Module Stdlib.Out_channel

Output channels.

This module provides functions for working with output channels.

See the example section below.

Channels

type t = out_channel;

The type of output channel.

type open_flag = open_flag =
  1. | Open_rdonly
    /*

    open for reading.

    */
  2. | Open_wronly
    /*

    open for writing.

    */
  3. | Open_append
    /*

    open for appending: always write at end of file.

    */
  4. | Open_creat
    /*

    create the file if it does not exist.

    */
  5. | Open_trunc
    /*

    empty the file if it already exists.

    */
  6. | Open_excl
    /*

    fail if Open_creat and the file already exists.

    */
  7. | Open_binary
    /*

    open in binary mode (no conversion).

    */
  8. | Open_text
    /*

    open in text mode (may perform conversions).

    */
  9. | Open_nonblock
    /*

    open in non-blocking mode.

    */
;

Opening modes for open_gen.

let stdout: t;

The standard output for the process.

let stderr: t;

The standard error output for the process.

let open_bin: string => t;

Open the named file for writing, and return a new output channel on that file, positioned at the beginning of the file. The file is truncated to zero length if it already exists. It is created if it does not already exists.

let open_text: string => t;

Same as open_bin, but the file is opened in text mode, so that newline translation takes place during writes. On operating systems that do not distinguish between text mode and binary mode, this function behaves like open_bin.

let open_gen: list(open_flag) => int => string => t;

open_gen mode perm filename opens the named file for writing, as described above. The extra argument mode specifies the opening mode. The extra argument perm specifies the file permissions, in case the file must be created. open_text and open_bin are special cases of this function.

let with_open_bin: string => (t => 'a) => 'a;

with_open_bin fn f opens a channel oc on file fn and returns f oc. After f returns, either with a value or by raising an exception, oc is guaranteed to be closed.

let with_open_text: string => (t => 'a) => 'a;

Like with_open_bin, but the channel is opened in text mode (see open_text).

let with_open_gen: list(open_flag) => int => string => (t => 'a) => 'a;

Like with_open_bin, but can specify the opening mode and file permission, in case the file must be created (see open_gen).

let close: t => unit;

Close the given channel, flushing all buffered write operations. Output functions raise a Sys_error exception when they are applied to a closed output channel, except close and flush, which do nothing when applied to an already closed channel. Note that close may raise Sys_error if the operating system signals an error when flushing or closing.

let close_noerr: t => unit;

Same as close, but ignore all errors.

Output

let output_char: t => char => unit;

Write the character on the given output channel.

let output_byte: t => int => unit;

Write one 8-bit integer (as the single character with that code) on the given output channel. The given integer is taken modulo 256.

let output_string: t => string => unit;

Write the string on the given output channel.

let output_bytes: t => bytes => unit;

Write the byte sequence on the given output channel.

Advanced output

let output: t => bytes => int => int => unit;

output oc buf pos len writes len characters from byte sequence buf, starting at offset pos, to the given output channel oc.

let output_substring: t => string => int => int => unit;

Same as output but take a string as argument instead of a byte sequence.

Flushing

let flush: t => unit;

Flush the buffer associated with the given output channel, performing all pending writes on that channel. Interactive programs must be careful about flushing standard output and standard error at the right time.

let flush_all: unit => unit;

Flush all open output channels; ignore errors.

Seeking

let seek: t => int64 => unit;

seek chan pos sets the current writing position to pos for channel chan. This works only for regular files. On files of other kinds (such as terminals, pipes and sockets), the behavior is unspecified.

let pos: t => int64;

Return the current writing position for the given channel. Does not work on channels opened with the Open_append flag (returns unspecified results).

For files opened in text mode under Windows, the returned position is approximate (owing to end-of-line conversion); in particular, saving the current position with pos, then going back to this position using seek will not work. For this programming idiom to work reliably and portably, the file must be opened in binary mode.

Attributes

let length: t => int64;

Return the size (number of characters) of the regular file on which the given channel is opened. If the channel is opened on a file that is not a regular file, the result is meaningless.

let set_binary_mode: t => bool => unit;

set_binary_mode oc true sets the channel oc to binary mode: no translations take place during output.

set_binary_mode oc false sets the channel oc to text mode: depending on the operating system, some translations may take place during output. For instance, under Windows, end-of-lines will be translated from \n to \r\n.

This function has no effect under operating systems that do not distinguish between text mode and binary mode.

let is_binary_mode: t => bool;

is_binary_mode oc returns whether the channel oc is in binary mode (see set_binary_mode).

  • since 5.2
let set_buffered: t => bool => unit;

set_buffered oc true sets the channel oc to buffered mode. In this mode, data output on oc will be buffered until either the internal buffer is full or the function flush or flush_all is called, at which point it will be sent to the output device.

set_buffered oc false sets the channel oc to unbuffered mode. In this mode, data output on oc will be sent to the output device immediately.

All channels are open in buffered mode by default.

let is_buffered: t => bool;

is_buffered oc returns whether the channel oc is buffered (see set_buffered).

let isatty: t => bool;

isatty oc is true if oc refers to a terminal or console window, false otherwise.

  • since 5.1

Examples

Writing the contents of a file:

let write_file file s =
  Out_channel.with_open_bin file
    (fun oc -> Out_channel.output_string oc s))