Warning: Reason support is experimental. We are looking for beta-tester and contributors.

Module Lwt_unix

module Lwt_unix : sig..end

Cooperative system calls

This modules maps system calls, like those of the standard library's Unix module, to cooperative ones, which will not block the program.

The semantics of all operations is the following: if the action (for example reading from a file descriptor) can be performed immediately, it is done and returns immediately, otherwise it returns a sleeping thread which is woken up when the operation completes.

Most operations on sockets and pipes (on Windows it is only sockets) are cancelable, meaning you can cancel them with Lwt.cancel. For example if you want to read something from a file descriptor with a timeout, you can cancel the action after the timeout and the reading will not be performed if not already done.

For example, consider that you have two sockets sock1 and sock2. You want to read something from sock1 or exclusively from sock2 and fail with an exception if a timeout of 1 second expires, without reading anything from sock1 and sock2, even if they become readable in the future.

Then you can do:

Lwt.pick [Lwt_unix.timeout 1.0; read sock1 buf1 ofs1 len1; read sock2 buf2 ofs2 len2]

In this case, it is guaranteed that exactly one of the three operations will complete, and the others will be cancelled.

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

Same as Unix.handle_unix_error but catches lwt-level exceptions


type async_method =  | Async_none(* <<div class="odocwiki_info"|~System calls are made synchronously~, and may block the
          entire program~.
>> *) | Async_detach(* <<div class="odocwiki_info"|~System calls are made in another system thread~, thus without
          blocking other ~Lwt threads~. ~The drawback is that it may
          degrade performance in some cases~.

          ~This is the default~.
>> *) | Async_switch(* <<div class="odocwiki_info"|~System calls are made in the main thread~, and if one blocks
          the execution continue in another system thread~. ~This method
          is the most efficient~, also you will get better performance
          if you force all threads to run on the same cpu~. ~On linux
          this can be done by using the command <<span class="odocwiki_inlinecode"|taskset>>~.

          ~Note that this method is still experimental~.
>> *)

For system calls that cannot be made asynchronously, Lwt uses one of the following method:

val default_async_method : unit -> async_method

Returns the default async method.

This can be initialized using the environment variable "LWT_ASYNC_METHOD" with possible values "none", "detach" and "switch".

val set_default_async_method : async_method -> unit

Sets the default async method.

val async_method : unit -> async_method

async_method () returns the async method used in the current thread.

val async_method_key : async_method Lwt.key

The key for storing the local async method.

val with_async_none : (unit -> 'a) -> 'a

with_async_none f is a shorthand for:

Lwt.with_value async_method_key (Some Async_none) f
val with_async_detach : (unit -> 'a) -> 'a

with_async_none f is a shorthand for:

Lwt.with_value async_method_key (Some Async_detach) f
val with_async_switch : (unit -> 'a) -> 'a

with_async_none f is a shorthand for:

Lwt.with_value async_method_key (Some Async_switch) f


val sleep : float -> unit Lwt.t

sleep d is a thread that remains suspended for d seconds and then terminates.

val yield : unit -> unit Lwt.t

yield () is a thread that suspends itself and then resumes as soon as possible and terminates.

val auto_yield : float -> unit -> unit Lwt.t

auto_yield timeout returns a function f that will yield every timeout seconds.

exception Timeout

Exception raised by timeout operations

val timeout : float -> 'a Lwt.t

timeout d is a thread that remains suspended for d seconds and then fails with Lwt_unix.Timeout.

val with_timeout : float -> (unit -> 'a Lwt.t) -> 'a Lwt.t

with_timeout d f is a short-hand for:

Lwt.pick [Lwt_unix.timeout d; f ()]

Operation on file-descriptors

type file_descr

The abstract type for file descriptors. A Lwt file descriptor is a pair of a unix file descriptor (of type Unix.file_descr) and a state.

