File manipulation

This is a guide to basic file manipulation in OCaml using only what the standard library provides.

Official documentation for the modules of interest: the core library including the initially opened module Pervasives, Printf.

The standard library doesn't provide functions that directly read a file into a string or save a string into a file. Such functions can be found in third-party libraries such as Extlib. See Std.input_file and Std.output_file.

Buffered channels

The normal way of opening a file in OCaml returns a channel. There are two kinds of channels:

  • channels that write to a file: type out_channel
  • channels that read from a file: type in_channel


For writing into a file, you would do this:

  1. Open the file to obtain an out_channel
  2. Write stuff to the channel
  3. If you want to force writing to the physical device, you must flush the channel, otherwise writing will not take place immediately.
  4. When you are done, you can close the channel. This flushes the channel automatically.

Commonly used functions: open_out, open_out_bin, flush, close_out, close_out_noerr

Standard out_channels: stdout, stderr


For reading data from a file you would do this:

  1. Open the file to obtain an in_channel
  2. Read characters from the channel. Reading consumes the channel, so if you read a character, the channel will point to the next character in the file.
  3. When there are no more characters to read, the End_of_file exception is raised. Often, this is where you want to close the channel.

Commonly used functions: open_in, open_in_bin, close_in, close_in_noerr

Standard in_channel: stdin


Whenever you write or read something to or from a channel, the current position changes to the next character after what you just wrote or read. Occasionally, you may want to skip to a particular position in the file, or restart reading from the beginning. This is possible for channels that point to regular files, use seek_in or seek_out.


  • Don't forget to flush your out_channels if you want to actually write something. This is particularly important if you are writing to non-files such as the standard output (stdout) or a socket.
  • Don't forget to close any unused channel, because operating systems have a limit on the number of files that can be opened simultaneously. You must catch any exception that would occur during the file manipulation, close the corresponding channel, and re-raise the exception.
  • The Unix module provides access to non-buffered file descriptors among other things. It provides standard file descriptors that have the same name as the corresponding standard channels: stdin, stdout and stderr. Therefore if you do open Unix, you may get type errors. If you want to be sure that you are using the stdout channel and not the stdout file descriptor, you can prepend it with the module name where it comes from: Pervasives.stdout. Note that most things that don't seem to belong to any module actually belong to the Pervasives module, which is automatically opened.
  • open_out and open_out_bin truncate the given file if it already exists! Use open_out_gen if you want an alternate behavior.


open Printf
let file = "example.dat"
let message = "Hello!"
let () =
  (* Write message to file *)
  let oc = open_out file in    (* create or truncate file, return channel *)
  fprintf oc "%s\n" message;   (* write something *)   
  close_out oc;                (* flush and close the channel *)
  (* Read file and display the first line *)
  let ic = open_in file in
    let line = input_line ic in  (* read line from in_channel and discard \n *)
    print_endline line;          (* write the result to stdout *)
    flush stdout;                (* write on the underlying device now *)
    close_in ic                  (* close the input channel *) 
  with e ->                      (* some unexpected exception occurs *)
    close_in_noerr ic;           (* emergency closing *)
    raise e                      (* exit with error: files are closed but
                                    channels are not flushed *)
  (* normal exit: all channels are flushed and closed *)