path: root/lib/kernel/doc/src/file.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2016</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>file</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>file</module>
  <modulesummary>File interface module.</modulesummary>
  <description>
    <p>This module provides an interface to the file system.</p>

    <p>On operating systems with thread support,
      file operations can be performed in threads of their own, allowing
      other Erlang processes to continue executing in parallel with
      the file operations. See command-line flag
      <c>+A</c> in <seealso marker="erts:erl"><c>erl(1)</c></seealso>.</p>

    <p>Regarding filename encoding, the Erlang VM can operate in
    two modes. The current mode can be queried using function
    <seealso marker="#native_name_encoding/0"><c>native_name_encoding/0</c></seealso>.
    It returns <c>latin1</c> or <c>utf8</c>.</p>

    <p>In <c>latin1</c> mode, the Erlang VM does not change the
    encoding of filenames. In <c>utf8</c> mode, filenames can
    contain Unicode characters greater than 255 and the VM
    converts filenames back and forth to the native filename encoding
    (usually UTF-8, but UTF-16 on Windows).</p>

    <p>The default mode depends on the operating system. Windows and
    MacOS X enforce consistent filename encoding and therefore the
    VM uses <c>utf8</c> mode.</p>

    <p>On operating systems with transparent naming (for example, all Unix
    systems except MacOS X), default is <c>utf8</c> if the
    terminal supports UTF-8, otherwise <c>latin1</c>. The default can
    be overridden using <c>+fnl</c> (to force <c>latin1</c> mode)
    or <c>+fnu</c> (to force <c>utf8</c> mode) when starting
    <seealso marker="erts:erl"><c>erl</c></seealso>.</p>

    <p>On operating systems with transparent naming, files can be
    inconsistently named, for example, some files are encoded in UTF-8 while
    others are encoded in ISO Latin-1. The concept of <em>raw filenames</em> is
    introduced to handle file systems with inconsistent naming when running in
    <c>utf8</c> mode.</p>

    <p>A <em>raw filename</em> is a filename specified as a binary. The Erlang VM
    does not translate a filename specified as a binary on
    systems with transparent naming.</p>

    <p>When running in <c>utf8</c> mode, functions
    <seealso marker="#list_dir/1"><c>list_dir/1</c></seealso> and
    <seealso marker="#read_link/1"><c>read_link/1</c></seealso>
    never return raw filenames. To return all filenames including raw filenames,
    use functions
    <seealso marker="#list_dir_all"><c>list_dir_all/1</c></seealso> and
    <seealso marker="#read_link_all"><c>read_link_all/1</c></seealso>.</p>

    <p>See also section <seealso marker="stdlib:unicode_usage#notes-about-raw-filenames">Notes About Raw Filenames</seealso> in the STDLIB User's Guide.</p>

    <note><p>
      File operations used to accept filenames containing
      null characters (integer value zero). This caused
      the name to be truncated and in some cases arguments
      to primitive operations to be mixed up. Filenames
      containing null characters inside the filename
      are now <em>rejected</em> and will cause primitive
      file operations fail.
    </p></note>
    <warning><p>
      Currently null characters at the end of the filename
      will be accepted by primitive file operations. Such
      filenames are however still documented as invalid. The
      implementation will also change in the future and
      reject such filenames.
    </p></warning>

  </description>

  <datatypes>
    <datatype>
      <name name="deep_list"/>
    </datatype>
    <datatype>
      <name>fd()</name>
      <desc>
        <p>A file descriptor representing a file opened in
	  <seealso marker="#raw"><c>raw</c></seealso> mode.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="filename"/>
      <desc>
        <p>
	  See also the documentation of the
	  <seealso marker="#type-name_all"><c>name_all()</c></seealso> type.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="filename_all"/>
      <desc>
        <p>
	  See also the documentation of the
	  <seealso marker="#type-name_all"><c>name_all()</c></seealso> type.
	</p>
      </desc>
    </datatype>
    <datatype>
      <name name="io_device"/>
      <desc>
        <p>As returned by
          <seealso marker="#open/2"><c>open/2</c></seealso>;
          <c>pid()</c> is a process handling I/O-protocols.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="name"/>
      <desc>
        <p>If VM is in Unicode filename mode, <c>string()</c> and <c>char()</c>
          are allowed to be &gt; 255. See also the documentation of the
	  <seealso marker="#type-name_all"><c>name_all()</c></seealso> type.
        </p>
      </desc>
    </datatype>
    <datatype>
      <name name="name_all"/>
      <desc>
        <p>If VM is in Unicode filename mode, characters
          are allowed to be &gt; 255.
          <c><anno>RawFilename</anno></c> is a filename not subject to
          Unicode translation,
          meaning that it can contain characters not conforming to
          the Unicode encoding expected from the file system
          (that is, non-UTF-8 characters although the VM is started
          in Unicode filename mode). Null characters (integer value zero)
	  are <em>not</em> allowed in filenames (not even at the end).
        </p>
      </desc>
    </datatype>
    <datatype>
      <name name="posix"/>
      <desc>
        <p>An atom that is named from the POSIX error codes used in
          Unix, and in the runtime libraries of most C compilers.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="date_time"/>
      <desc>
        <p>Must denote a valid date and time.</p>
      </desc>
    </datatype>
    <datatype>
      <name name="file_info"/>
    </datatype>
    <datatype>
      <name name="location"/>
    </datatype>
    <datatype>
      <name name="mode"/>
    </datatype>
    <datatype>
      <name name="file_info_option"/>
    </datatype>
   </datatypes>