A file descriptor may be:

  • opened, in which case it is fully usable
  • closed or aborted, in which case it is no longer usable
type state =  | Opened(* <<div class="odocwiki_info"|~The **file descriptor** is opened
>> *) | Closed(* <<div class="odocwiki_info"|~The **file descriptor** has been closed by <<a_api | val Lwt_unix.close >>~. ~It must
          not be used for any operation~.
>> *) | Aborted of exn(* <<div class="odocwiki_info"|~The **file descriptor** has been aborted~, the only operation
          possible is <<a_api | val Lwt_unix.close >>~, all others will fail~.
>> *)

State of a file descriptor

val state : file_descr -> state

state fd returns the state of fd

val unix_file_descr : file_descr -> Unix.file_descr

Returns the underlying unix file descriptor. It always succeeds, even if the file descriptor's state is not Open.

val of_unix_file_descr : 
  ?blocking:bool ->
  ?set_flags:bool -> Unix.file_descr -> file_descr

Creates a lwt file descriptor from a unix one.

blocking is the blocking mode of the file-descriptor, and describes how Lwt will use it. In non-blocking mode, read/write on this file descriptor are made using non-blocking IO; in blocking mode they are made using the current async method. If blocking is not specified it is guessed according to the file kind: socket and pipes are in non-blocking mode and others are in blocking mode.

If set_flags is true (the default) then the file flags are modified according to the blocking argument, otherwise they are left unchanged.

Note that the blocking mode is less efficient than the non-blocking one, so it should be used only for file descriptors that does not support asynchronous operations, such as regular files, or for shared descriptors such as Lwt_unix.stdout, Lwt_unix.stderr or Lwt_unix.stdin.

val blocking : file_descr -> bool Lwt.t

blocking fd returns whether fd is used in blocking or non-blocking mode.

val set_blocking : ?set_flags:bool -> file_descr -> bool -> unit

set_blocking fd b puts fd in blocking or non-blocking mode. If set_flags is true (the default) then the file flags are modified, otherwise the modification is only done at the application level.

val abort : file_descr -> exn -> unit

abort fd exn makes all current and further uses of the file descriptor fail with the given exception. This put the file descriptor into the Aborted state.

If the file descriptor is closed, this does nothing, if it is aborted, this replace the abort exception by exn.

Note that this only works for reading and writing operations on file descriptors supporting non-blocking mode.

Process handling

val fork : unit -> int

fork () does the same as Unix.fork. You must use this function instead of Unix.fork when you want to use Lwt in the child process.


  • in the child process all pending jobs are canceled,
  • if you are going to use Lwt in the parent and the child, it is a good idea to call Lwt_io.flush_all before callling Lwt_unix.fork to avoid double-flush.
type process_status = Unix.process_status  =  | WEXITED of int | WSIGNALED of int | WSTOPPED of int
type wait_flag = Unix.wait_flag  =  | WNOHANG | WUNTRACED
val wait : unit -> (int * process_status) Lwt.t

Wrapper for Unix.wait

val waitpid : 
  wait_flag list ->
  int -> (int * process_status) Lwt.t

Wrapper for Unix.waitpid

type resource_usage = {
  ru_utime: float;(* <<div class="odocwiki_info"|~User time used
>> *)  ru_stime: float;(* <<div class="odocwiki_info"|~System time used
>> *)}

Resource usages

val wait4 : 
  wait_flag list ->
  int -> (int * process_status * resource_usage) Lwt.t

wait4 flags pid returns (pid, status, rusage) where (pid, status) is the same result as Unix.waitpid flags pid, and rusage contains accounting information about the child.

On windows it will always returns { utime = 0.0; stime = 0.0 }.

val wait_count : unit -> int

Returns the number of threads waiting for a child to terminate.

val system : string -> process_status Lwt.t

Executes the given command, waits until it terminates, and return its termination status. The string is interpreted by the shell /bin/sh on Unix and cmd.exe on Windows. The result WEXITED 127 indicates that the shell couldn't be executed.

Basic file input/output

val stdin : file_descr

The standard file descriptor for input. This one is usually a terminal is the program is started from a terminal.

val stdout : file_descr

The standard file descriptor for output

val stderr : file_descr

The standard file descriptor for printing error messages

type file_perm = Unix.file_perm
val openfile : 
  string ->
  open_flag list ->
  file_perm -> file_descr Lwt.t

Wrapper for Unix.openfile.

val close : file_descr -> unit Lwt.t

Close a file descriptor. This close the underlying unix file descriptor and set its state to Closed

val read : file_descr -> Bytes.t -> int -> int -> int Lwt.t

read fd buf ofs len has the same semantic as Unix.read, but is cooperative

val write : file_descr -> Bytes.t -> int -> int -> int Lwt.t

write fd buf ofs len has the same semantic as Unix.write, but is cooperative

val write_string : file_descr -> string -> int -> int -> int Lwt.t
val readable : file_descr -> bool

