# Module Out_channel

    module Out_channel: sig .. end

Output channels.

This module provides functions for working with output channels.

See the example section below.

- Since 4.14

## Channels

    type t = out_channel

The type of output channel.

    type open_flag = open_flag =

`|` | `Open_rdonly` | `(*` | open for reading. | `*)`
`|` | `Open_wronly` | `(*` | open for writing. | `*)`
`|` | `Open_append` | `(*` | open for appending: always write at end of file. | `*)`
`|` | `Open_creat` | `(*` | create the file if it does not exist. | `*)`
`|` | `Open_trunc` | `(*` | empty the file if it already exists. | `*)`
`|` | `Open_excl` | `(*` | fail if Open_creat and the file already exists. | `*)`
`|` | `Open_binary` | `(*` | open in binary mode (no conversion). | `*)`
`|` | `Open_text` | `(*` | open in text mode (may perform conversions). | `*)`
`|` | `Open_nonblock` | `(*` | open in non-blocking mode. | `*)`

Opening modes for `Out_channel.open_gen`.

    val stdout : t

The standard output for the process.

    val stderr : t

The standard error output for the process.

    val 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.

    val open_text : string -> t

Same as `Out_channel.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 `Out_channel.open_bin`.

    val open_gen : open_flag list -> 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. `Out_channel.open_text` and `Out_channel.open_bin` are special cases of this function.

    val 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.

    val with_open_text : string -> (t -> 'a) -> 'a

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

    val with_open_gen : open_flag list -> int -> string -> (t -> 'a) -> 'a

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

    val 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 `Out_channel.close` and `Out_channel.flush`, which do nothing when applied to an already closed channel. Note that `Out_channel.close` may raise `Sys_error` if the operating system signals an error when flushing or closing.

    val close_noerr : t -> unit

Same as `Out_channel.close`, but ignore all errors.

## Output

    val output_char : t -> char -> unit

Write the character on the given output channel.

    val 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.

    val output_string : t -> string -> unit

Write the string on the given output channel.

    val output_bytes : t -> bytes -> unit

Write the byte sequence on the given output channel.

## Advanced output

    val 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`.

- Raises `Invalid_argument` if `pos` and `len` do not designate a valid range of `buf`.

    val output_substring : t -> string -> int -> int -> unit

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

## Flushing

    val 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.

    val flush_all : unit -> unit

Flush all open output channels; ignore errors.

## Seeking

    val 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.

    val 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 `Out_channel.pos`, then going back to this position using `Out_channel.seek` will not work. For this programming idiom to work reliably and portably, the file must be opened in binary mode.

## Attributes

    val 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.

    val 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.

    val 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 `Out_channel.flush` or `Out_channel.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.

    val is_buffered : t -> bool

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

    val 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))