  <funcs>
    <func>
      <name name="advise" arity="4"/>
      <fsummary>Predeclare an access pattern for file data.</fsummary>
      <type name="posix_file_advise"/>
      <desc>
        <p><c>advise/4</c> can be used to announce an intention to access file
          data in a specific pattern in the future, thus allowing the
          operating system to perform appropriate optimizations.</p>
        <p>On some platforms, this function might have no effect.</p>
      </desc>
    </func>
    <func>
      <name name="allocate" arity="3"/>
      <fsummary>Allocate file space.</fsummary>
      <desc>
        <p><c>allocate/3</c> can be used to preallocate space for a file.</p>
        <p>This function only succeeds in platforms that provide this
          feature. When it succeeds, space is preallocated for the file but
          the file size might not be updated. This behaviour depends on the
          preallocation implementation. To guarantee that the file size is updated,
          truncate the file to the new size.</p>
      </desc>
    </func>
    <func>
      <name name="change_group" arity="2"/>
      <fsummary>Change group of a file.</fsummary>
      <desc>
        <p>Changes group of a file. See
          <seealso marker="#write_file_info/2"><c>write_file_info/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="change_mode" arity="2"/>
      <fsummary>Change permissions of a file.</fsummary>
      <desc>
        <p>Changes permissions of a file. See
          <seealso marker="#write_file_info/2"><c>write_file_info/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="change_owner" arity="2"/>
      <fsummary>Change owner of a file.</fsummary>
      <desc>
        <p>Changes owner of a file. See
          <seealso marker="#write_file_info/2"><c>write_file_info/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="change_owner" arity="3"/>
      <fsummary>Change owner and group of a file.</fsummary>
      <desc>
        <p>Changes owner and group of a file. See
          <seealso marker="#write_file_info/2"><c>write_file_info/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="change_time" arity="2"/>
      <fsummary>Change the modification time of a file.</fsummary>
      <desc>
        <p>Changes the modification and access times of a file. See
          <seealso marker="#write_file_info/2"><c>write_file_info/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="change_time" arity="3"/>
      <fsummary>Change the modification and last access time of a file.</fsummary>
      <desc>
        <p>Changes the modification and last access times of a file. See
          <seealso marker="#write_file_info/2"><c>write_file_info/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="close" arity="1"/>
      <fsummary>Close a file.</fsummary>
      <desc>
        <p>Closes the file referenced by <c><anno>IoDevice</anno></c>. It mostly
          returns <c>ok</c>, except for some severe errors such as out
          of memory.</p>
        <p>Notice that if option <c>delayed_write</c> was
          used when opening the file, <c>close/1</c> can return an
          old write error and not even try to close the file. See
          <seealso marker="#open/2"><c>open/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="consult" arity="1"/>
      <fsummary>Read Erlang terms from a file.</fsummary>
      <desc>
        <p>Reads Erlang terms, separated by '.', from
           <c><anno>Filename</anno></c>. Returns one of the following:</p>
        <taglist>
          <tag><c>{ok, <anno>Terms</anno>}</c></tag>
          <item>
            <p>The file was successfully read.</p>
          </item>
          <tag><c>{error, atom()}</c></tag>
          <item>
            <p>An error occurred when opening the file or reading it.
              For a list of typical error codes, see
	      <seealso marker="#open/2"><c>open/2</c></seealso>.</p>
          </item>
          <tag><c>{error, {<anno>Line</anno>, <anno>Mod</anno>,
                           <anno>Term</anno>}}</c></tag>
          <item>
            <p>An error occurred when interpreting the Erlang terms in
              the file. To convert the three-element tuple to an English
	      description of the error, use
	    <seealso marker="#format_error/1"><c>format_error/1</c></seealso>.</p>
          </item>
        </taglist>
        <p><em>Example:</em></p>
<code type="none">
f.txt:  {person, "kalle", 25}.
        {person, "pelle", 30}.</code>
<pre>
1> <input>file:consult("f.txt").</input>
{ok,[{person,"kalle",25},{person,"pelle",30}]}</pre>
        <p>The encoding of <c><anno>Filename</anno></c> can be set
          by a comment, as described in
	  <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="copy" arity="2"/>
      <name name="copy" arity="3"/>
      <fsummary>Copy file contents.</fsummary>
      <desc>
        <p>Copies <c><anno>ByteCount</anno></c> bytes from
          <c><anno>Source</anno></c> to <c><anno>Destination</anno></c>.
          <c><anno>Source</anno></c> and <c><anno>Destination</anno></c> refer
          to either filenames or IO devices from, for example, <c>open/2</c>.
          <c><anno>ByteCount</anno></c> defaults to <c>infinity</c>, denoting an
          infinite number of bytes.</p>
        <p>Argument <c><anno>Modes</anno></c> is a list of possible modes,
          see <seealso marker="#open/2"><c>open/2</c></seealso>, and defaults to
          <c>[]</c>.</p>
        <p>If both <c><anno>Source</anno></c> and
          <c><anno>Destination</anno></c> refer to
          filenames, the files are opened with <c>[read, binary]</c>
          and <c>[write, binary]</c> prepended to their mode lists,
          respectively, to optimize the copy.</p>
        <p>If <c><anno>Source</anno></c> refers to a filename, it is opened with
          <c>read</c> mode prepended to the mode list before the copy,
          and closed when done.</p>
        <p>If <c><anno>Destination</anno></c> refers to a filename, it is opened
          with <c>write</c> mode prepended to the mode list before
          the copy, and closed when done.</p>
        <p>Returns <c>{ok, <anno>BytesCopied</anno>}</c>, where
          <c><anno>BytesCopied</anno></c> is
          the number of bytes that was copied, which can be
          less than <c><anno>ByteCount</anno></c> if end of file was
          encountered on the source. If the operation fails,
          <c>{error, <anno>Reason</anno>}</c> is returned.</p>
        <p>Typical error reasons: as for
	<seealso marker="#open/2"><c>open/2</c></seealso> if a file
	had to be opened, and as for
	<seealso marker="#read/2"><c>read/2</c></seealso> and
	<seealso marker="#write/2"><c>write/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="datasync" arity="1"/>
      <fsummary>Synchronize the in-memory data of a file, ignoring most of its metadata, with that on the physical medium.</fsummary>
      <desc>
        <p>Ensures that any buffers kept by the operating system
          (not by the Erlang runtime system) are written to disk. In
          many ways it resembles <c>fsync</c> but it does not update
          some of the metadata of the file, such as the access time. On
          some platforms this function has no effect.</p>
        <p>Applications that access databases or log files often write
          a tiny data fragment (for example, one line in a log file) and then
          call <c>fsync()</c> immediately to ensure that the written
          data is physically stored on the hard disk. Unfortunately, <c>fsync()</c>
          always initiates two write operations: one for the newly
          written data and another one to update the modification
          time stored in the <c>inode</c>. If the modification time is not a part
          of the transaction concept, <c>fdatasync()</c> can be used to avoid
          unnecessary <c>inode</c> disk write operations.</p>
        <p>Available only in some POSIX systems, this call results in a
          call to <c>fsync()</c>, or has no effect in systems not providing
          the <c>fdatasync()</c> syscall.</p>
      </desc>
    </func>
    <func>
      <name name="del_dir" arity="1"/>
      <fsummary>Delete a directory.</fsummary>
      <desc>
        <p>Tries to delete directory <c><anno>Dir</anno></c>.
          The directory must
          be empty before it can be deleted. Returns <c>ok</c> if
          successful.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing search or write permissions for the parent
              directories of <c><anno>Dir</anno></c>.</p>
          </item>
          <tag><c>eexist</c></tag>
          <item>
            <p>The directory is not empty.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>The directory does not exist.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of <c><anno>Dir</anno></c> is not a directory.
              On some platforms, <c>enoent</c> is returned instead.</p>
          </item>
          <tag><c>einval</c></tag>
          <item>
            <p>Attempt to delete the current directory. On some
              platforms, <c>eacces</c> is returned instead.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="delete" arity="1"/>
      <fsummary>Delete a file.</fsummary>
      <desc>
        <p>Tries to delete file <c><anno>Filename</anno></c>.
          Returns <c>ok</c> if successful.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>enoent</c></tag>
          <item>
            <p>The file does not exist.</p>
          </item>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing permission for the file or one of its parents.</p>
          </item>
          <tag><c>eperm</c></tag>
          <item>
            <p>The file is a directory and the user is not superuser.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of the filename is not a directory. On some
              platforms, <c>enoent</c> is returned instead.</p>
          </item>
          <tag><c>einval</c></tag>
          <item>
            <p><c><anno>Filename</anno></c> has an improper type, such as tuple.</p>
          </item>
        </taglist>
        <warning>
          <p>In a future release, a bad type for argument
            <c><anno>Filename</anno></c> will probably generate
            an exception.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="eval" arity="1"/>
      <fsummary>Evaluate Erlang expressions in a file.</fsummary>
      <desc>
        <p>Reads and evaluates Erlang expressions, separated by '.' (or
          ',', a sequence of expressions is also an expression) from
          <c><anno>Filename</anno></c>. The result of the evaluation
          is not returned; any expression sequence in the file must be there
          for its side effect. Returns one of the following:</p>
        <taglist>
          <tag><c>ok</c></tag>
          <item>
            <p>The file was read and evaluated.</p>
          </item>
          <tag><c>{error, atom()}</c></tag>
          <item>
            <p>An error occurred when opening the file or reading it.
            For a list of typical error codes, see
	    <seealso marker="#open/2"><c>open/2</c></seealso>.</p>
          </item>
          <tag><c>{error, {<anno>Line</anno>, <anno>Mod</anno>,
                           <anno>Term</anno>}}</c></tag>
          <item>
            <p>An error occurred when interpreting the Erlang
              expressions in the file. To convert the three-element tuple
	      to an English description of the error, use
	      <seealso marker="#format_error/1"><c>format_error/1</c></seealso>.</p>
          </item>
        </taglist>
        <p>The encoding of <c><anno>Filename</anno></c> can be set
          by a comment, as described in
	  <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="eval" arity="2"/>
      <fsummary>Evaluate Erlang expressions in a file.</fsummary>
      <desc>
        <p>The same as <c>eval/1</c>, but the variable bindings
          <c><anno>Bindings</anno></c> are used in the evaluation. For information
	  about the variable bindings, see
          <seealso marker="stdlib:erl_eval"><c>erl_eval(3)</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="format_error" arity="1"/>
      <fsummary>Return a descriptive string for an error reason.</fsummary>
      <desc>
        <p>Given the error reason returned by any function in this
          module, returns a descriptive string of the error in English.</p>
      </desc>
    </func>
    <func>
      <name name="get_cwd" arity="0"/>
      <fsummary>Get the current working directory.</fsummary>
      <desc>
        <p>Returns <c>{ok, <anno>Dir</anno>}</c>, where <c><anno>Dir</anno></c>
          is the current
          working directory of the file server.</p>
        <note>
          <p>In rare circumstances, this function can fail on Unix.
            It can occur if read permission does not exist for
            the parent directories of the current directory.</p>
        </note>
        <p>A typical error reason:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing read permission for one of the parents of
              the current directory.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="get_cwd" arity="1"/>
      <fsummary>Get the current working directory for the specified drive.</fsummary>
      <desc>
	<p>Returns <c>{ok, <anno>Dir</anno>}</c> or
          <c>{error, <anno>Reason</anno>}</c>, where <c><anno>Dir</anno></c>
          is the current working directory of the specified drive.</p>