Returns whether the given file descriptor is currently readable.

val writable : file_descr -> bool

Returns whether the given file descriptor is currently writable.

val wait_read : file_descr -> unit Lwt.t

waits (without blocking other threads) until there is something to read on the file descriptor

val wait_write : file_descr -> unit Lwt.t

waits (without blocking other threads) until it is possible to write on the file descriptor

Seeking and truncating

type seek_command = Unix.seek_command  =  | SEEK_SET | SEEK_CUR | SEEK_END
val lseek : file_descr -> int -> seek_command -> int Lwt.t

Wrapper for Unix.lseek

val truncate : string -> int -> unit Lwt.t

Wrapper for Unix.truncate

val ftruncate : file_descr -> int -> unit Lwt.t

Wrapper for Unix.ftruncate


val fsync : file_descr -> unit Lwt.t

Synchronise all data and metadata of the file descriptor with the disk. On Windows it uses FlushFileBuffers.

val fdatasync : file_descr -> unit Lwt.t

Synchronise all data (but not metadata) of the file descriptor with the disk.

Note that fdatasync is not available on all platforms.

File status

type file_kind = Unix.file_kind  =  | S_REG | S_DIR | S_CHR | S_BLK | S_LNK | S_FIFO | S_SOCK
type stats = Unix.stats  = {
  st_dev: int;  st_ino: int;  st_kind: file_kind;  st_perm: file_perm;  st_nlink: int;  st_uid: int;  st_gid: int;  st_rdev: int;  st_size: int;  st_atime: float;  st_mtime: float;  st_ctime: float;}
val stat : string -> stats Lwt.t

Wrapper for Unix.stat

val lstat : string -> stats Lwt.t

Wrapper for Unix.lstat

val fstat : file_descr -> stats Lwt.t

Wrapper for Unix.fstat

val file_exists : string -> bool Lwt.t

file_exists name tests if a file named name exists.

val isatty : file_descr -> bool Lwt.t

Wrapper for Unix.isatty

File operations on large files

module LargeFile : sig..end

Operations on file names

Wrapper for Unix.unlink

val rename : string -> string -> unit Lwt.t

Wrapper for Unix.rename

Wrapper for Unix.link

File permissions and ownership

val chmod : string -> file_perm -> unit Lwt.t

Wrapper for Unix.chmod

val fchmod : file_descr -> file_perm -> unit Lwt.t

Wrapper for Unix.fchmod

val chown : string -> int -> int -> unit Lwt.t

Wrapper for Unix.chown

val fchown : file_descr -> int -> int -> unit Lwt.t

Wrapper for Unix.fchown

type access_permission = Unix.access_permission  =  | R_OK | W_OK | X_OK | F_OK
val access : string -> access_permission list -> unit Lwt.t

Wrapper for Unix.access

Operations on file descriptors

val dup : file_descr -> file_descr

Wrapper for Unix.dup

val dup2 : file_descr -> file_descr -> unit

Wrapper for Unix.dup2

val set_close_on_exec : file_descr -> unit

Wrapper for Unix.set_close_on_exec

val clear_close_on_exec : file_descr -> unit

Wrapper for Unix.clear_close_on_exec


val mkdir : string -> file_perm -> unit Lwt.t

Wrapper for Unix.mkdir

val rmdir : string -> unit Lwt.t

Wrapper for Unix.rmdir

val chdir : string -> unit Lwt.t

Wrapper for Unix.chdir

val chroot : string -> unit Lwt.t

Wrapper for Unix.chroot

type dir_handle = Unix.dir_handle
val opendir : string -> dir_handle Lwt.t

Wrapper for Unix.opendir

val readdir : dir_handle -> string Lwt.t

Wrapper for Unix.readdir.

val readdir_n : dir_handle -> int -> string array Lwt.t

readdir_n handle count reads at most count entry from the given directory. It is more efficient than calling readdir count times. If the length of the returned array is smaller than count, this means that the end of the directory has been reached.

val rewinddir : dir_handle -> unit Lwt.t

Wrapper for Unix.rewinddir

val closedir : dir_handle -> unit Lwt.t

Wrapper for Unix.closedir

val files_of_directory : string -> string Lwt_stream.t

files_of_directory dir returns the stream of all files of dir.

