Writer is Async's main API for output to a file descriptor. It is the analog of Core.Std.Out_channel.

Each writer has an internal buffer, to which Writer.write* adds data. Each writer uses an Async microthread that makes write() system calls to move the data from the writer's buffer to an OS buffer via the file descriptor. There is no guarantee that the data sync on the other side of the writer can keep up with the rate at which you are writing. If it cannot, the OS buffer will fill up and the writer's micro-thread will be unable to send any bytes. In that case, calls to Writer.write* will grow the writer's buffer without bound, as long as your program produces data. One solution to this problem is to call Writer.flushed and not continue until that becomes determined, which will only happen once the bytes in the writer's buffer have been successfully transferred to the OS buffer. Another solution is to check Writer.bytes_to_write and not produce any more data if that is beyond some bound.

There are two kinds of errors that one can handle with writers. First, a writer can be closed, which will cause future writes (and other operations) to synchronously raise an excecption. Second, the writer's microthread can fail due to a write() system call failing. This will cause an exception to be sent to the writer's monitor, which will be a child of the monitor in effect when the writer is created. One can deal with such asynchronous exceptions in the usual way, by handling the stream returned by Monitor.errors (Writer.monitor writer).

module Id : Core.Std.Unique_id
type t
include Core.Std.Invariant.S with type t := t
val io_stats : Io_stats.t
io_stats Overall IO statistics for all writers
val stdout : t Core.Std.Lazy.t
stdout and stderr are writers for file descriptors 1 and 2. They are lazy because we don't want to create them in all programs that happen to link with Async.

When either stdout or stderr is created, they both are created. Furthermore, if they point to the same inode, then they will be the same writer to Fd.stdout. This can be confusing, because fd (force stderr) will be Fd.stdout, not Fd.stderr. And subsequent modifications of Fd.stderr will have no effect on Writer.stderr.

Unfortunately, the sharing is necessary because Async uses OS threads to do write() syscalls using the writer buffer. When calling a program that redirects stdout and stderr to the same file, as in:

      foo.exe >/tmp/z.file 2>&1

if Writer.stdout and Writer.stderr weren't the same writer, then they could have threads simultaneously writing to the same file, which could easily cause data loss.

val stderr : t Core.Std.Lazy.t
type buffer_age_limit = [ `At_most of Core.Std.Time.Span.t | `Unlimited ]
val create : ?buf_len:int ->
?syscall:[ `Per_cycle | `Periodic of Core.Std.Time.Span.t ] ->
?buffer_age_limit:buffer_age_limit ->
?raise_when_consumer_leaves:bool -> Fd.t -> t
create ?buf_len ?syscall ?buffer_age_limit fd creates a new writer. The file descriptor fd should not be in use for writing by anything else.

By default, a write system call occurs at the end of a cycle in which bytes were written. One can supply ~syscall:(`Periodic span) to get better performance. This batches writes together, doing the write system call periodically according to the supplied span.

A writer can asynchronously fail if the underlying write syscall returns an error, e.g. EBADF, EPIPE, ECONNRESET, ....

buffer_age_limit specifies how backed up you can get before raising an exception. The default is `Unlimited for files, and 2 minutes for other kinds of file descriptors. You can supply `Unlimited to turn off buffer-age checks.

raise_when_consumer_leaves specifies whether the writer should raise an exception when the consumer receiving bytes from the writer leaves, i.e. in Unix, the write syscall returns EPIPE or ECONNRESET. If not raise_when_consumer_leaves, then the writer will silently drop all writes after the consumer leaves, and the writer will eventually fail with a writer-buffer-older-than error if the application remains open long enough.

val raise_when_consumer_leaves : t -> bool
val set_raise_when_consumer_leaves : t -> bool -> unit
set_raise_when_consumer_leaves t bool sets the raise_when_consumer_leaves flag of t, which determies how t responds to a write system call raising EPIPE and ECONNRESET (see create).
val set_buffer_age_limit : t -> buffer_age_limit -> unit
set_buffer_age_limit t buffer_age_limit replaces the existing buffer age limit with the new one. This is useful for stdout and stderr, which are lazily created in a context that does not allow applications to specify buffer_age_limit.
val consumer_left : t -> unit Import.Deferred.t
consumer_left t returns a deferred that becomes determined when t attempts to write to a pipe that broke because the consumer on the other side left.
val of_out_channel : out_channel -> Fd.Kind.t -> t
val open_file : ?append:bool ->
?close_on_exec:bool -> ?perm:int -> string -> t Import.Deferred.t
open_file file opens file for writing and returns a writer for it. It uses Unix_syscalls.openfile to open the file.
val with_file : ?perm:int ->
?append:bool ->
?exclusive:bool ->
string -> f:(t -> 'a Import.Deferred.t) -> 'a Import.Deferred.t
with_file ~file f opens file for writing, creates a writer t, and runs f t to obtain a deferred d. When d becomes determined, the writer is closed. When the close completes, the result of with_file becomes determined with the value of d.