	<p><c><anno>Drive</anno></c> is to be of the form
          "<c>Letter</c><c>:</c>", for example, "c:".</p>

	<p>Returns <c>{error, enotsup}</c> on platforms
          that have no concept of current drive (Unix, for example).</p>

        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>enotsup</c></tag>
          <item>
            <p>The operating system has no concept of drives.</p>
          </item>
          <tag><c>eacces</c></tag>
          <item>
            <p>The drive does not exist.</p>
          </item>
          <tag><c>einval</c></tag>
          <item>
            <p>The format of <c><anno>Drive</anno></c> is invalid.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="list_dir" arity="1"/>
      <fsummary>List files in a directory.</fsummary>
      <desc>
        <p>Lists all files in a directory, <em>except</em> files
	with raw filenames. Returns
          <c>{ok, <anno>Filenames</anno>}</c> if successful,
          otherwise <c>{error, <anno>Reason</anno>}</c>.
          <c><anno>Filenames</anno></c> is a list of
          the names of all the files in the directory. The names are
          not sorted.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing search or write permissions for <c><anno>Dir</anno></c>
              or one of its parent directories.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>The directory does not exist.</p>
          </item>
          <tag><c>{no_translation, <anno>Filename</anno>}</c></tag>
          <item>
            <p><c><anno>Filename</anno></c> is a <c>binary()</c> with
              characters coded in ISO Latin-1 and the VM was started
              with parameter <c>+fnue</c>.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="list_dir_all" arity="1"/>
      <fsummary>List all files in a directory.</fsummary>
      <desc>
        <p><marker id="list_dir_all"/>Lists all the files in a directory,
	including files with raw filenames.
	Returns <c>{ok, <anno>Filenames</anno>}</c> if successful,
          otherwise <c>{error, <anno>Reason</anno>}</c>.
          <c><anno>Filenames</anno></c> is a list of
          the names of all the files in the directory. The names are
          not sorted.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing search or write permissions for <c><anno>Dir</anno></c>
              or one of its parent directories.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>The directory does not exist.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="make_dir" arity="1"/>
      <fsummary>Make a directory.</fsummary>
      <desc>
        <p>Tries to create directory <c><anno>Dir</anno></c>. Missing parent
          directories are <em>not</em> created. Returns <c>ok</c> if
          successful.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing search or write permissions for the parent
              directories of <c><anno>Dir</anno></c>.</p>
          </item>
          <tag><c>eexist</c></tag>
          <item>
            <p>A file or directory named <c><anno>Dir</anno></c> exists already.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>A component of <c><anno>Dir</anno></c> does not exist.</p>
          </item>
          <tag><c>enospc</c></tag>
          <item>
            <p>No space is left on the device.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of <c><anno>Dir</anno></c> is not a directory.
              On some platforms, <c>enoent</c> is returned instead.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="make_link" arity="2"/>
      <fsummary>Make a hard link to a file.</fsummary>
      <desc>
        <p>Makes a hard link from <c><anno>Existing</anno></c> to
          <c><anno>New</anno></c> on
          platforms supporting links (Unix and Windows). This function returns
          <c>ok</c> if the link was successfully created, otherwise
          <c>{error, <anno>Reason</anno>}</c>. On platforms not supporting
          links, <c>{error,enotsup}</c> is returned.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing read or write permissions for the parent
              directories of <c><anno>Existing</anno></c> or
              <c><anno>New</anno></c>.</p>
          </item>
          <tag><c>eexist</c></tag>
          <item>
            <p><c><anno>New</anno></c> already exists.</p>
          </item>
          <tag><c>enotsup</c></tag>
          <item>
            <p>Hard links are not supported on this platform.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="make_symlink" arity="2"/>
      <fsummary>Make a symbolic link to a file or directory.</fsummary>
      <desc>
        <p>Creates a symbolic link <c><anno>New</anno></c> to
          the file or directory <c><anno>Existing</anno></c> on platforms
          supporting symbolic links (most Unix systems and Windows, beginning with
	  Vista).
	  <c><anno>Existing</anno></c> does not need to exist.
          Returns <c>ok</c> if the link is
          successfully created, otherwise <c>{error, <anno>Reason</anno>}</c>.
          On platforms not supporting symbolic links, <c>{error, enotsup}</c>
          is returned.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing read or write permissions for the parent directories
            of <c><anno>Existing</anno></c> or <c><anno>New</anno></c>.</p>
          </item>
          <tag><c>eexist</c></tag>
          <item>
            <p><c><anno>New</anno></c> already exists.</p>
          </item>
          <tag><c>enotsup</c></tag>
          <item>
            <p>Symbolic links are not supported on this platform.</p>
          </item>
          <tag><c>eperm</c></tag>
          <item>
            <p>User does not have privileges to create symbolic links
	    (<c>SeCreateSymbolicLinkPrivilege</c> on Windows).</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="native_name_encoding" arity="0"/>
      <fsummary>Return the configured filename encoding of the VM.</fsummary>
      <desc>
	<p><marker id="native_name_encoding"/>Returns
	the filename encoding mode. If it is <c>latin1</c>, the
	system translates no filenames. If it is
	<c>utf8</c>, filenames are converted back and forth to
	the native filename encoding (usually UTF-8, but UTF-16 on
	Windows).</p>
      </desc>
    </func>
    <func>
      <name name="open" arity="2"/>
      <fsummary>Open a file.</fsummary>
      <desc>
        <p>Opens file <c><anno>File</anno></c> in the mode determined
          by <c><anno>Modes</anno></c>, which can contain one or more of the
          following options:</p>
        <taglist>
          <tag><c>read</c></tag>
          <item>
            <p>The file, which must exist, is opened for reading.</p>
          </item>
          <tag><c>write</c></tag>
          <item>
            <p>The file is opened for writing. It is created if it does
              not exist. If the file exists and <c>write</c> is not
              combined with <c>read</c>, the file is truncated.</p>
          </item>
          <tag><c>append</c></tag>
          <item>
            <p>The file is opened for writing. It is created if it does
	      not exist. Every write operation to a file opened with
	      <c>append</c> takes place at the end of the file.</p>
          </item>
          <tag><c>exclusive</c></tag>
          <item>
            <p>The file is opened for writing. It is created if it does
	      not exist. If the file exists, <c>{error, eexist}</c>
	      is returned.</p>
	    <warning><p>This option does not guarantee exclusiveness on
	    file systems not supporting <c>O_EXCL</c> properly,
	    such as NFS. Do not depend on this option unless you
	    know that the file system supports it (in general, local
	    file systems are safe).</p></warning>
          </item>
          <tag><c>raw</c></tag>
          <item>
            <p><marker id="raw"/>
              Allows faster access to a file,
              as no Erlang process is needed to handle the file.
              However, a file opened in this way has the following
              limitations:</p>
            <list type="bulleted">
              <item><p>The functions in the <c>io</c> module cannot be used,
               as they can only talk to an Erlang process.
               Instead, use functions
	       <seealso marker="#read/2"><c>read/2</c></seealso>,
	       <seealso marker="#read_line/1"><c>read_line/1</c></seealso>, and
	       <seealso marker="#write/2"><c>write/2</c></seealso>.</p></item>
	      <item><p>Especially if <c>read_line/1</c> is to be used on a <c>raw</c>
	      file, it is recommended to combine this option with option
	      <c>{read_ahead, Size}</c> as line-oriented I/O is inefficient
	      without buffering.</p></item>
              <item><p>Only the Erlang process that opened the file can use
               it.</p></item>
              <item><p>A remote Erlang file server cannot be used.
               The computer on which the Erlang node is running must
               have access to the file system (directly or through
               NFS).</p></item>
            </list>
          </item>
          <tag><c>binary</c></tag>
          <item>
            <p>Read operations on the file return binaries rather than lists.</p>
          </item>
          <tag><c>{delayed_write, Size, Delay}</c></tag>
          <item>
            <p>Data in subsequent
              <c>write/2</c> calls is buffered until at least
              <c>Size</c> bytes are buffered, or until the oldest buffered
              data is <c>Delay</c> milliseconds old. Then all buffered
              data is written in one operating system call.
              The buffered data is also flushed before some other file