Pipes and redirections

val pipe : unit -> file_descr * file_descr

pipe () creates pipe using Unix.pipe and returns two lwt file descriptors created from unix file_descriptor

val pipe_in : unit -> file_descr * Unix.file_descr

pipe_in () is the same as Lwt_unix.pipe but maps only the unix file descriptor for reading into a lwt one. The second is not put into non-blocking mode. You usually want to use this before forking to receive data from the child process.

val pipe_out : unit -> Unix.file_descr * file_descr

pipe_out () is the inverse of Lwt_unix.pipe_in. You usually want to use this before forking to send data to the child process

val mkfifo : string -> file_perm -> unit Lwt.t

Wrapper for Unix.mkfifo

Wrapper for Unix.symlink

Wrapper for Unix.readlink


type lock_command = Unix.lock_command  =  | F_ULOCK | F_LOCK | F_TLOCK | F_TEST | F_RLOCK | F_TRLOCK
val lockf : file_descr -> lock_command -> int -> unit Lwt.t

Wrapper for Unix.lockf

User id, group id

type passwd_entry = Unix.passwd_entry  = {
  pw_name: string;  pw_passwd: string;  pw_uid: int;  pw_gid: int;  pw_gecos: string;  pw_dir: string;  pw_shell: string;}
type group_entry = Unix.group_entry  = {
  gr_name: string;  gr_passwd: string;  gr_gid: int;  gr_mem: string array;}
val getlogin : unit -> string Lwt.t

Wrapper for Unix.getlogin

val getpwnam : string -> passwd_entry Lwt.t

Wrapper for Unix.getpwnam

val getgrnam : string -> group_entry Lwt.t

Wrapper for Unix.getgrnam

val getpwuid : int -> passwd_entry Lwt.t

Wrapper for Unix.getpwuid

val getgrgid : int -> group_entry Lwt.t

Wrapper for Unix.getgrgid


type signal_handler_id

Id of a signal handler, used to cancel it

val on_signal : int -> (int -> unit) -> signal_handler_id

on_signal signum f calls f each time the signal with numnber signum is received by the process. It returns a signal handler identifier that can be used to stop monitoring signum.

val on_signal_full : 
  int ->
  (signal_handler_id -> int -> unit) ->

on_signal_full f is the same as on_signal f except that f also receive the signal handler identifier as argument so it can disable it.

val disable_signal_handler : signal_handler_id -> unit

Stops receiving this signal

val signal_count : unit -> int

Returns the number of registered signal handler.

val reinstall_signal_handler : int -> unit

reinstall_signal_handler signum if any signal handler is registered for this signal with Lwt_unix.on_signal, it reinstall the signal handler (with Sys.set_signal). This is usefull in case another part of the program install another signal handler.


type inet_addr = Unix.inet_addr
type socket_domain = Unix.socket_domain  =  | PF_UNIX | PF_INET | PF_INET6
type socket_type = Unix.socket_type  =  | SOCK_STREAM | SOCK_DGRAM | SOCK_RAW | SOCK_SEQPACKET
type sockaddr = Unix.sockaddr  =  | ADDR_UNIX of string | ADDR_INET of inet_addr * int
val socket : 
  socket_domain ->
  socket_type -> int -> file_descr

socket domain type proto is the same as Unix.socket but maps the result into a lwt file descriptor

val socketpair : 
  socket_domain ->
  socket_type ->
  int -> file_descr * file_descr

Wrapper for Unix.socketpair

val bind : file_descr -> sockaddr -> unit

Wrapper for Unix.bind

val listen : file_descr -> int -> unit

Wrapper for Unix.listen

val accept : file_descr -> (file_descr * sockaddr) Lwt.t

Wrapper for Unix.accept

val accept_n : 
  file_descr ->
  int ->
  ((file_descr * sockaddr) list * exn option) Lwt.t

accept_n fd count accepts up to count connections at one time.