There is no need to call Writer.flushed to ensure that with_file waits for the writer to be flushed before closing it. Writer.close will already wait for the flush.

val id : t -> Id.t
id t Returns an id for this writer that is unique among all other writers
val fd : t -> Fd.t
fd t Returns the Fd.t used to create this writer
val set_fd : t -> Fd.t -> unit Import.Deferred.t
set_fd t fd sets the fd used by t for its underlying system calls. It first waits until everything being sent to the current fd is flushed. Of course, one must understand how the writer works and what one is doing to use this.
val write_gen : length:('a -> int) ->
blit_to_bigstring:('a, Core.Std.Bigstring.t) Core.Std.Blit.blit ->
?pos:int -> ?len:int -> t -> 'a -> unit
write_gen t a writes a to writer t, with length specifying the number of bytes needed and blit_to_bigstring blitting a directly into the t's buffer. If one has a type that has length and blit_to_bigstring functions, like:
      module A : sig
        type t
        val length : t -> int
        val blit_to_bigstring : (t, Bigstring.t) Blit.blit
      end

then one can use write_gen to implement a custom analog of Writer.write, like:

      module Write_a : sig
        val write : ?pos:int -> ?len:int -> A.t -> Writer.t -> unit
      end = struct
        let write ?pos ?len a writer =
          Writer.write_gen
            ~length:A.length
            ~blit_to_bigstring:A.blit_to_bigstring
            ?pos ?len writer a
      end

If it is difficult to write only part of a value, one can choose to not support ?pos and ?len:

      module Write_a : sig
        val write : A.t -> Writer.t -> unit
      end = struct
        let write a writer =
          Writer.write_gen
            ~length:A.length
            ~blit_to_bigstring:A.blit_to_bigstring
            writer a
      end
val write : ?pos:int -> ?len:int -> t -> string -> unit
write ?pos ?len t s adds a job to the writer's queue of pending writes. The contents of the string are copied to an internal buffer before write returns, so clients can do whatever they want with s after that.
val write_bigstring : ?pos:int -> ?len:int -> t -> Core.Std.Bigstring.t -> unit
val write_iobuf : ?pos:int -> ?len:int -> t -> ('a, 'b) Core.Std.Iobuf.t -> unit
val write_substring : t -> Core.Std.Substring.t -> unit
val write_bigsubstring : t -> Core.Std.Bigsubstring.t -> unit
val writef : t -> ('a, unit, string, unit) format4 -> 'a
val to_formatter : t -> Format.formatter
to_formatter t Returns an OCaml-formatter that one can print to using Format.fprintf. Note that flushing the formatter will only submit all buffered data to the writer, but does _not_ guarantee flushing to the operating system.
val write_char : t -> char -> unit
write_char t c writes the character
val newline : t -> unit
newline t is write_char t '\n'
val write_line : t -> string -> unit
write_line t s is write t s; newline t.
val write_byte : t -> int -> unit
write_byte t i writes one 8-bit integer (as the single character with that code). The given integer is taken modulo 256.
val write_sexp : ?hum:bool -> t -> Core.Std.Sexp.t -> unit
val write_bin_prot : t -> 'a Core.Std.Bin_prot.Type_class.writer -> 'a -> unit
write_bin_prot writes out a value using its bin_prot sizer/writer pair. The format is the "size-prefixed binary protocol", in which the length of the data is written before the data itself. This is the format that Reader.read_bin_prot reads.
val write_marshal : t -> flags:Marshal.extern_flags list -> 'a -> unit
Serialize data using marshal and write it to the writer
Unlike the write_ functions, all functions starting with schedule_ require flushing or closing of the writer after returning before it is safe to modify the bigstrings which were directly or indirectly passed to these functions. The reason is that these bigstrings will be read from directly when writing; their contents is not copied to internal buffers.