              operation than <c>write/2</c> is executed.</p>
            <p>The purpose of this option is to increase performance
              by reducing the number of operating system calls. Thus, the
              <c>write/2</c> calls must be for sizes significantly
              less than <c>Size</c>, and not interspersed by too many
              other file operations.</p>
            <p>When this option is used, the result of <c>write/2</c>
              calls can prematurely be reported as successful, and if
	      a write error occurs, the error is reported as the result
	      of the next file operation, which is not executed.</p>
            <p>For example, when <c>delayed_write</c> is used, after a
              number of <c>write/2</c> calls, <c>close/1</c> can
              return <c>{error, enospc}</c>, as there is not enough
              space on the disc for previously written data.
              <c>close/1</c> must probably be called again, as the
              file is still open.</p>
          </item>
          <tag><c>delayed_write</c></tag>
          <item>
            <p>The same as <c>{delayed_write, Size, Delay}</c> with
              reasonable default values for <c>Size</c> and
              <c>Delay</c> (roughly some 64 KB, 2 seconds).</p>
          </item>
          <tag><c>{read_ahead, Size}</c></tag>
          <item>
            <p>Activates read data buffering. If
              <c>read/2</c> calls are for significantly less than
              <c>Size</c> bytes, read operations to the operating
              system are still performed for blocks of <c>Size</c>
              bytes. The extra data is buffered and returned in
              subsequent <c>read/2</c> calls, giving a performance gain
              as the number of operating system calls is reduced.</p>
	    <p>The <c>read_ahead</c> buffer is also highly used
	      by function <c>read_line/1</c> in <c>raw</c> mode,
	      therefore this option is recommended
	      (for performance reasons)
	      when accessing raw files using that function.</p>
            <p>If <c>read/2</c> calls are for sizes not significantly
              less than, or even greater than <c>Size</c> bytes, no
              performance gain can be expected.</p>
          </item>
          <tag><c>read_ahead</c></tag>
          <item>
            <p>The same as <c>{read_ahead, Size}</c> with a reasonable
              default value for <c>Size</c> (roughly some 64 KB).</p>
          </item>
          <tag><c>compressed</c></tag>
          <item>
            <p>Makes it possible to read or write gzip compressed
              files. Option <c>compressed</c> must be combined
	      with <c>read</c> or <c>write</c>, but not both.
	      Notice that the file size obtained with
              <seealso marker="#read_file_info/1"><c>read_file_info/1</c></seealso>
	      does probably not match the number of bytes that can be
	      read from a compressed file.</p>
          </item>
          <tag><c>{encoding, Encoding}</c></tag>
          <item>
            <p>Makes the file perform automatic translation of characters to
	    and from a specific (Unicode) encoding. Notice that the data supplied
	    to
	    <seealso marker="#write/2"><c>write/2</c></seealso>
	    or returned by
	    <seealso marker="#read/2"><c>read/2</c></seealso>
	    still is byte-oriented; this option
	    denotes only how data is stored in the disk file.</p>
	    <p>Depending on the encoding, different methods of reading and writing
	    data is preferred. The default encoding of <c>latin1</c> implies using
	    this module (<c>file</c>) for reading and writing data as the interfaces
	    provided here work with byte-oriented data. Using other (Unicode)
	    encodings makes the
	    <seealso marker="stdlib:io"><c>io(3)</c></seealso> functions
	    <c>get_chars</c>, <c>get_line</c>, and <c>put_chars</c> more suitable,
	    as they can work with the full Unicode range.</p>
	    <p>If data is sent to an <c>io_device()</c> in a format that cannot be
	    converted to the specified encoding, or if data is read by a function
	    that returns data in a format that cannot cope with the character range
	    of the data, an error occurs and the file is closed.</p>
	    <p>Allowed values for <c>Encoding</c>:</p>
	    <taglist>
	    <tag><c>latin1</c></tag>
	    <item>
	    <p>The default encoding. Bytes supplied to the file, that is,
	    <seealso marker="#write/2"><c>write/2</c></seealso>
	    are written "as is" on the file. Likewise, bytes read from the file,
	    that is,
	    <seealso marker="#read/2"><c>read/2</c></seealso> are
	    returned "as is". If module
	    <seealso marker="stdlib:io"><c>io(3)</c></seealso> is used for
	    writing, the file can only cope with Unicode characters up to code point
	    255 (the ISO Latin-1 range).</p>
	    </item>
	    <tag><c>unicode or utf8</c></tag>
	    <item>
	    <p>Characters are translated to and from UTF-8 encoding before they are
	    written to or read from the file. A file opened in this way can be
	    readable using function
	    <seealso marker="#read/2"><c>read/2</c></seealso>,
	    as long as no data stored on
	    the file lies beyond the ISO Latin-1 range (0..255), but failure occurs
	    if the data contains Unicode code points beyond that range. The file is
	    best read with the functions in the Unicode aware module
	    <seealso marker="stdlib:io"><c>io(3)</c></seealso>.</p>
	    <p>Bytes written to the file by any means are translated to UTF-8 encoding
	    before being stored on the disk file.</p>
	    </item>
	    <tag><c>utf16 or {utf16,big}</c></tag>
	    <item>
	    <p>Works like <c>unicode</c>, but translation is done to and from big
	    endian UTF-16 instead of UTF-8.</p>
	    </item>
	    <tag><c>{utf16,little}</c></tag>
	    <item>
	    <p>Works like <c>unicode</c>, but translation is done to and from little
	    endian UTF-16 instead of UTF-8.</p>
	    </item>
	    <tag><c>utf32 or {utf32,big}</c></tag>
	    <item>
	    <p>Works like <c>unicode</c>, but translation is done to and from big
	    endian UTF-32 instead of UTF-8.</p>
	    </item>
	    <tag><c>{utf32,little}</c></tag>
	    <item>
	    <p>Works like <c>unicode</c>, but translation is done to and from little
	    endian UTF-32 instead of UTF-8.</p>
	    </item>
	    </taglist>
	    <p>The Encoding can be changed for a file "on the fly" by using function
	    <seealso marker="stdlib:io#setopts/2"><c>io:setopts/2</c></seealso>.
	    So a file can be analyzed in latin1 encoding for, for example, a BOM,
	    positioned beyond the BOM and then be set for the right encoding before
	    further reading. For functions identifying BOMs, see module
	    <seealso marker="stdlib:unicode"><c>unicode(3)</c></seealso>. </p>
	    <p>This option is not allowed on <c>raw</c> files.</p>
          </item>
          <tag><c>ram</c></tag>
          <item>
            <p><c>File</c> must be <c>iodata()</c>. Returns an <c>fd()</c>, which lets
	    module <c>file</c> operate on the data in-memory as if it is a file.</p>
          </item>
          <tag><c>sync</c></tag>
          <item>
            <p>On platforms supporting it, enables the POSIX <c>O_SYNC</c> synchronous
	    I/O flag or its platform-dependent equivalent (for example,
	    <c>FILE_FLAG_WRITE_THROUGH</c> on Windows) so that writes to the file
	    block until the data is physically written to disk. However, be aware
	    that the exact semantics of this flag differ from platform to
            platform. For example, none of Linux or Windows guarantees that all file
	    metadata are also written before the call returns. For precise semantics,
	    check the details of your platform documentation. On platforms with no
            support for POSIX <c>O_SYNC</c> or equivalent, use of the <c>sync</c>
	    flag causes <c>open</c> to return <c>{error, enotsup}</c>.</p>
          </item>
        </taglist>
        <p>Returns:</p>
        <taglist>
          <tag><c>{ok, <anno>IoDevice</anno>}</c></tag>
          <item>
            <p>The file is opened in the requested mode.
              <c><anno>IoDevice</anno></c> is a reference to the file.</p>
          </item>
          <tag><c>{error, <anno>Reason</anno>}</c></tag>
          <item>
            <p>The file cannot be opened.</p>
          </item>
        </taglist>
        <p><c><anno>IoDevice</anno></c> is really the pid of the process that
          handles the file. This process is linked to the process
          that originally opened the file. If any process to which
          the <c><anno>IoDevice</anno></c> is linked terminates, the file is
          closed and the process itself is terminated.
          An <c><anno>IoDevice</anno></c> returned from this call can be used
          as an argument to the I/O functions (see
          <seealso marker="stdlib:io"><c>io(3)</c></seealso>).</p>
        <note>
          <p>In previous versions of <c>file</c>, modes were specified
            as one of the atoms <c>read</c>, <c>write</c>, or
            <c>read_write</c> instead of a list. This is still allowed
            for reasons of backwards compatibility, but is not to be
            used for new code. Also note that <c>read_write</c> is not
            allowed in a mode list.</p>
        </note>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>enoent</c></tag>
          <item>
            <p>The file does not exist.</p>
          </item>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing permission for reading the file or searching one
              of the parent directories.</p>
          </item>
          <tag><c>eisdir</c></tag>
          <item>
