Writer

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).

io_stats Overall IO statistics for all writers

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.

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. Default is 2 minutes. You can supply `Unlimited to turn off buffer-age checks.

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).

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.

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.

open_file file opens file for writing and returns a writer for it. It uses Unix_syscalls.openfile to open the file.

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.

id t
Returns an id for this writer that is unique among all other writers

fd t
Returns the Fd.t used to create this writer

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.

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.

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.

write_char t c writes the character

newline t is write_char t '\n'

write_line t s is write t s; newline t.

write_byte t i writes one 8-bit integer (as the single character with that code). The given integer is taken modulo 256.

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.

Serialize data using marshal and write it to the writer

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.

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.

schedule_iovecs t iovecs like Writer.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.

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.

send t s writes a string to the channel that can be read back using Reader.recv

monitor t returns the writer's monitor.

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)

bytes_to_write t returns how many bytes have been requested to write but have not yet been written.

bytes_written t returns how many bytes have been written.

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.

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.

save_lines file lines writes all lines in lines to file, with each line followed by a newline.

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 result becomes determined when pipe_r reaches its EOF.

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.

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.

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:

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.

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. Default is 2 minutes. 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.

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).

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.

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.

open_file file opens file for writing and returns a writer for it. It uses Unix_syscalls.openfile to open the file.

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.

id t

fd 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.

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.

to_formatter t

write_char t c writes the character

newline t is write_char t '\n'

write_line t s is write t s; newline t.

write_byte t i writes one 8-bit integer (as the single character with that code). The given integer is taken modulo 256.

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.

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.

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.

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.

schedule_iovecs t iovecs like Writer.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.

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.

send t s writes a string to the channel that can be read back using Reader.recv

monitor t returns the writer's monitor.

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.

is_open t is not (is_closed t)

bytes_to_write t returns how many bytes have been requested to write but have not yet been written.

bytes_written t returns how many bytes have been written.

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.

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.

save_lines file lines writes all lines in lines to file, with each line followed by a newline.

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 result becomes determined when pipe_r reaches its EOF.

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.

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.

Module Writer