This is important if users need to send the same large data string to a huge number of clients simultaneously (e.g. on a cluster), because these functions then avoid needlessly exhausting memory by sharing the data.

val schedule_bigstring : t -> ?pos:int -> ?len:int -> Core.Std.Bigstring.t -> unit
schedule_bigstring t bstr schedules a write of bigstring bstr. It is not safe to change the bigstring until the writer has been successfully flushed or closed after this operation.
val schedule_bigsubstring : t -> Core.Std.Bigsubstring.t -> unit
val schedule_iovec : t -> Core.Std.Bigstring.t Core.Std.Unix.IOVec.t -> unit
schedule_iovec t iovec schedules a write of I/O-vector iovec. It is not safe to change the bigstrings underlying the I/O-vector until the writer has been successfully flushed or closed after this operation.
val schedule_iovecs : t -> Core.Std.Bigstring.t Core.Std.Unix.IOVec.t Core.Std.Queue.t -> unit
schedule_iovecs t iovecs like schedule_iovec, but takes a whole queue iovecs of I/O-vectors as argument. The queue is guaranteed to be empty when this function returns and can be modified. It is not safe to change the bigstrings underlying the I/O-vectors until the writer has been successfully flushed or closed after this operation.
val flushed : t -> unit Import.Deferred.t
flushed t returns a deferred that will become determined when all prior writes complete (i.e. the write() system call returns). If a prior write fails, then the deferred will never become determined.

It is OK to call flushed t after t has been closed.

val flushed_time : t -> Core.Std.Time.t Import.Deferred.t
val fsync : t -> unit Import.Deferred.t
val fdatasync : t -> unit Import.Deferred.t
val send : t -> string -> unit
send t s writes a string to the channel that can be read back using Reader.recv
val monitor : t -> Import.Monitor.t
monitor t returns the writer's monitor.
val close : ?force_close:unit Import.Deferred.t -> t -> unit Import.Deferred.t
close ?force_close t waits for the writer to be flushed, and then calls Unix.close on the underlying file descriptor. force_close causes the Unix.close to happen even if the flush hangs. By default force_close is Deferred.never () for files and after (sec 5) for other types of file descriptors (e.g. sockets). If the close is forced, data in the writer's buffer may not be written to the file descriptor. You can check this by calling bytes_to_write after close finishes.

close will raise an exception if the Unix.close on the underlying file descriptor fails.

It is required to call close on a writer in order to close the underlying file descriptor. Not doing so will cause a file descriptor leak. It also will cause a space leak, because until the writer is closed, it is held on to in order to flush the writer on shutdown.

It is an error to call other operations on t after close t has been called, except that calls of close subsequent to the original call to close will return the same deferred as the original call.

close_finished t becomes determined after t's underlying file descriptor has been closed, i.e. it is the same as the result of close. close_finished differs from close in that it does not have the side effect of initiating a close.

is_closed t returns true iff close t has been called.

is_open t is not (is_closed t)

with_close t ~f runs f (), and closes t after f finishes or raises.

val close_finished : t -> unit Import.Deferred.t
val is_closed : t -> bool
val is_open : t -> bool
val with_close : t -> f:(unit -> 'a Import.Deferred.t) -> 'a Import.Deferred.t
val with_flushed_at_close : t ->
flushed:(unit -> unit Import.Deferred.t) ->
f:(unit -> 'a Import.Deferred.t) -> 'a Import.Deferred.t
val bytes_to_write : t -> int
bytes_to_write t returns how many bytes have been requested to write but have not yet been written.
val bytes_written : t -> Core.Std.Int63.t
bytes_written t returns how many bytes have been written.
val bytes_received : t -> Core.Std.Int63.t
bytes_received t returns how many bytes have been received by the writer. As long as the writer is running, bytes_received = bytes_written + bytes_to_write.
with_file_atomic ?temp_file ?perm ?fsync file ~f creates a writer to a temp file, feeds that writer to f, and when the result of f becomes determined, atomically moves (i.e. uses Unix.rename) the temp file to file. If file currently exists, it will be replaced, even if it is read only. The temp file will be file (or temp_file if supplied) suffixed by a unique random sequence of six characters. The temp file may need to be removed in case of a crash so it may be prudent to choose a temp file that can be easily found by cleanup tools.

