package lwt

  1. Overview
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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 performed and returns an already resolved promise, otherwise it returns a pending promise which is resolved 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

Sleeping

val sleep : float -> unit Lwt.t

sleep d is a promise that remains in a pending state for d seconds after which it is resolved with value ().

val yield : unit -> unit Lwt.t

yield () is a promise in a pending state. It resumes itself as soon as possible and resolves with value ().

  • deprecated

    Since 5.5.0 yield is deprecated. Use the more general Lwt.pause instead. See Lwt_main.yield for additional details.

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

auto_pause timeout returns a function f, and f () has the following behavior:

  • If it has been more than timeout seconds since the last time f () behaved like Lwt.pause, f () calls Lwt.pause.
  • Otherwise, if it has been less than timeout seconds, f () behaves like Lwt.return_unit, i.e. it does not yield.
exception Timeout

Exception raised by timeout operations

val timeout : float -> 'a Lwt.t

timeout d is a promise that remains pending for d seconds and then is rejected with Timeout.

  • raises Timeout

    The promise timeout d is rejected with Timeout unless it is cancelled.

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 ()]
  • raises Timeout

    The promise with_timeout d f raises Timeout if the promise returned by f () takes more than d seconds to resolve.

Operations 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 =
  1. | Opened
    (*

    The file descriptor is opened

    *)
  2. | Closed
    (*

    The file descriptor has been closed by close. It must not be used for any operation.

    *)
  3. | Aborted of exn
    (*

    The file descriptor has been aborted, the only operation possible is 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 Opened.

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

Wraps a Unix file descriptor fd in an Lwt file_descr fd'.

~blocking controls the internal strategy Lwt uses to perform I/O on the underlying fd. Regardless of ~blocking, at the API level, Lwt_unix.read, Lwt_unix.write, etc. on fd' always block the Lwt promise, but never block the whole process. However, for performance reasons, it is important that ~blocking match the actual blocking mode of fd.

If ~blocking is not specified, of_unix_file_descr chooses non-blocking mode for Unix sockets, Unix pipes, and Windows sockets, and blocking mode for everything else. Note: not specifying ~blocking causes fstat to be lazily called on fd, the first time your code performs I/O on fd'. This fstat call can be expensive, so if you use of_unix_file_descr a lot, be sure to specify ~blocking explicitly.

of_unix_file_descr runs a system call to set the specified or chosen blocking mode on the underlying fd.

To prevent of_unix_file_descr from running this system call, you can pass ~set_flags:false. Note that, in this case, if ~blocking, whether passed explicitly or chosen by Lwt, does not match the true blocking mode of the underlying fd, I/O on fd' will suffer performance degradation.

Note that ~set_flags is effectively always false if running on Windows and fd is not a socket.

Generally, non-blocking I/O is faster: for blocking I/O, Lwt typically has to run system calls in worker threads to avoid blocking the process. See your system documentation for whether particular kinds of file descriptors support non-blocking I/O.

val blocking : file_descr -> bool Lwt.t

blocking fd indicates whether Lwt is internally using blocking or non-blocking I/O with fd.

Note that this may differ from the blocking mode of the underlying Unix file descriptor (i.e. unix_file_descr fd).

See of_unix_file_descr for details.

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

set_blocking fd b causes Lwt to internally use blocking or non-blocking I/O with fd, according to the value of b.

If ~set_flags is true (the default), Lwt also makes a system call to set the underlying file descriptor's blocking mode to match. Otherwise, set_blocking is only informational for Lwt.

It is important that the underlying file descriptor actually have the same blocking mode as that indicated by b.

See of_unix_file_descr for details.

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, even if you have not started using Lwt before the fork.

Notes:

  • In the child process all pending Lwt_unix I/O jobs are abandoned. This may cause the child's copy of their associated promises to remain forever pending.
  • 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 fork to avoid double-flush.
  • Otherwise, if you will not use Lwt in the child, call Lwt_main.Exit_hooks.remove_all to avoid Lwt calling Lwt_main.run during process exit.
  • None of the above is necessary if you intend to call exec. Indeed, in that case, it is not even necessary to use Lwt_unix.fork. You can use Unix.fork.
  • To abandon some more promises, see Lwt_main.abandon_yielded_and_paused.
type process_status = Unix.process_status =
  1. | WEXITED of int
  2. | WSIGNALED of int
  3. | WSTOPPED of int
type wait_flag = Unix.wait_flag =
  1. | WNOHANG
  2. | WUNTRACED
val wait : unit -> (int * process_status) Lwt.t

