Module Lwt_io

Type of buffered byte channels

Input mode

Output mode

Channel mode

input input mode representation

output output mode representation

Type of input channels

Type of output channels

mode ch returns the mode of a channel

The standard input, it reads data from Lwt_unix.stdin

The standard output, it writes data to Lwt_unix.stdout

The standard output for error messages, it writes data to Lwt_unix.stderr

Inputs which returns always '\x00'

Output which drops everything

pipe ?in_buffer ?out_buffer () creates a pipe using Lwt_unix.pipe and makes two channels from the two returned file descriptors

make ?buffer ?close ~mode perform_io is the main function for creating new channels.

parameter buffer

user-supplied buffer. When this argument is present, its value will be used as the buffer for the created channel. The size of buffer must conform to the limitations described in set_default_buffer_size. When this argument is not present, a new internal buffer of default size will be allocated for this channel.

Warning: do not use the same buffer for simultaneous work with more than one channel.

There are other functions in this module that take a buffer argument, sharing the same semantics.

parameter close

close function of the channel. It defaults to Lwt.return

parameter seek

same meaning as Unix.lseek

parameter mode

either input or output

parameter perform_io

is the read or write function. It is called when more input is needed or when the buffer need to be flushed.

Create a channel from a byte array. Reading/writing is done directly on the provided array.

of_fd ?buffer ?close ~mode fd creates a channel from a file descriptor.

parameter close

defaults to closing the file descriptor.

of_unix_fd ?buffer ?close ~mode fd is a short-hand for:

of_fd ?buffer ?close (Lwt_unix.of_unix_file_descr fd)

close ch closes the given channel. If ch is an output channel, it performs all pending actions, flushes it and closes it. If ch is an input channel, it just closes it immediately.

close returns the result of the close function of the channel. Multiple calls to close will return exactly the same value.

Note: you cannot use close on channels obtained with atomic.

abort ch abort current operations and close the channel immediately.

atomic f transforms a sequence of io operations into one single atomic io operation.

Note:

• the channel passed to f is invalid after f terminates
• atomic can be called inside another atomic

Retrieves the length of the file at the given path. If the path refers to a directory, the returned promise is rejected with Unix.(Unix_error (EISDIR, _, _)).

buffered oc returns the number of bytes in the buffer

flush oc performs all pending writes on oc

flush_all () flushes all open output channels

Returns the size of the internal buffer.

Resize the internal buffer to the given size

is_busy channel returns whether the given channel is currently busy. A channel is busy when there is at least one job using it that has not yet terminated.

is_closed channel returns whether the given channel is currently closed.

since

4.2.0

position ch Returns the current position in the channel.

set_position ch pos Sets the position in the output channel. This does not work if the channel does not support random access.

Returns the length of the channel in bytes

read_char ic reads the next character of ic.

raises End_of_file

if the end of the file is reached

Same as Lwt_io.read_char, but does not raise End_of_file on end of input

read_chars ic returns a stream holding all characters of ic

read_line ic reads one complete line from ic and returns it without the end of line. End of line is either "\n" or "\r\n".

If the end of input is reached before reading any character, End_of_file is raised. If it is reached before reading an end of line but characters have already been read, they are returned.

Same as read_line but do not raise End_of_file on end of input.

read_lines ic returns a stream holding all lines of ic

If ~count is specified, read ~count ic reads at most ~count characters from ic. Note that fewer than ~count characters can be read; check the size of the resulting string. read returns "" if the end of input is reached.

If ~count is not specified, read ic reads all bytes until the end of input.

read_into ic buffer offset length reads up to length bytes, stores them in buffer at offset offset, and returns the number of bytes read.

Note: read_into does not raise End_of_file, it returns a length of 0 instead.

read_into_exactly ic buffer offset length reads exactly length bytes and stores them in buffer at offset offset.

raises End_of_file

on end of input

read_value channel reads a marshaled value from channel; it corresponds to the standard library's Marshal.from_channel. The corresponding writing function is write_value.

