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.
Types ¶
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
mode : either Lwt_io.input or Lwt_io.output
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.
Note:
- 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
Reading ¶
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
If ~count is specified, read ~count ic reads at most ~count characters from ic. Note that fewer than ~count characters can be read; check the size of the resulting string. read returns "" if the end of input is reached.
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_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.
Writing ¶
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
See Lwt_io.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_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.
Printing ¶
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
%! 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).
Utilities ¶
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 ~mode filename f opens a file and passes the channel to f. It is ensured that the channel is closed when f ch terminates (even if it fails).
val open_temp_file :
?buffer:Lwt_bytes.t ->
?flags:Unix.open_flag list ->
?perm:Unix.file_perm ->
?temp_dir:string ->
?prefix: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.
val with_temp_file :
?buffer:Lwt_bytes.t ->
?flags:Unix.open_flag list ->
?perm:Unix.file_perm ->
?temp_dir:string ->
?prefix:string ->
(string * output_channel -> 'b Lwt.t) -> 'b Lwt.t
with_temp_file f calls Lwt_io.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.
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 completly closed when you close both
channels.
Raises Unix.Unix_error on error.
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_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
establish_server_with_client_address 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 (in_channel, out_channel)where client_address is the address (peer name) of the new client, and in_channel and out_channel are two channels wrapping the socket for communicating with that 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_address automatically closes in_channel and out_channel. 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. If the channels are still open when f completes, and their automatic closing raises an exception, establish_server_with_client_address 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_address can do nothing with that exception, except pass it to Lwt.async_exception_hook.
~fd can be specified to use an existing file descriptor for listening. Otherwise, a fresh socket is created internally.
~backlog is the argument passed to Lwt_unix.listen.
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 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 Lwt_io.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 Lwt_io.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
include Lwt_io.NumberIO
Reading/writing of numbers in the system endianness.
type byte_order = Lwt_sys.byte_order = | Little_endian | Big_endian(* <<div class="odocwiki_info"|~Type of byte order >> *)
val system_byte_order : byte_order
Same as Lwt_sys.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 = { da_buffer: Lwt_bytes.t;(* <<div class="odocwiki_info"|~The internal buffer >> *) mutable da_ptr: int;(* <<div class="odocwiki_info"|~The pointer to~: * the beginning of free space for output channels * the beginning of data for input channels >> *) mutable da_max: int;(* <<div class="odocwiki_info"|~The maximum offset >> *) da_perform: unit -> int Lwt.t;(* <<div class="odocwiki_info"|~- 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 Lwt_io.direct_access structure. f must use it and update da_ptr to reflect how many bytes have been read/written.
Misc ¶
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
Deprecated ¶
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
Deprecated.Use Lwt_io.establish_server_with_client_address.
Like establish_server_with_client_address, but does not pass the client
address to the callback f.
Since 3.0.0
module Versioned : sig..end
Versioned variants of APIs undergoing breaking changes.