            <p>The named file is not a regular file. It can be a
              directory, a FIFO, or a device.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of the filename is not a directory. On some
              platforms, <c>enoent</c> is returned instead.</p>
          </item>
          <tag><c>enospc</c></tag>
          <item>
            <p>There is no space left on the device (if <c>write</c>
              access was specified).</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="path_consult" arity="2"/>
      <fsummary>Read Erlang terms from a file.</fsummary>
      <desc>
        <p>Searches the path <c><anno>Path</anno></c> (a list of directory
          names) until the file <c><anno>Filename</anno></c> is found.
          If <c><anno>Filename</anno></c>
          is an absolute filename, <c><anno>Path</anno></c> is ignored.
          Then reads Erlang terms, separated by '.', from the file.</p>
          <p>Returns one of the following:</p>
        <taglist>
          <tag><c>{ok, <anno>Terms</anno>, <anno>FullName</anno>}</c></tag>
          <item>
            <p>The file is successfully read. <c><anno>FullName</anno></c> is
              the full name of the file.</p>
          </item>
          <tag><c>{error, enoent}</c></tag>
          <item>
            <p>The file cannot be found in any of the directories in
              <c><anno>Path</anno></c>.</p>
          </item>
          <tag><c>{error, atom()}</c></tag>
          <item>
            <p>An error occurred when opening the file or reading it.
              For a list of typical error codes, see
	    <seealso marker="#open/2"><c>open/2</c></seealso>.</p>
          </item>
          <tag><c>{error, {<anno>Line</anno>, <anno>Mod</anno>,
                           <anno>Term</anno>}}</c></tag>
          <item>
            <p>An error occurred when interpreting the Erlang terms in
              the file. Use
	      <seealso marker="#format_error/1"><c>format_error/1</c></seealso>
	      to convert the three-element tuple to an English description of
              the error.</p>
          </item>
        </taglist>
        <p>The encoding of <c><anno>Filename</anno></c> can be set
          by a comment as described in
	  <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="path_eval" arity="2"/>
      <fsummary>Evaluate Erlang expressions in a file.</fsummary>
      <desc>
        <p>Searches the path <c><anno>Path</anno></c> (a list of directory
          names) until the file <c><anno>Filename</anno></c> is found.
          If <c><anno>Filename</anno></c> is an absolute filename,
          <c><anno>Path</anno></c> is ignored. Then reads
          and evaluates Erlang expressions, separated by '.' (or ',', a
          sequence of expressions is also an expression), from the file.
          The result of evaluation is not returned; any
          expression sequence in the file must be there for its side
          effect.</p>
	  <p>Returns one of the following:</p>
        <taglist>
          <tag><c>{ok, <anno>FullName</anno>}</c></tag>
          <item>
            <p>The file is read and evaluated. <c><anno>FullName</anno></c> is
              the full name of the file.</p>
          </item>
          <tag><c>{error, enoent}</c></tag>
          <item>
            <p>The file cannot be found in any of the directories in
              <c><anno>Path</anno></c>.</p>
          </item>
          <tag><c>{error, atom()}</c></tag>
          <item>
            <p>An error occurred when opening the file or reading it.
              For a list of typical error codes, see
	    <seealso marker="#open/2"><c>open/2</c></seealso>.</p>
          </item>
          <tag><c>{error, {<anno>Line</anno>, <anno>Mod</anno>,
                           <anno>Term</anno>}}</c></tag>
          <item>
            <p>An error occurred when interpreting the Erlang
              expressions in the file. Use
	      <seealso marker="#format_error/1"><c>format_error/1</c></seealso>
	      to convert the three-element tuple to an English description
              of the error.</p>
          </item>
        </taglist>
        <p>The encoding of <c><anno>Filename</anno></c> can be set
          by a comment as described in
	  <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="path_open" arity="3"/>
      <fsummary>Open a file.</fsummary>
      <desc>
        <p>Searches the path <c><anno>Path</anno></c> (a list of directory
          names) until the file <c><anno>Filename</anno></c> is found.
          If <c><anno>Filename</anno></c>
          is an absolute filename, <c><anno>Path</anno></c> is ignored.
          Then opens the file in the mode determined by <c><anno>Modes</anno></c>.</p>
        <p>Returns one of the following:</p>
        <taglist>
          <tag><c>{ok, <anno>IoDevice</anno>, <anno>FullName</anno>}</c></tag>
          <item>
            <p>The file is opened in the requested mode.
              <c><anno>IoDevice</anno></c> is a reference to the file and
              <c><anno>FullName</anno></c> is the full name of the file.</p>
          </item>
          <tag><c>{error, enoent}</c></tag>
          <item>
            <p>The file cannot be found in any of the directories in
              <c><anno>Path</anno></c>.</p>
          </item>
          <tag><c>{error, atom()}</c></tag>
          <item>
            <p>The file cannot be opened.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="path_script" arity="2"/>
      <fsummary>Evaluate and return the value of Erlang expressions in a file.</fsummary>
      <desc>
        <p>Searches the path <c><anno>Path</anno></c> (a list of directory
          names) until the file <c><anno>Filename</anno></c> is found.
          If <c><anno>Filename</anno></c> is an absolute filename,
          <c><anno>Path</anno></c> is ignored. Then reads
          and evaluates Erlang expressions, separated by '.' (or ',', a
          sequence of expressions is also an expression), from the file.</p>
          <p>Returns one of the following:</p>
        <taglist>
          <tag><c>{ok, <anno>Value</anno>, <anno>FullName</anno>}</c></tag>
          <item>
            <p>The file is read and evaluated. <c><anno>FullName</anno></c> is
              the full name of the file and <c><anno>Value</anno></c> the value of
              the last expression.</p>
          </item>
          <tag><c>{error, enoent}</c></tag>
          <item>
            <p>The file cannot be found in any of the directories in
              <c><anno>Path</anno></c>.</p>
          </item>
          <tag><c>{error, atom()}</c></tag>
          <item>
            <p>An error occurred when opening the file or reading it.
              For a list of typical error codes, see
	      <seealso marker="#open/2"><c>open/2</c></seealso>.</p>
          </item>
          <tag><c>{error, {<anno>Line</anno>, <anno>Mod</anno>,
                           <anno>Term</anno>}}</c></tag>
          <item>
            <p>An error occurred when interpreting the Erlang
              expressions in the file. Use
	      <seealso marker="#format_error/1"><c>format_error/1</c></seealso>
	      to convert the three-element tuple to an English description
              of the error.</p>
          </item>
        </taglist>
        <p>The encoding of <c><anno>Filename</anno></c> can be set
          by a comment as described in
	  <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>.</p>
   </desc>
    </func>
    <func>
      <name name="path_script" arity="3"/>
      <fsummary>Evaluate and return the value of Erlang expressions in a file.</fsummary>
      <desc>
        <p>The same as <c>path_script/2</c> but the variable bindings
          <c><anno>Bindings</anno></c> are used in the evaluation. See
          <seealso marker="stdlib:erl_eval"><c>erl_eval(3)</c></seealso> about
          variable bindings.</p>
      </desc>
    </func>
    <func>
      <name name="pid2name" arity="1"/>
      <fsummary>Return the name of the file handled by a pid.</fsummary>
      <desc>
        <p>If <c><anno>Pid</anno></c> is an I/O device, that is, a pid returned from
          <c>open/2</c>, this function returns the filename, or rather:</p>
        <taglist>
          <tag><c>{ok, <anno>Filename</anno>}</c></tag>
          <item>
            <p>If the file server of this node is not a slave, the file was
              opened by the file server of this node (this implies that
              <c><anno>Pid</anno></c> must be a local pid) and the file is not
              closed. <c><anno>Filename</anno></c> is the filename in flat string
              format.</p>
          </item>
          <tag><c>undefined</c></tag>
          <item>
            <p>In all other cases.</p>
          </item>
        </taglist>
        <warning>
          <p>This function is intended for debugging only.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="position" arity="2"/>
      <fsummary>Set position in a file.</fsummary>
      <desc>
        <p>Sets the position of the file referenced by <c><anno>IoDevice</anno></c>
          to <c><anno>Location</anno></c>. Returns <c>{ok, <anno>NewPosition</anno>}</c>
	  (as absolute offset) if successful, otherwise
	  <c>{error, <anno>Reason</anno>}</c>. <c><anno>Location</anno></c> is
          one of the following:</p>
        <taglist>
          <tag><c>Offset</c></tag>
          <item>
            <p>The same as <c>{bof, Offset}</c>.</p>
          </item>
          <tag><c>{bof, Offset}</c></tag>
          <item>
            <p>Absolute offset.</p>
          </item>
          <tag><c>{cur, Offset}</c></tag>
          <item>
            <p>Offset from the current position.</p>