  • if no connection is available right now, it returns a sleeping thread
  • if more than 1 and less than count are available, it returns all of them
  • if more than count are available, it returns the next count of them
  • if an error happens, it returns the connections that have been successfully accepted so far and the error

accept_n has the advantage of improving performance. If you want a more detailed description, you can have a look at:

Acceptable strategies for improving web server performance

val connect : file_descr -> sockaddr -> unit Lwt.t

Wrapper for Unix.connect

type shutdown_command = Unix.shutdown_command  =  | SHUTDOWN_RECEIVE | SHUTDOWN_SEND | SHUTDOWN_ALL
val shutdown : file_descr -> shutdown_command -> unit

Wrapper for Unix.shutdown

val getsockname : file_descr -> sockaddr

Wrapper for Unix.getsockname

val getpeername : file_descr -> sockaddr

Wrapper for Unix.getpeername

type msg_flag = Unix.msg_flag  =  | MSG_OOB | MSG_DONTROUTE | MSG_PEEK
val recv : 
  file_descr ->
  Bytes.t -> int -> int -> msg_flag list -> int Lwt.t

Wrapper for Unix.recv

val recvfrom : 
  file_descr ->
  Bytes.t ->
  int ->
  int -> msg_flag list -> (int * sockaddr) Lwt.t

Wrapper for Unix.recvfrom

val send : 
  file_descr ->
  Bytes.t -> int -> int -> msg_flag list -> int Lwt.t

Wrapper for Unix.send

val sendto : 
  file_descr ->
  Bytes.t ->
  int -> int -> msg_flag list -> sockaddr -> int Lwt.t

Wrapper for Unix.sendto

type io_vector = {
  iov_buffer: string;  iov_offset: int;  iov_length: int;}

An io-vector. Used by Lwt_unix.recv_msg and Lwt_unix.send_msg.

val io_vector : buffer:string -> offset:int -> length:int -> io_vector

Creates an io-vector

val recv_msg : 
  socket:file_descr ->
  io_vectors:io_vector list ->
  (int * Unix.file_descr list) Lwt.t

recv_msg ~socket ~io_vectors receives data into a list of io-vectors, plus any file-descriptors that may accompany the messages. It returns a tuple whose first field is the number of bytes received and second is a list of received file descriptors. The messages themselves will be recorded in the provided io_vectors list.

This call is not available on windows.

val send_msg : 
  socket:file_descr ->
  io_vectors:io_vector list ->
  fds:Unix.file_descr list -> int Lwt.t

send_msg ~socket ~io_vectors ~fds sends data from a list of io-vectors, accompanied with a list of file-descriptors. It returns the number of bytes sent. If fd-passing is not possible on the current system and fds is not empty, it raises Lwt_sys.Not_available "fd_passing".

This call is not available on windows.

type credentials = {
  cred_pid: int;  cred_uid: int;  cred_gid: int;}
val get_credentials : file_descr -> credentials

get_credentials fd returns credentials information from the given socket. On some platforms, obtaining the peer pid is not possible and it will be set to -1. If obtaining credentials is not possible on the current system, it raises Lwt_sys.Not_available "get_credentials".

This call is not available on windows.

Socket options

type socket_int_option = Unix.socket_int_option  =  | SO_SNDBUF | SO_RCVBUF | SO_ERROR | SO_TYPE | SO_RCVLOWAT | SO_SNDLOWAT
type socket_optint_option = Unix.socket_optint_option  =  | SO_LINGER
type socket_float_option = Unix.socket_float_option  =  | SO_RCVTIMEO | SO_SNDTIMEO
val getsockopt : file_descr -> socket_bool_option -> bool

Wrapper for Unix.getsockopt

val setsockopt : file_descr -> socket_bool_option -> bool -> unit

Wrapper for Unix.setsockopt

val getsockopt_int : file_descr -> socket_int_option -> int

Wrapper for Unix.getsockopt_int

val setsockopt_int : file_descr -> socket_int_option -> int -> unit

Wrapper for Unix.setsockopt_int

val getsockopt_optint : file_descr -> socket_optint_option -> int option

Wrapper for Unix.getsockopt_optint

val setsockopt_optint : 
  file_descr ->
  socket_optint_option -> int option -> unit

Wrapper for Unix.setsockopt_optint

val getsockopt_float : file_descr -> socket_float_option -> float

Wrapper for Unix.getsockopt_float

val setsockopt_float : file_descr -> socket_float_option -> float -> unit

Wrapper for Unix.setsockopt_float

val getsockopt_error : file_descr -> Unix.error option

Wrapper for Unix.getsockopt_error

Multicast functions

val mcast_set_loop : file_descr -> bool -> unit

Whether sent multicast messages are received by the sending host

val mcast_set_ttl : file_descr -> int -> unit

Set TTL/hops value

val mcast_add_membership : 
  file_descr ->
  ?ifname:Unix.inet_addr -> Unix.inet_addr -> unit

mcast_add_membership fd ~ifname addr joins the multicast group addr on the network interface ifname.

val mcast_drop_membership : 
  file_descr ->
  ?ifname:Unix.inet_addr -> Unix.inet_addr -> unit

mcast_drop_membership fd ~ifname addr leaves the multicast group addr on the network interface ifname.

