions. The default permissions are 0755.

~parent is the directory in which the temporary directory is created. If not specified, the default value is the result of Filename.get_temp_dir_name ().

~prefix is prepended to the directory name, and ~suffix is appended to it.

  • since 4.4.0
val with_temp_dir : ?perm:Unix.file_perm -> ?parent:string -> ?prefix:string -> ?suffix:string -> (string -> 'a Lwt.t) -> 'a Lwt.t

with_temp_dir f first calls create_temp_dir, forwarding all optional arguments to it. Once the temporary directory is created at path, with_temp_dir f calls f path. When the promise returned by f path is resolved, with_temp_dir f recursively deletes the temporary directory and all its contents.

  • since 4.4.0
val open_connection : ?fd:Lwt_unix.file_descr -> ?in_buffer:Lwt_bytes.t -> ?out_buffer:Lwt_bytes.t -> Unix.sockaddr -> (input_channel * output_channel) Lwt.t

open_connection ?fd ?in_buffer ?out_buffer addr opens a connection to the given address and returns two channels for using it. If fd is not specified, a fresh one will be used.

The connection is completely closed when you close both channels.

val with_connection : ?fd:Lwt_unix.file_descr -> ?in_buffer:Lwt_bytes.t -> ?out_buffer:Lwt_bytes.t -> Unix.sockaddr -> ((input_channel * output_channel) -> 'a Lwt.t) -> 'a Lwt.t

with_connection ?fd ?in_buffer ?out_buffer addr f opens a connection to the given address and passes the channels to f

type server

Type of a server

val establish_server_with_client_socket : ?server_fd:Lwt_unix.file_descr -> ?backlog:int -> ?no_close:bool -> Unix.sockaddr -> (Lwt_unix.sockaddr -> Lwt_unix.file_descr -> unit Lwt.t) -> server Lwt.t

establish_server_with_client_socket listen_address f creates a server which listens for incoming connections on listen_address. When a client makes a new connection, it is passed to f: more precisely, the server calls

f client_address client_socket

where client_address is the address (peer name) of the new client, and client_socket is the socket connected to the client.

The server does not block waiting for f to complete: it concurrently tries to accept more client connections while f is handling the client.

When the promise returned by f completes (i.e., f is done handling the client), establish_server_with_client_socket automatically closes client_socket. This is a default behavior that is useful for simple cases, but for a robust application you should explicitly close these channels yourself, and handle any exceptions as appropriate. If the channels are still open when f completes, and their automatic closing raises an exception, establish_server_with_client_socket treats it as an unhandled exception reaching the top level of the application: it passes that exception to Lwt.async_exception_hook, the default behavior of which is to print the exception and terminate your process.

Automatic closing can be completely disabled by passing ~no_close:true.

Similarly, if f raises an exception (or the promise it returns fails with an exception), establish_server_with_client_socket can do nothing with that exception, except pass it to Lwt.async_exception_hook.

~server_fd can be specified to use an existing file descriptor for listening. Otherwise, a fresh socket is created internally. In either case, establish_server_with_client_socket will internally assign listen_address to the server socket.

~backlog is the argument passed to Lwt_unix.listen. Its default value is SOMAXCONN, which varies by platform and socket kind.

The returned promise (a server Lwt.t) resolves when the server has just started listening on listen_address: right after the internal call to listen, and right before the first internal call to accept.

  • since 4.1.0
val establish_server_with_client_address : ?fd:Lwt_unix.file_descr -> ?buffer_size:int -> ?backlog:int -> ?no_close:bool -> Unix.sockaddr -> (Lwt_unix.sockaddr -> (input_channel * output_channel) -> unit Lwt.t) -> server Lwt.t

Like Lwt_io.establish_server_with_client_socket, but passes two buffered channels to the connection handler f. These channels wrap the client socket.

The channels are closed automatically when the promise returned by f resolves. To avoid this behavior, pass ~no_close:true.