          </item>
          <tag><c>{eof, Offset}</c></tag>
          <item>
            <p>Offset from the end of file.</p>
          </item>
          <tag><c>bof | cur | eof</c></tag>
          <item>
            <p>The same as above with <c>Offset</c> 0.</p>
          </item>
        </taglist>
	<p>Notice that offsets are counted in bytes, not in characters. If the file
	is opened using some other <c>encoding</c> than <c>latin1</c>, one byte
	does not correspond to one character. Positioning in such a file can only
	be done to known character boundaries. That is, to a position earlier retrieved
	by getting a current position, to the beginning/end of the file or to some
	other position <em>known</em> to be on a correct character boundary by some
	other means (typically beyond a byte order mark in the file, which has a
	known byte-size).</p>
        <p>A typical error reason is:</p>
        <taglist>
          <tag><c>einval</c></tag>
          <item>
            <p>Either <c><anno>Location</anno></c> is illegal, or it is
              evaluated to a
              negative offset in the file. Notice that if the resulting
              position is a negative value, the result is an error, and
              after the call the file position is undefined.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="pread" arity="2"/>
      <fsummary>Read from a file at certain positions.</fsummary>
      <desc>
        <p>Performs a sequence of <c>pread/3</c> in one operation, 
          which is more efficient than calling them one at a time.
          Returns <c>{ok, [<anno>Data</anno>, ...]}</c> or
          <c>{error, <anno>Reason</anno>}</c>,
          where each <c><anno>Data</anno></c>, the result of the corresponding
          <c>pread</c>, is either a list or a binary depending on
          the mode of the file, or <c>eof</c> if the requested position
          is beyond end of file.</p>
	  <p>As the position is specified as a byte-offset, take special caution
	  when working with files where <c>encoding</c> is set to something else
	  than <c>latin1</c>, as not every byte position is a valid character
	  boundary on such a file.</p>
      </desc>
    </func>
    <func>
      <name name="pread" arity="3"/>
      <fsummary>Read from a file at a certain position.</fsummary>
      <desc>
        <p>Combines <c>position/2</c> and <c>read/2</c> in one
          operation, which is more efficient than calling them one at a
          time. If <c><anno>IoDevice</anno></c> is opened in <c>raw</c> mode,
          some restrictions apply:</p>
	  <list type="bulleted">
	    <item><c><anno>Location</anno></c> is only allowed to be an
            integer.</item>
	    <item>The current position of the file is undefined after the
	    operation.</item>
	  </list>
	  <p>As the position is specified as a byte-offset, take special caution
	  when working with files where <c>encoding</c> is set to something else
	  than <c>latin1</c>, as not every byte position is a valid character
	  boundary on such a file.</p>
        </desc>
    </func>
    <func>
      <name name="pwrite" arity="2"/>
      <fsummary>Write to a file at certain positions.</fsummary>
      <desc>
        <p>Performs a sequence of <c>pwrite/3</c> in one operation, 
          which is more efficient than calling them one at a time.
          Returns <c>ok</c> or <c>{error, {<anno>N</anno>,
          <anno>Reason</anno>}}</c>, where
          <c><anno>N</anno></c> is the number of successful writes done
          before the failure.</p>
	<p>When positioning in a file with other <c>encoding</c> than <c>latin1</c>,
	caution must be taken to set the position on a correct character boundary.
	For details, see <seealso marker="#position/2"><c>position/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="pwrite" arity="3"/>
      <fsummary>Write to a file at a certain position.</fsummary>
      <desc>
        <p>Combines <c>position/2</c> and <c>write/2</c> in one
          operation, which is more efficient than calling them one at a
          time. If <c><anno>IoDevice</anno></c> has been opened in <c>raw</c> mode,
          some restrictions apply:</p>
	  <list type="bulleted">
	    <item><c><anno>Location</anno></c> is only allowed to be an
            integer.</item>
	    <item>The current position of the file is undefined after the
	    operation.</item>
	  </list>
	<p>When positioning in a file with other <c>encoding</c> than <c>latin1</c>,
	caution must be taken to set the position on a correct character boundary.
	For details, see <seealso marker="#position/2"><c>position/2</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="read" arity="2"/>
      <fsummary>Read from a file.</fsummary>
      <desc>
        <p>Reads <c><anno>Number</anno></c> bytes/characters from the file
          referenced by <c><anno>IoDevice</anno></c>. The functions
          <seealso marker="#read/2"><c>read/2</c></seealso>,
	  <seealso marker="#pread/3"><c>pread/3</c></seealso>, and
	  <seealso marker="#read_line/1"><c>read_line/1</c></seealso>
	  are the only ways to read from a file opened in <c>raw</c> mode
	  (although they work for normally opened files, too).</p>
	  <p>For files where <c>encoding</c> is set to something else than <c>latin1</c>,
	  one character can be represented by more than one byte on the file.
	  The parameter <c>Number</c> always denotes the number of <em>characters</em>
	  read from the file, while the position in the file can be moved much more than
	  this number when reading a Unicode file.</p>
	  <p>Also, if <c>encoding</c> is set to something else than <c>latin1</c>,
	  the <c>read/3</c> call fails if the data contains characters larger than 255,
	  which is why module <seealso marker="stdlib:io"><c>io(3)</c></seealso>
	  is to be preferred when reading such a file.</p>
	  <p>The function returns:</p>
        <taglist>
          <tag><c>{ok, <anno>Data</anno>}</c></tag>
          <item>
            <p>If the file was opened in binary mode, the read bytes are
              returned in a binary, otherwise in a list. The list or
              binary is shorter than the number of bytes requested
              if end of file was reached.</p>
          </item>
          <tag><c>eof</c></tag>
          <item>
            <p>Returned if <c><anno>Number</anno>>0</c> and end of file was
              reached before anything at all could be read.</p>
          </item>
          <tag><c>{error, <anno>Reason</anno>}</c></tag>
          <item>
            <p>An error occurred.</p>
          </item>
        </taglist>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>ebadf</c></tag>
          <item>
            <p>The file is not opened for reading.</p>
          </item>
          <tag><c>{no_translation, unicode, latin1}</c></tag>
          <item>
            <p>The file is opened with another <c>encoding</c> than <c>latin1</c> and
	    the data in the file cannot be translated to the byte-oriented data that
	    this function returns.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="read_file" arity="1"/>
      <fsummary>Read a file.</fsummary>
      <desc>
        <p>Returns <c>{ok, <anno>Binary</anno>}</c>, where
          <c><anno>Binary</anno></c> is a binary
          data object that contains the contents of
          <c><anno>Filename</anno></c>, or
          <c>{error, <anno>Reason</anno>}</c> if an error occurs.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>enoent</c></tag>
          <item>
            <p>The file does not exist.</p>
          </item>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing permission for reading the file, or for
              searching one of the parent directories.</p>
          </item>
          <tag><c>eisdir</c></tag>
          <item>
            <p>The named file is a directory.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of the filename is not a directory. On some
              platforms, <c>enoent</c> is returned instead.</p>
          </item>
          <tag><c>enomem</c></tag>
          <item>
            <p>There is not enough memory for the contents of the file.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="read_file_info" arity="1"/>
      <name name="read_file_info" arity="2"/>
      <fsummary>Retrieve information about a file.</fsummary>
      <desc>
        <p>Retrieves information about a file. Returns
          <c>{ok, <anno>FileInfo</anno>}</c> if successful, otherwise
          <c>{error, <anno>Reason</anno>}</c>. <c><anno>FileInfo</anno></c>
          is a record
          <c>file_info</c>, defined in the Kernel include file
          <c>file.hrl</c>. Include the following directive in the module
          from which the function is called:</p>
        <code type="none">
 -include_lib("kernel/include/file.hrl").</code>
	<p>The time type returned in <c>atime</c>, <c>mtime</c>, and <c>ctime</c>
	  is dependent on the time type set in <c>Opts :: {time, Type}</c> as
	  follows:</p>
	  <taglist>
	    <tag><c>local</c></tag>
	    <item><p>Returns local time.</p></item>
	    <tag><c>universal</c></tag>
	    <item><p>Returns universal time.</p></item>
	    <tag><c>posix</c></tag>
	    <item><p>Returns seconds since or before Unix time epoch,
	    which is 1970-01-01 00:00 UTC.</p></item>
	  </taglist>
	<p>Default is <c>{time, local}</c>.</p>
        <p>If the option <c>raw</c> is set, the file server is not called
          and only information about local files is returned.</p>
	  <note>
          <p>As file times are stored in POSIX time on most OS, it is faster to
	  query file information with option <c>posix</c>.</p>
          </note>
        