Host and protocol databases

type host_entry = Unix.host_entry  = {
  h_name: string;  h_aliases: string array;  h_addrtype: socket_domain;  h_addr_list: inet_addr array;}
type protocol_entry = Unix.protocol_entry  = {
  p_name: string;  p_aliases: string array;  p_proto: int;}
type service_entry = Unix.service_entry  = {
  s_name: string;  s_aliases: string array;  s_port: int;  s_proto: string;}
val gethostname : unit -> string Lwt.t

Wrapper for Unix.gethostname

val gethostbyname : string -> host_entry Lwt.t

Wrapper for Unix.gethostbyname

val gethostbyaddr : inet_addr -> host_entry Lwt.t

Wrapper for Unix.gethostbyaddr

val getprotobyname : string -> protocol_entry Lwt.t

Wrapper for Unix.getprotobyname

val getprotobynumber : int -> protocol_entry Lwt.t

Wrapper for Unix.getprotobynumber

val getservbyname : string -> string -> service_entry Lwt.t

Wrapper for Unix.getservbyname

val getservbyport : int -> string -> service_entry Lwt.t

Wrapper for Unix.getservbyport

type addr_info = Unix.addr_info  = {
  ai_family: socket_domain;  ai_socktype: socket_type;  ai_protocol: int;  ai_addr: sockaddr;  ai_canonname: string;}
type getaddrinfo_option = Unix.getaddrinfo_option  =  | AI_FAMILY of socket_domain | AI_SOCKTYPE of socket_type | AI_PROTOCOL of int | AI_NUMERICHOST | AI_CANONNAME | AI_PASSIVE
val getaddrinfo : 
  string ->
  string ->
  getaddrinfo_option list -> addr_info list Lwt.t

Wrapper for Unix.getaddrinfo

type name_info = Unix.name_info  = {
  ni_hostname: string;  ni_service: string;}
type getnameinfo_option = Unix.getnameinfo_option  =  | NI_NOFQDN | NI_NUMERICHOST | NI_NAMEREQD | NI_NUMERICSERV | NI_DGRAM
val getnameinfo : 
  sockaddr ->
  getnameinfo_option list -> name_info Lwt.t

Wrapper for Unix.getnameinfo

Terminal interface

type terminal_io = Unix.terminal_io  = {
  mutable c_ignbrk: bool;  mutable c_brkint: bool;  mutable c_ignpar: bool;  mutable c_parmrk: bool;  mutable c_inpck: bool;  mutable c_istrip: bool;  mutable c_inlcr: bool;  mutable c_igncr: bool;  mutable c_icrnl: bool;  mutable c_ixon: bool;  mutable c_ixoff: bool;  mutable c_opost: bool;  mutable c_obaud: int;  mutable c_ibaud: int;  mutable c_csize: int;  mutable c_cstopb: int;  mutable c_cread: bool;  mutable c_parenb: bool;  mutable c_parodd: bool;  mutable c_hupcl: bool;  mutable c_clocal: bool;  mutable c_isig: bool;  mutable c_icanon: bool;  mutable c_noflsh: bool;  mutable c_echo: bool;  mutable c_echoe: bool;  mutable c_echok: bool;  mutable c_echonl: bool;  mutable c_vintr: char;  mutable c_vquit: char;  mutable c_verase: char;  mutable c_vkill: char;  mutable c_veof: char;  mutable c_veol: char;  mutable c_vmin: int;  mutable c_vtime: int;  mutable c_vstart: char;  mutable c_vstop: char;}
val tcgetattr : file_descr -> terminal_io Lwt.t

Wrapper for Unix.tcgetattr

type setattr_when = Unix.setattr_when  =  | TCSANOW | TCSADRAIN | TCSAFLUSH
val tcsetattr : 
  file_descr ->
  setattr_when -> terminal_io -> unit Lwt.t

Wrapper for Unix.tcsetattr

val tcsendbreak : file_descr -> int -> unit Lwt.t

Wrapper for Unix.tcsendbreak

val tcdrain : file_descr -> unit Lwt.t

Wrapper for Unix.tcdrain

type flush_queue = Unix.flush_queue  =  | TCIFLUSH | TCOFLUSH | TCIOFLUSH
val tcflush : file_descr -> flush_queue -> unit Lwt.t