Wrapper for Unix.wait

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

A promise-returning analog to Unix.waitpid. This call is non-blocking on Unix-like systems, but is always blocking on Windows.

type resource_usage = {
  1. ru_utime : float;
    (*

    User time used

    *)
  2. ru_stime : float;
    (*

    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 promises waiting for a child process 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 file descriptor for standard input.

val stdout : file_descr

The file descriptor for standard output.

val stderr : file_descr

The file descriptor for standard error.

type file_perm = Unix.file_perm
type open_flag = Unix.open_flag =
  1. | O_RDONLY
  2. | O_WRONLY
  3. | O_RDWR
  4. | O_NONBLOCK
  5. | O_APPEND
  6. | O_CREAT
  7. | O_TRUNC
  8. | O_EXCL
  9. | O_NOCTTY
  10. | O_DSYNC
  11. | O_SYNC
  12. | O_RSYNC
  13. | O_SHARE_DELETE
  14. | O_CLOEXEC
  15. | O_KEEPEXEC
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 -> int -> int -> int Lwt.t

read fd buf ofs len reads up to len bytes from fd, and writes them to buf, starting at offset ofs. The function immediately evaluates to an Lwt promise which waits for the operation to complete. If it completes successfully, the promise resolves to the number of bytes actually read, or zero if the end of file has been reached.

Note that the Lwt promise waits for data (or end of file) even if the underlying file descriptor is in non-blocking mode. See of_unix_file_descr for a discussion of non-blocking I/O and Lwt.

If Lwt is using blocking I/O on fd, read writes data into a temporary buffer, then copies it into buf.

The promise can be rejected with any exception that can be raised by Unix.read, except Unix.Unix_error Unix.EAGAIN, Unix.Unix_error Unix.EWOULDBLOCK or Unix.Unix_error Unix.EINTR.

val pread : file_descr -> bytes -> file_offset:int -> int -> int -> int Lwt.t

pread fd buf ~file_offset ofs len on file descriptors allowing seek, reads up to len bytes from fd at offset file_offset from the beginning of the file, and writes them to buf, starting at offset ofs.

On Unix systems, the file descriptor position is unaffected. On Windows it is changed to be just after the last read position.

The promise can be rejected with any exception that can be raised by read or lseek.

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

write fd buf ofs len writes up to len bytes to fd from buf, starting at buffer offset ofs. The function immediately evaluates to an Lwt promise which waits for the operation to complete. If the operation completes successfully, the promise resolves to the number of bytes actually written, which may be less than len.

Note that the Lwt promise waits to write even if the underlying file descriptor is in non-blocking mode. See of_unix_file_descr for a discussion of non-blocking I/O and Lwt.

If Lwt is using blocking I/O on fd, buf is copied before writing.

The promise can be rejected with any exception that can be raised by Unix.single_write, except Unix.Unix_error Unix.EAGAIN, Unix.Unix_error Unix.EWOULDBLOCK or Unix.Unix_error Unix.EINTR.

val pwrite : file_descr -> bytes -> file_offset:int -> int -> int -> int Lwt.t

pwrite fd buf ~file_offset ofs len on file descriptors allowing seek, writes up to len bytes to fd from buf, starting at buffer offset ofs. The data is written at offset file_offset from the beginning of fd.

On Unix systems, the file descriptor position is unaffected. On Windows it is changed to be just after the last written position.

The promise can be rejected with any exception that can be raised by write or lseek.

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

See write.

val pwrite_string : file_descr -> string -> file_offset:int -> int -> int -> int Lwt.t

See pwrite.

module IO_vectors : sig ... end

Sequences of buffer slices for writev.

val readv : file_descr -> IO_vectors.t -> int Lwt.t

readv fd vs reads bytes from fd into the buffer slices vs. If the operation completes successfully, the resulting promise resolves to the number of bytes read.

Data is always read directly into Bigarray slices. If the Unix file descriptor underlying fd is in non-blocking mode, data is also read directly into bytes slices. Otherwise, data for bytes slices is first read into temporary buffers, then copied.

Note that the returned Lwt promise is pending until failure or a successful read, even if the underlying file descriptor is in non-blocking mode. See of_unix_file_descr for a discussion of non-blocking I/O and Lwt.

If IO_vectors.system_limit is Some n and the count of slices in vs exceeds n, then Lwt_unix.readv reads only into the first n slices of vs.

Not implemented on Windows. It should be possible to implement, upon request, for Windows sockets only.

See readv(3p).