Note that reading marshaled values is not, in general, type-safe. See the warning in the description of module Marshal for details. The short version is: if you read a value of one type, such as string, when a value of another type, such as int has actually been marshaled to channel, you may get arbitrary behavior, including segmentation faults, access violations, security bugs, etc.

write_char oc char writes char on oc

write_chars oc chars writes all characters of chars on oc

write oc str writes all characters of str on oc

write_line oc str writes str on oc followed by a new-line.

write_lines oc lines writes all lines of lines to oc

write_from oc buffer offset length writes up to length bytes to oc, from buffer at offset offset and returns the number of bytes actually written

See write.

write_from_exactly oc buffer offset length writes all length bytes from buffer at offset offset to oc

See write_from_exactly.

write_value channel ?flags v writes v to channel using the Marshal module of the standard library. See Marshal.to_channel for an explanation of ?flags.

The corresponding reading function is read_value. See warnings about type safety in the description of read_value.

%! does nothing here. To flush the channel, use Lwt_io.flush channel.

%! does nothing here. To flush the channel, use Lwt_io.flush channel.

%! does nothing here. To flush the channel, use Lwt_io.(flush stdout).

%! does nothing here. To flush the channel, use Lwt_io.(flush stdout).

%! does nothing here. To flush the channel, use Lwt_io.(flush stderr).

%! does nothing here. To flush the channel, use Lwt_io.(flush stderr).

hexdump_stream oc byte_stream produces the same output as the command hexdump -C.

hexdump oc str = hexdump_stream oc (Lwt_stream.of_string str)

Type of file names

Lwt_io.open_file ~mode file opens the given file, either for reading (with ~mode:Input) or for writing (with ~mode:Output). The returned channel provides buffered I/O on the file.

If ~buffer is supplied, it is used as the I/O buffer.

If ~flags is supplied, the file is opened with the given flags (see Unix.open_flag). Note that ~flags is used exactly as given. For example, opening a file with ~flags and ~mode:Input does not implicitly add O_RDONLY. So, you should include O_RDONLY when opening for reading (~mode:Input), and O_WRONLY when opening for writing (~mode:Input). It is also recommended to include O_NONBLOCK, unless you are sure that the file cannot be a socket or a named pipe.

The default permissions used for creating new files are 0o666, i.e. reading and writing are allowed for the file owner, group, and everyone. These default permissions can be overridden by supplying ~perm.

Note: if opening for writing (~mode:Output), and the file already exists, open_file truncates (clears) the file by default. If you would like to keep the pre-existing contents of the file, use the ~flags parameter to pass a custom flags list that does not include Unix.O_TRUNC.

raises Unix.Unix_error

on error.

Lwt_io.with_file ~mode filename f opens the given using Lwt_io.open_file, and passes the resulting channel to f. Lwt_io.with_file ensures that the channel is closed when the promise returned by f resolves, or if f raises an exception.

See Lwt_io.open_file for a description of the arguments, warnings, and other notes.

open_temp_file () starts creating a new temporary file, and evaluates to a promise for the pair of the file's name, and an output channel for writing to the file.

The caller should take care to delete the file later. Alternatively, see Lwt_io.with_temp_file.

The ?buffer and ?perm arguments are passed directly to an internal call to Lwt_io.open_file.

If not specified, ?flags defaults to [O_CREATE; O_EXCL; O_WRONLY; O_CLOEXEC]. If specified, the specified flags are used exactly. Note that these should typically contain at least O_CREAT and O_EXCL, otherwise open_temp_file may open an existing file.

?temp_dir can be used to choose the directory in which the file is created. For the current directory, use Filename.current_dir_name. If not specified, the directory is taken from Filename.get_temp_dir_name, which is typically set to your system temporary file directory.

?prefix helps determine the name of the file. It will be the prefix concatenated with a random sequence of characters. If not specified, open_temp_file uses some default prefix.

?suffix is like prefix, but it is appended at the end of the filename. In particular, it can be used to set the extension. This argument is supported since Lwt 4.4.0.

since

3.2.0

with_temp_file f calls open_temp_file (), passing all optional arguments directly to it. It then attaches f to run after the file is created, passing the filename and output channel to f. When the promise returned by f is resolved, with_temp_file closes the channel and deletes the temporary file by calling Lwt_unix.unlink.

since

3.2.0

Creates a temporary directory, and returns a promise that resolves to its path. The caller must take care to remove the directory. Alternatively, see Lwt_io.with_temp_dir.