Wrapper for Unix.tcflush

type flow_action = Unix.flow_action  =  | TCOOFF | TCOON | TCIOFF | TCION
val tcflow : file_descr -> flow_action -> unit Lwt.t

Wrapper for Unix.tcflow

Low-level interaction

exception Retry

If an action raises Lwt_unix.Retry, it will be requeued until the file descriptor becomes readable/writable again.

exception Retry_read

If an action raises Lwt_unix.Retry_read, it will be requeued until the file descriptor becomes readable.

exception Retry_write

If an action raises Lwt_unix.Retry_read, it will be requeued until the file descriptor becomes writables.

type io_event =  | Read | Write
val wrap_syscall : io_event -> file_descr -> (unit -> 'a) -> 'a Lwt.t

wrap_syscall set fd action wrap an action on a file descriptor. It tries to execute action, and if it can not be performed immediately without blocking, it is registered for later.

In the latter case, if the thread is canceled, action is removed from set.

val check_descriptor : file_descr -> unit

check_descriptor fd raise an exception if fd is not in the state Open

val register_action : io_event -> file_descr -> (unit -> 'a) -> 'a Lwt.t

register_action set fd action registers action on fd. When fd becomes readable/writable action is called.


  • you must call check_descriptor fd before calling register_action
type 'a job

Type of job descriptions. A job description describe how to call a C function and how to get its result. The C function may be executed in another system thread.

val execute_job : 
  ?async_method:async_method ->
  job:'a job ->
  result:('a job -> 'b) ->
  free:('a job -> unit) -> 'b Lwt.t

This is the old and deprecated way of running a job. Use Lwt_unix.run_job in new code.

val run_job : ?async_method:async_method -> 'a job -> 'a Lwt.t

run_job ?async_method job starts job and wait for its termination.

The async method is choosen follow:

If the method is Async_none then the job is run synchronously and may block the current system thread, thus blocking all Lwt threads.

If the method is Async_detach then the job is run in another system thread, unless the the maximum number of worker threads has been reached (as given by Lwt_unix.pool_size).

If the method is Async_switch then the job is run synchronously and if it blocks, execution will continue in another system thread (unless the limit is reached).

val abort_jobs : exn -> unit

abort_jobs exn make all pending jobs to fail with exn. Note that this does not abort the real job (i.e. the C function executing it), just the lwt thread for it.

val cancel_jobs : unit -> unit

cancel_jobs () is the same as abort_jobs Lwt.Canceled.

val wait_for_jobs : unit -> unit Lwt.t

Wait for all pending jobs to terminate.


Lwt internally use a pipe to send notification to the main thread. The following functions allow to use this pipe.

val make_notification : ?once:bool -> (unit -> unit) -> int

new_notifier ?once f registers a new notifier. It returns the id of the notifier. Each time a notification with this id is received, f is called.

if once is specified, then the notification is stopped after the first time it is received. It defaults to false.

val send_notification : int -> unit

send_notification id sends a notification.

This function is thread-safe.

val stop_notification : int -> unit

Stop the given notification. Note that you should not reuse the id after the notification has been stopped, the result is unspecified if you do so.

val call_notification : int -> unit

Call the handler associated to the given notification. Note that if the notification was defined with once = true it is removed.

val set_notification : int -> (unit -> unit) -> unit

set_notification id f replace the function associated to the notification by f. It raises Not_found if the given notification is not found.

System threads pool

If the program is using the async method Async_detach or Async_switch, Lwt will launch system threads to execute blocking system calls asynchronously.

val pool_size : unit -> int

Maximum number of system threads that can be started. If this limit is reached, jobs will be executed synchronously.

val set_pool_size : int -> unit

Change the size of the pool.

val thread_count : unit -> int

The number of system threads running (excluding this one).

val thread_waiting_count : unit -> int

The number threads waiting for a job.


val get_cpu : unit -> int

get_cpu () returns the number of the CPU the current thread is running on.

val get_affinity : ?pid:int -> unit -> int list

get_affinity ?pid () returns the list of CPUs the process with pid pid is allowed to run on. If pid is not specified then the affinity of the current process is returned.

val set_affinity : ?pid:int -> int list -> unit

set_affinity ?pid cpus sets the list of CPUs the given process is allowed to run on.