  • since 2.7.0
val writev : file_descr -> IO_vectors.t -> int Lwt.t

writev fd vs writes the bytes in the buffer slices vs to the file descriptor fd. If the operation completes successfully, the resulting promise resolves to the number of bytes written.

If the Unix file descriptor underlying fd is in non-blocking mode, writev does not make a copy the bytes before writing. Otherwise, it copies bytes slices, but not Bigarray slices.

Note that the returned Lwt promise is pending until failure or a successful write, even if the underlying descriptor is in non-blocking mode. See of_unix_file_descr for a discussion of non-blocking I/O and Lwt.

If IO_vectors.system_limit is Some n and the count of slices in vs exceeds n, then Lwt_unix.writev passes only the first n slices in vs to the underlying writev system call.

Not implemented on Windows. It should be possible to implement, upon request, for Windows sockets only.

The behavior of writev when vs has zero slices depends on the system, and may change in future versions of Lwt. On Linux, writev will succeed and write zero bytes. On BSD (including macOS), writev will fail with Unix.Unix_error (Unix.EINVAL, "writev", ...).

See writev(3p).

  • since 2.7.0
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 promises) until there is something to read from the file descriptor.

Note that you don't need to use this function if you are using Lwt I/O functions for reading, since they provide non-blocking waiting automatically.

The intended use case for this function is interfacing with existing libraries that are known to be blocking.

val wait_write : file_descr -> unit Lwt.t

Waits (without blocking other promises) until it is possible to write on the file descriptor.

Note that you don't need to use this function if you are using Lwt I/O functions for writing, since they provide non-blocking waiting automatically.

The intended use case for this function is interfacing with existing libraries that are known to be blocking.

Seeking and truncating

type seek_command = Unix.seek_command =
  1. | SEEK_SET
  2. | SEEK_CUR
  3. | 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

Syncing

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 Windows and OS X.

File status

type file_kind = Unix.file_kind =
  1. | S_REG
  2. | S_DIR
  3. | S_CHR
  4. | S_BLK
  5. | S_LNK
  6. | S_FIFO
  7. | S_SOCK
type stats = Unix.stats = {
  1. st_dev : int;
  2. st_ino : int;
  3. st_kind : file_kind;
  4. st_perm : file_perm;
  5. st_uid : int;
  6. st_gid : int;
  7. st_rdev : int;
  8. st_size : int;
  9. st_atime : float;
  10. st_mtime : float;
  11. 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.

Note that file_exists behaves similarly to Sys.file_exists:

  • “file” is interpreted as “directory entry” in this context
  • file_exists name will return false in circumstances that would make stat raise a Unix.Unix_error exception.
val utimes : string -> float -> float -> unit Lwt.t

utimes path atime mtime updates the access and modification times of the file at path. The access time is set to atime and the modification time to mtime. To set both to the current time, call utimes path 0. 0..

This function corresponds to Unix.utimes. See also utimes(3p).

  • since 2.6.0
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 =
  1. | R_OK
  2. | W_OK
  3. | X_OK
  4. | F_OK
val access : string -> access_permission list -> unit Lwt.t

Wrapper for Unix.access

Operations on file descriptors

val dup : ?cloexec:bool -> file_descr -> file_descr

Wrapper for Unix.dup

val dup2 : ?cloexec:bool -> 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

Directories

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 getcwd : unit -> string Lwt.t

Wrapper for Unix.getcwd

  • since 3.1.0
val chroot : string -> unit Lwt.t

Wrapper for Unix.chroot

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

Opens a directory for listing. Directories opened with this function must be explicitly closed with closedir. This is a cooperative analog of Unix.opendir.

val readdir : dir_handle -> string Lwt.t

Reads the next directory entry from the given directory. Special entries such as . and .. are included. If all entries have been read, raises End_of_file. This is a cooperative analog of Unix.readdir.

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

readdir_n handle count reads at most count entries 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

Resets the given directory handle, so that directory listing can be restarted. Cooperative analog of Unix.rewinddir.

val closedir : dir_handle -> unit Lwt.t

Closes a directory handle. Cooperative analog of 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 : ?cloexec:bool -> 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 : ?cloexec:bool -> unit -> file_descr * Unix.file_descr

pipe_in () is the same as 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 : ?cloexec:bool -> unit -> Unix.file_descr * file_descr

pipe_out () is the inverse of 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

Locking

type lock_command = Unix.lock_command =
  1. | F_ULOCK
  2. | F_LOCK
  3. | F_TLOCK
  4. | F_TEST
  5. | F_RLOCK
  6. | 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 = {
  1. pw_name : string;
  2. pw_passwd : string;
  3. pw_uid : int;
  4. pw_gid : int;
  5. pw_gecos : string;
  6. pw_dir : string;
  7. pw_shell : string;
}
type group_entry = Unix.group_entry = {
  1. gr_name : string;
  2. gr_passwd : string;
  3. gr_gid : int;
  4. 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