If ~perm is specified, the directory is created with the given permissions. The default permissions are 0755.

~parent is the directory in which the temporary directory is created. If not specified, the default value is the result of Filename.get_temp_dir_name ().

~prefix is prepended to the directory name, and ~suffix is appended to it.

since

4.4.0

with_temp_dir f first calls create_temp_dir, forwarding all optional arguments to it. Once the temporary directory is created at path, with_temp_dir f calls f path. When the promise returned by f path is resolved, with_temp_dir f recursively deletes the temporary directory and all its contents.

since

4.4.0

open_connection ?fd ?in_buffer ?out_buffer addr opens a connection to the given address and returns two channels for using it. If fd is not specified, a fresh one will be used.

The connection is completely closed when you close both channels.

raises Unix.Unix_error

on error.

with_connection ?fd ?in_buffer ?out_buffer addr f opens a connection to the given address and passes the channels to f

Type of a server

establish_server_with_client_socket listen_address f creates a server which listens for incoming connections on listen_address. When a client makes a new connection, it is passed to f: more precisely, the server calls

f client_address client_socket

where client_address is the address (peer name) of the new client, and client_socket is the socket connected to the client.

The server does not block waiting for f to complete: it concurrently tries to accept more client connections while f is handling the client.

When the promise returned by f completes (i.e., f is done handling the client), establish_server_with_client_socket automatically closes client_socket. This is a default behavior that is useful for simple cases, but for a robust application you should explicitly close these channels yourself, and handle any exceptions as appropriate. If the channels are still open when f completes, and their automatic closing raises an exception, establish_server_with_client_socket treats it as an unhandled exception reaching the top level of the application: it passes that exception to Lwt.async_exception_hook, the default behavior of which is to print the exception and terminate your process.

Automatic closing can be completely disabled by passing ~no_close:true.

Similarly, if f raises an exception (or the promise it returns fails with an exception), establish_server_with_client_socket can do nothing with that exception, except pass it to Lwt.async_exception_hook.

~server_fd can be specified to use an existing file descriptor for listening. Otherwise, a fresh socket is created internally. In either case, establish_server_with_client_socket will internally assign listen_address to the server socket.

~backlog is the argument passed to Lwt_unix.listen. Its default value is SOMAXCONN, which varies by platform and socket kind.

The returned promise (a server Lwt.t) resolves when the server has just started listening on listen_address: right after the internal call to listen, and right before the first internal call to accept.

since

4.1.0

Like Lwt_io.establish_server_with_client_socket, but passes two buffered channels to the connection handler f. These channels wrap the client socket.

The channels are closed automatically when the promise returned by f resolves. To avoid this behavior, pass ~no_close:true.

since

3.1.0

Closes the given server's listening socket. The returned promise resolves when the close(2) system call completes. This function does not affect the sockets of connections that have already been accepted, i.e. passed to f by establish_server.

since

3.0.0

lines_of_file name returns a stream of all lines of the file with name name. The file is automatically closed when all lines have been read.

lines_to_file name lines writes all lines of lines to file with name name.

chars_of_file name returns a stream of all characters of the file with name name. As for lines_of_file the file is closed when all characters have been read.

chars_to_file name chars writes all characters of chars to name

Common interface for reading/writing integers in binary

Reading/writing of numbers in little-endian

Reading/writing of numbers in big-endian

Same as Lwt_sys.byte_order.

block ch size f pass to f the internal buffer and an offset. The buffer contains size chars at offset. f may read or write these chars. size must satisfy 0 <= size <= 16

Information for directly accessing the internal buffer of a channel

direct_access ch f passes to f a direct_access structure. f must use it and update da_ptr to reflect how many bytes have been read/written.

Return the default size for buffers. Channels that are created without a specific buffer use new buffer of this size.

Change the default buffer size.

raises Invalid_argument

if the given size is smaller than 16 or greater than Sys.max_string_length

Like establish_server_with_client_address, but does not pass the client address or fd to the callback f.

deprecated

Use establish_server_with_client_address.

since

3.0.0

Versioned variants of APIs undergoing breaking changes.