        <p>The record <c>file_info</c> contains the following fields:</p>
        <taglist>
          <tag><c>size = integer() >= 0</c></tag>
          <item>
            <p>Size of file in bytes.</p>
          </item>
          <tag><c>type = device | directory | other | regular | symlink</c></tag>
          <item>
            <p>The type of the file.</p>
          </item>
          <tag><c>access = read | write | read_write | none</c></tag>
          <item>
            <p>The current system access to the file.</p>
          </item>
          <tag><c>atime = </c>
	  <seealso marker="#type-date_time"><c>date_time()</c></seealso><c> |
	  integer() >= 0</c></tag>
          <item>
            <p>The last time the file was read.</p>
          </item>
          <tag><c>mtime = </c>
	  <seealso marker="#type-date_time"><c>date_time()</c></seealso><c> |
	  integer() >= 0</c></tag>
          <item>
            <p>The last time the file was written.</p>
          </item>
          <tag><c>ctime = </c>
	  <seealso marker="#type-date_time"><c>date_time()</c></seealso><c> |
	  integer() >=0</c></tag>
          <item>
            <p>The interpretation of this time field depends on
              the operating system. On Unix, it is the last time
              the file or the <c>inode</c> was changed. In Windows, it is
              the create time.</p>
          </item>
          <tag><c>mode = integer() >= 0</c></tag>
          <item>
            <p>The file permissions as the sum of the following bit
              values:</p>
            <taglist>
              <tag><c>8#00400</c></tag>
              <item><p>read permission: owner</p></item>
              <tag><c>8#00200</c></tag>
              <item><p>write permission: owner</p></item>
              <tag><c>8#00100</c></tag>
              <item><p>execute permission: owner</p></item>
              <tag><c>8#00040</c></tag>
              <item><p>read permission: group</p></item>
              <tag><c>8#00020</c></tag>
              <item><p>write permission: group</p></item>
              <tag><c>8#00010</c></tag>
              <item><p>execute permission: group</p></item>
              <tag><c>8#00004</c></tag>
              <item><p>read permission: other</p></item>
              <tag><c>8#00002</c></tag>
              <item><p>write permission: other</p></item>
              <tag><c>8#00001</c></tag>
              <item><p>execute permission: other</p></item>
              <tag><c>16#800</c></tag>
              <item><p>set user id on execution</p></item>
              <tag><c>16#400</c></tag>
              <item><p>set group id on execution</p></item>
            </taglist>
            <p>On Unix platforms, other bits than those listed above
		may be set.</p>
          </item>
          <tag><c>links = integer() >= 0</c></tag>
          <item>
            <p>Number of links to the file (this is always 1 for
              file systems that have no concept of links).</p>
          </item>
          <tag><c>major_device = integer() >= 0</c></tag>
          <item>
            <p>Identifies the file system where the file is located.
              In Windows, the number indicates a drive as follows:
              0 means A:, 1 means B:, and so on.</p>
          </item>
          <tag><c>minor_device = integer() >= 0</c></tag>
          <item>
            <p>Only valid for character devices on Unix. In all other
              cases, this field is zero.</p>
          </item>
          <tag><c>inode = integer() >= 0</c></tag>
          <item>
            <p>Gives the <c>inode</c> number. On non-Unix file systems,
              this field is zero.</p>
          </item>
          <tag><c>uid = integer() >= 0</c></tag>
          <item>
            <p>Indicates the owner of the file. On non-Unix file systems,
              this field is zero.</p>
          </item>
          <tag><c>gid = integer() >= 0</c></tag>
          <item>
            <p>Gives the group that the owner of the file belongs to.
              On non-Unix file systems, this field is zero.</p>
          </item>
        </taglist>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing search permission for one of the parent
              directories of the file.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>The file does not exist.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of the filename is not a directory. On some
              platforms, <c>enoent</c> is returned instead.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="read_line" arity="1"/>
      <fsummary>Read a line from a file.</fsummary>
      <desc>
        <p>Reads a line of bytes/characters from the file referenced by
          <c><anno>IoDevice</anno></c>. Lines are defined to be delimited by the
	  linefeed (LF, <c>\n</c>) character, but any carriage return (CR, <c>\r</c>)
	  followed by a newline is also treated as a single LF character (the carriage
	  return is silently ignored). The line is returned <em>including</em> the LF,
	  but excluding any CR immediately followed by an LF. This behaviour is
	  consistent with the behaviour of
	  <seealso marker="stdlib:io#get_line/2"><c>io:get_line/2</c></seealso>.
	  If end of file is reached without any LF ending the last line, a line with no
	  trailing LF is returned.</p>
	  <p>The function can be used on files opened in <c>raw</c> mode. However, it is
	  inefficient to use it on <c>raw</c> files if the file is not opened with
	  option <c>{read_ahead, Size}</c> specified. Thus, combining <c>raw</c> and
	  <c>{read_ahead, Size}</c> is highly recommended when opening a text file for
	  raw line-oriented reading.</p>
	  <p>If <c>encoding</c> is set to something else than <c>latin1</c>, the
	  <c>read_line/1</c> call fails if the data contains characters larger than 255,
	  why module <seealso marker="stdlib:io"><c>io(3)</c></seealso> is to be
	  preferred when reading such a file.</p>
	  <p>The function returns:</p>
        <taglist>
          <tag><c>{ok, <anno>Data</anno>}</c></tag>
          <item>
	    <p>One line from the file is returned, including the trailing LF,
	    but with CRLF sequences replaced by a single LF (see above).</p>
            <p>If the file is opened in binary mode, the read bytes are
              returned in a binary, otherwise in a list.</p>
          </item>
          <tag><c>eof</c></tag>
          <item>
            <p>Returned if end of file was reached
              before anything at all could be read.</p>
          </item>
          <tag><c>{error, <anno>Reason</anno>}</c></tag>
          <item>
            <p>An error occurred.</p>
          </item>
        </taglist>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>ebadf</c></tag>
          <item>
            <p>The file is not opened for reading.</p>
          </item>
          <tag><c>{no_translation, unicode, latin1}</c></tag>
          <item>
            <p>The file is opened with another <c>encoding</c> than <c>latin1</c> and
	    the data on the file cannot be translated to the byte-oriented data that
	    this function returns.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="read_link" arity="1"/>
      <fsummary>See what a link is pointing to.</fsummary>
      <desc>
        <p><marker id="read_link_all"/>Returns
	<c>{ok, <anno>Filename</anno>}</c> if
          <c><anno>Name</anno></c> refers to a symbolic link that is
	  not a raw filename, or <c>{error, <anno>Reason</anno>}</c>
	  otherwise.
          On platforms that do not support symbolic links, the return
          value is <c>{error,enotsup}</c>.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>einval</c></tag>