  • since 3.1.0
val shutdown_server : server -> unit Lwt.t

Closes the given server's listening socket. The returned promise resolves when the close(2) system call completes. This function does not affect the sockets of connections that have already been accepted, i.e. passed to f by establish_server.

  • since 3.0.0
val lines_of_file : file_name -> string Lwt_stream.t

lines_of_file name returns a stream of all lines of the file with name name. The file is automatically closed when all lines have been read.

val lines_to_file : file_name -> string Lwt_stream.t -> unit Lwt.t

lines_to_file name lines writes all lines of lines to file with name name.

val chars_of_file : file_name -> char Lwt_stream.t

chars_of_file name returns a stream of all characters of the file with name name. As for lines_of_file the file is closed when all characters have been read.

val chars_to_file : file_name -> char Lwt_stream.t -> unit Lwt.t

chars_to_file name chars writes all characters of chars to name

Input/output of integers

module type NumberIO = sig ... end

Common interface for reading/writing integers in binary

module LE : NumberIO

Reading/writing of numbers in little-endian

module BE : NumberIO

Reading/writing of numbers in big-endian

Reading/writing of numbers in the system endianness.

include NumberIO


val read_int : input_channel -> int Lwt.t

Reads a 32-bits integer as an ocaml int

val read_int16 : input_channel -> int Lwt.t
val read_int32 : input_channel -> int32 Lwt.t
val read_int64 : input_channel -> int64 Lwt.t
val read_float32 : input_channel -> float Lwt.t

Reads an IEEE single precision floating point value

val read_float64 : input_channel -> float Lwt.t

Reads an IEEE double precision floating point value


val write_int : output_channel -> int -> unit Lwt.t

Writes an ocaml int as a 32-bits integer

val write_int16 : output_channel -> int -> unit Lwt.t
val write_int32 : output_channel -> int32 -> unit Lwt.t
val write_int64 : output_channel -> int64 -> unit Lwt.t
val write_float32 : output_channel -> float -> unit Lwt.t

Writes an IEEE single precision floating point value

val write_float64 : output_channel -> float -> unit Lwt.t

Writes an IEEE double precision floating point value

type byte_order = Lwt_sys.byte_order =
  1. | Little_endian
  2. | Big_endian

    Type of byte order

val system_byte_order : byte_order

Low-level access to the internal buffer

val block : 'a channel -> int -> (Lwt_bytes.t -> int -> 'b Lwt.t) -> 'b Lwt.t

block ch size f pass to f the internal buffer and an offset. The buffer contains size chars at offset. f may read or write these chars. size must satisfy 0 <= size <= 16

type direct_access = {
  1. da_buffer : Lwt_bytes.t;

    The internal buffer

  2. mutable da_ptr : int;

    The pointer to:

    • the beginning of free space for output channels
    • the beginning of data for input channels
  3. mutable da_max : int;

    The maximum offset

  4. da_perform : unit -> int Lwt.t;
    • for input channels: refills the buffer and returns how many bytes have been read
    • for output channels: flush partially the buffer and returns how many bytes have been written

Information for directly accessing the internal buffer of a channel

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

direct_access ch f passes to f a direct_access structure. f must use it and update da_ptr to reflect how many bytes have been read/written.


val default_buffer_size : unit -> int

Return the default size for buffers. Channels that are created without a specific buffer use new buffer of this size.

val set_default_buffer_size : int -> unit

Change the default buffer size.

  • raises Invalid_argument

    if the given size is smaller than 16 or greater than Sys.max_string_length


val establish_server : ?fd:Lwt_unix.file_descr -> ?buffer_size:int -> ?backlog:int -> ?no_close:bool -> Unix.sockaddr -> ((input_channel * output_channel) -> unit Lwt.t) -> server Lwt.t

Like establish_server_with_client_address, but does not pass the client address or fd to the callback f.

  • since 3.0.0
module Versioned : sig ... end

Versioned variants of APIs undergoing breaking changes.


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