Signals

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) -> signal_handler_id

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 on_signal, it reinstall the signal handler (with Sys.set_signal). This is useful in case another part of the program install another signal handler.

val handle_signal : int -> unit

handle_signal signum acts as if Lwt had received the signum signal. This allows another IO library to install the handler, perform its own handling, but still notify Lwt. It is particularly useful for SIGCHLD, where several IO libraries may be spawning sub-processes.

This function is thread-safe.

Sockets

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

Wrapper for Unix.socketpair

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

Binds an address to the given socket. This is the cooperative analog of Unix.bind. See also bind(3p).

  • since 3.0.0
val listen : file_descr -> int -> unit

Wrapper for Unix.listen

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

Wrapper for Unix.accept

val accept_n : ?cloexec:bool -> 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 pending promise
  • 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 =
  1. | SHUTDOWN_RECEIVE
  2. | SHUTDOWN_SEND
  3. | 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 =
  1. | MSG_OOB
  2. | MSG_DONTROUTE
  3. | MSG_PEEK
val recv : file_descr -> bytes -> int -> int -> msg_flag list -> int Lwt.t

Wrapper for Unix.recv.

On Windows, recv writes data into a temporary buffer, then copies it into the given one.

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

Wrapper for Unix.recvfrom.

On Windows, recvfrom writes data into a temporary buffer, then copies it into the given one.

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

Wrapper for Unix.send.

On Windows, send copies the given buffer before writing.

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

Wrapper for Unix.sendto.

On Windows, sendto copies the given buffer before writing.

val recv_msg : socket:file_descr -> io_vectors:IO_vectors.t -> (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. Data is written directly into the iov_buffer buffers.

Not implemented on Windows.

  • since 5.0.0
val send_msg : socket:file_descr -> io_vectors:IO_vectors.t -> 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". Data is written directly from the io_vectors buffers.

Not implemented on Windows.

  • since 5.0.0
val send_msgto : socket:file_descr -> io_vectors:IO_vectors.t -> fds:Unix.file_descr list -> dest:Unix.sockaddr -> int Lwt.t

send_msgto ~socket ~io_vectors ~fds ~dest is similar to send_msg but takes an additional dest argument to set the address when using a connection-less socket.

Not implemented on Windows.

  • since 5.4.0
type credentials = {
  1. cred_pid : int;
  2. cred_uid : int;
  3. 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_bool_option = Unix.socket_bool_option =
  1. | SO_DEBUG
  2. | SO_BROADCAST
  3. | SO_REUSEADDR
  4. | SO_KEEPALIVE
  5. | SO_DONTROUTE
  6. | SO_OOBINLINE
  7. | SO_ACCEPTCONN
  8. | TCP_NODELAY
  9. | IPV6_ONLY
  10. | SO_REUSEPORT
type socket_int_option = Unix.socket_int_option =
  1. | SO_SNDBUF
  2. | SO_RCVBUF
  3. | SO_ERROR
    (*
    • deprecated Use Unix.getsockopt_error instead.
    *)
  4. | SO_TYPE
  5. | SO_RCVLOWAT
  6. | SO_SNDLOWAT
type socket_optint_option = Unix.socket_optint_option =
  1. | SO_LINGER
type socket_float_option = Unix.socket_float_option =
  1. | SO_RCVTIMEO
  2. | SO_SNDTIMEO
    (*

    Note: these options are provided for the sake of completeness only. Lwt places all sockets in non-blocking mode, for which these options are meaningless. Use Lwt.pick with Lwt_unix.sleep or Lwt_unix.timeout for timeouts.

    *)
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 = {
  1. h_name : string;
  2. h_aliases : string array;
  3. h_addrtype : socket_domain;
  4. h_addr_list : inet_addr array;
}
type protocol_entry = Unix.protocol_entry = {
  1. p_name : string;
  2. p_aliases : string array;
  3. p_proto : int;
}
type service_entry = Unix.service_entry = {
  1. s_name : string;
  2. s_aliases : string array;
  3. s_port : int;
  4. 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 = {
  1. ai_family : socket_domain;
  2. ai_socktype : socket_type;
  3. ai_protocol : int;
  4. ai_addr : sockaddr;
  5. ai_canonname : string;
}
type getaddrinfo_option = Unix.getaddrinfo_option =
  1. | AI_FAMILY of socket_domain
  2. | AI_SOCKTYPE of socket_type
  3. | AI_PROTOCOL of int
  4. | AI_NUMERICHOST
  5. | AI_CANONNAME
  6. | AI_PASSIVE
val getaddrinfo : string -> string -> getaddrinfo_option list -> addr_info list Lwt.t