If fsync is true, the temp file will be flushed to disk before it takes the place of the target file, thus guaranteeing that the target file will always be in a sound state, even after a machine crash. Since synchronization is extremely slow, this is not the default. Think carefully about the event of machine crashes and whether you may need this option!

We intend for with_file_atomic to preserve the behavior of the open system call, so if file does not exist, we will apply the umask to perm. If file does exist, perm will default to the file's current permissions rather than 0o666.

save is a special case of with_file_atomic that atomically writes the given string to the specified file.

save_sexp is a special case of with_file_atomic that atomically writes the given sexp to the specified file.

val with_file_atomic : ?temp_file:string ->
?perm:Core.Std.Unix.file_perm ->
?fsync:bool ->
string -> f:(t -> 'a Import.Deferred.t) -> 'a Import.Deferred.t
val save : ?temp_file:string ->
?perm:Core.Std.Unix.file_perm ->
?fsync:bool -> string -> contents:string -> unit Import.Deferred.t
val save_lines : ?temp_file:string ->
?perm:Core.Std.Unix.file_perm ->
?fsync:bool -> string -> string list -> unit Import.Deferred.t
save_lines file lines writes all lines in lines to file, with each line followed by a newline.
val save_sexp : ?temp_file:string ->
?perm:Core.Std.Unix.file_perm ->
?fsync:bool ->
?hum:bool -> string -> Core.Std.Sexp.t -> unit Import.Deferred.t
save_sexp t sexp writes sexp to t, followed by a newline. To read a file produced using save_sexp, one would typically use Reader.load_sexp, which deals with the additional whitespace and works nicely with converting the sexp to a value.
val transfer' : ?stop:unit Import.Deferred.t ->
t ->
'a Import.Pipe.Reader.t ->
('a Core.Std.Queue.t -> unit Import.Deferred.t) -> unit Import.Deferred.t
transfer' t pipe_r f repeatedly pulls values from pipe_r, and feeds them to f, which should in turn write them to t. It provides pushback to pipe_r by not reading when t cannot keep up with the data being pushed in.

The transfer' stops and the result becomes determined when pipe_r reaches its EOF, when t's consumer leaves, or when stop becomes determined.

transfer' causes Pipe.flushed on pipe_r's writer to ensure that the bytes have been flushed to t before returning. It also waits on Pipe.upstream_flushed at shutdown.

transfer t pipe_r f is equivalent to:

      transfer' t pipe_r (fun q -> Queue.iter q ~f; Deferred.unit)
val transfer : ?stop:unit Import.Deferred.t ->
t -> 'a Import.Pipe.Reader.t -> ('a -> unit) -> unit Import.Deferred.t
val pipe : t -> string Import.Pipe.Writer.t
pipe t returns the writing end of a pipe attached to t that pushes back when t cannot keep up with the data being pushed in. Closing the pipe will close t.
val of_pipe : Core.Std.Info.t ->
string Import.Pipe.Writer.t ->
(t * [ `Closed_and_flushed_downstream of unit Import.Deferred.t ])
Import.Deferred.t
of_pipe info pipe_w returns a writer t such that data written to t will appear on pipe_w. If either t or pipe_w are closed, the other is closed as well.

of_pipe is implemented by attaching t to the write-end of a Unix pipe, and shuttling bytes from the read-end of the Unix pipe to pipe_w.

val sexp_of_t : t -> Sexplib.Sexp.t
val buffer_age_limit_of_sexp : Sexplib.Sexp.t -> buffer_age_limit
val __buffer_age_limit_of_sexp__ : Sexplib.Sexp.t -> buffer_age_limit
val sexp_of_buffer_age_limit : buffer_age_limit -> Sexplib.Sexp.t
val bin_buffer_age_limit : buffer_age_limit Core.Std.Bin_prot.Type_class.t
val bin_read_buffer_age_limit : buffer_age_limit Core.Std.Bin_prot.Read.reader
val __bin_read_buffer_age_limit__ : (int -> buffer_age_limit) Core.Std.Bin_prot.Read.reader
val bin_reader_buffer_age_limit : buffer_age_limit Core.Std.Bin_prot.Type_class.reader
val bin_size_buffer_age_limit : buffer_age_limit Core.Std.Bin_prot.Size.sizer
val bin_write_buffer_age_limit : buffer_age_limit Core.Std.Bin_prot.Write.writer
val bin_writer_buffer_age_limit : buffer_age_limit Core.Std.Bin_prot.Type_class.writer