Wrapper for Unix.getaddrinfo

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

Wrapper for Unix.getnameinfo

Terminal interface

type terminal_io = Unix.terminal_io = {
  1. mutable c_ignbrk : bool;
  2. mutable c_brkint : bool;
  3. mutable c_ignpar : bool;
  4. mutable c_parmrk : bool;
  5. mutable c_inpck : bool;
  6. mutable c_istrip : bool;
  7. mutable c_inlcr : bool;
  8. mutable c_igncr : bool;
  9. mutable c_icrnl : bool;
  10. mutable c_ixon : bool;
  11. mutable c_ixoff : bool;
  12. mutable c_opost : bool;
  13. mutable c_obaud : int;
  14. mutable c_ibaud : int;
  15. mutable c_csize : int;
  16. mutable c_cstopb : int;
  17. mutable c_cread : bool;
  18. mutable c_parenb : bool;
  19. mutable c_parodd : bool;
  20. mutable c_hupcl : bool;
  21. mutable c_clocal : bool;
  22. mutable c_isig : bool;
  23. mutable c_icanon : bool;
  24. mutable c_noflsh : bool;
  25. mutable c_echo : bool;
  26. mutable c_echoe : bool;
  27. mutable c_echok : bool;
  28. mutable c_echonl : bool;
  29. mutable c_vintr : char;
  30. mutable c_vquit : char;
  31. mutable c_verase : char;
  32. mutable c_vkill : char;
  33. mutable c_veof : char;
  34. mutable c_veol : char;
  35. mutable c_vmin : int;
  36. mutable c_vtime : int;
  37. mutable c_vstart : char;
  38. mutable c_vstop : char;
}
val tcgetattr : file_descr -> terminal_io Lwt.t

Wrapper for Unix.tcgetattr

type setattr_when = Unix.setattr_when =
  1. | TCSANOW
  2. | TCSADRAIN
  3. | 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 =
  1. | TCIFLUSH
  2. | TCOFLUSH
  3. | TCIOFLUSH
val tcflush : file_descr -> flush_queue -> unit Lwt.t

Wrapper for Unix.tcflush

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

Wrapper for Unix.tcflow

Configuration (deprecated)

type async_method =
  1. | Async_none
    (*

    System calls are made synchronously, and may block the entire program.

    *)
  2. | Async_detach
    (*

    System calls are made in another system thread, thus without blocking other Lwt promises. The drawback is that it may degrade performance in some cases.

    This is the default.

    *)
  3. | Async_switch
    (*
    • deprecated

      A synonym for Async_detach. This was a different method in the past.

    *)

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".

  • deprecated

    Will always return Async_detach in Lwt 5.0.0.

val set_default_async_method : async_method -> unit

Sets the default async method.

  • deprecated

    Will be a no-op in Lwt 5.0.0.

val async_method : unit -> async_method

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

  • deprecated

    Will always return Async_detach in Lwt 5.0.0.

val async_method_key : async_method Lwt.key

The key for storing the local async method.

  • deprecated

    Will be ignored in Lwt 5.0.0.

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
  • deprecated

    Will have no effect in Lwt 5.0.0.

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

with_async_detach f is a shorthand for:

Lwt.with_value async_method_key (Some Async_detach) f
  • deprecated

    Will have no effect in Lwt 5.0.0.

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

with_async_switch f is a shorthand for:

Lwt.with_value async_method_key (Some Async_switch) f
  • deprecated

    Will have no effect in Lwt 5.0.0.

Low-level interaction

exception Retry

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

exception Retry_read

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

exception Retry_write

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

type io_event =
  1. | Read
  2. | 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 promise 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.

Note:

  • 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 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 argument will be ignored in Lwt 5.0.0, and this function will always act as if ~async_method:Async_detach is passed.

The async method is chosen follow:

  • if the optional parameter async_method is specified, it is used,
  • otherwise if the local key async_method_key is set in the current thread, it is used,
  • otherwise the default method (returned by default_async_method) is used.

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 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.

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

    Use run_job.

Notifications

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

make_notification ?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.

CPUs

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.

Versioned interfaces

module Versioned : sig ... end

Versioned variants of APIs undergoing breaking changes.

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