          <item>
            <p><c><anno>Name</anno></c> does not refer to a symbolic link
	    or the name of the file that it refers to does not conform
	    to the expected encoding.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>The file does not exist.</p>
          </item>
          <tag><c>enotsup</c></tag>
          <item>
            <p>Symbolic links are not supported on this platform.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="read_link_all" arity="1"/>
      <fsummary>See what a link is pointing to.</fsummary>
      <desc>
        <p>Returns <c>{ok, <anno>Filename</anno>}</c> if
          <c><anno>Name</anno></c> refers to a symbolic link or
          <c>{error, <anno>Reason</anno>}</c> otherwise.
          On platforms that do not support symbolic links, the return
          value is <c>{error,enotsup}</c>.</p>
	  <p>Notice that <c><anno>Filename</anno></c> can be either a list
	  or a binary.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>einval</c></tag>
          <item>
            <p><c><anno>Name</anno></c> does not refer to a symbolic link.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>The file does not exist.</p>
          </item>
          <tag><c>enotsup</c></tag>
          <item>
            <p>Symbolic links are not supported on this platform.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="read_link_info" arity="1"/>
      <name name="read_link_info" arity="2"/>
      <fsummary>Retrieve information about a link or file.</fsummary>
      <desc>
	<p>Works like
	<seealso marker="#read_file_info/2"><c>read_file_info/1,2</c></seealso>
	except that if <c><anno>Name</anno></c> is a symbolic link, information
	about the link is returned in the <c>file_info</c> record and
          the <c>type</c> field of the record is set to <c>symlink</c>.</p>
      <p>If the option <c>raw</c> is set, the file server is not called
        and only information about local files is returned.</p>
        <p>If <c><anno>Name</anno></c> is not a symbolic link, this function returns
          the same result as <c>read_file_info/1</c>.
          On platforms that do not support symbolic links, this function
          is always equivalent to <c>read_file_info/1</c>.</p>
      </desc>
    </func>
    <func>
      <name name="rename" arity="2"/>
      <fsummary>Rename a file.</fsummary>
      <desc>
        <p>Tries to rename the file <c><anno>Source</anno></c> to
          <c><anno>Destination</anno></c>.
          It can be used to move files (and directories) between
          directories, but it is not sufficient to specify
          the destination only. The destination filename must also be
          specified. For example, if <c>bar</c> is a normal file and
          <c>foo</c> and <c>baz</c> are directories,
          <c>rename("foo/bar", "baz")</c> returns an error, but
          <c>rename("foo/bar", "baz/bar")</c> succeeds. Returns
          <c>ok</c> if it is successful.</p>
        <note>
          <p>Renaming of open files is not allowed on most platforms
            (see <c>eacces</c> below).</p>
        </note>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing read or write permissions for the parent
              directories of <c><anno>Source</anno></c> or
              <c><anno>Destination</anno></c>. On
              some platforms, this error is given if either
              <c><anno>Source</anno></c> or <c><anno>Destination</anno></c>
              is open.</p>
          </item>
          <tag><c>eexist</c></tag>
          <item>
            <p><c><anno>Destination</anno></c> is not an empty directory.
              On some platforms, also given when <c><anno>Source</anno></c> and
              <c><anno>Destination</anno></c> are not of the same type.</p>
          </item>
          <tag><c>einval</c></tag>
          <item>
            <p><c><anno>Source</anno></c> is a root directory, or
              <c><anno>Destination</anno></c>
              is a subdirectory of <c><anno>Source</anno></c>.</p>
          </item>
          <tag><c>eisdir</c></tag>
          <item>
            <p><c><anno>Destination</anno></c> is a directory, but
              <c><anno>Source</anno></c> is not.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p><c>Source</c> does not exist.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p><c><anno>Source</anno></c> is a directory, but
              <c><anno>Destination</anno></c> is not.</p>
          </item>
          <tag><c>exdev</c></tag>
          <item>
            <p><c><anno>Source</anno></c> and <c><anno>Destination</anno></c>
              are on different file systems.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="script" arity="1"/>
      <fsummary>Evaluate and return the value of Erlang expressions in a file.</fsummary>
      <desc>
        <p>Reads and evaluates Erlang expressions, separated by '.' (or
          ',', a sequence of expressions is also an expression), from
          the file.</p>
	<p>Returns one of the following:</p>
        <taglist>
          <tag><c>{ok, <anno>Value</anno>}</c></tag>
          <item>
            <p>The file is read and evaluated. <c><anno>Value</anno></c> is
              the value of the last expression.</p>
          </item>
          <tag><c>{error, atom()}</c></tag>
          <item>
            <p>An error occurred when opening the file or reading it.
              For a list of typical error codes, see
	    <seealso marker="#open/2"><c>open/2</c></seealso>.</p>
          </item>
          <tag><c>{error, {<anno>Line</anno>, <anno>Mod</anno>,
                           <anno>Term</anno>}}</c></tag>
          <item>
            <p>An error occurred when interpreting the Erlang
              expressions in the file. Use
	      <seealso marker="#format_error/1"><c>format_error/1</c></seealso>
	      to convert the three-element tuple to an English description
              of the error.</p>
          </item>
        </taglist>
        <p>The encoding of <c><anno>Filename</anno></c> can be set
          by a comment as described in
	  <seealso marker="stdlib:epp#encoding"><c>epp(3)</c></seealso>.</p>
      </desc>
    </func>
    <func>
      <name name="script" arity="2"/>
      <fsummary>Evaluate and return the value of Erlang expressions in a file.</fsummary>
      <desc>
        <p>The same as <c>script/1</c> but the variable bindings
          <c><anno>Bindings</anno></c> are used in the evaluation. See
          <seealso marker="stdlib:erl_eval"><c>erl_eval(3)</c></seealso> about
          variable bindings.</p>
      </desc>
    </func>
    <func>
      <name name="sendfile" arity="2"/>
      <fsummary>Send a file to a socket.</fsummary>
      <desc>
        <p>Sends the file <c>Filename</c> to <c>Socket</c>.
        Returns <c>{ok, BytesSent}</c> if successful,
        otherwise <c>{error, Reason}</c>.</p>
      </desc>
    </func>
    <func>
      <name name="sendfile" arity="5"/>
      <fsummary>Send a file to a socket.</fsummary>
      <type name="sendfile_option"/>
      <desc>
        <p>Sends <c>Bytes</c> from the file
        referenced by <c>RawFile</c> beginning at <c>Offset</c> to
        <c>Socket</c>.
        Returns <c>{ok, BytesSent}</c> if successful,
        otherwise <c>{error, Reason}</c>. If <c>Bytes</c> is set to
	<c>0</c> all data after the specified <c>Offset</c> is sent.</p>
	<p>The file used must be opened using the <c>raw</c> flag, and the process
	calling <c>sendfile</c> must be the controlling process of the socket.
	See <seealso marker="gen_tcp#controlling_process-2"><c>gen_tcp:controlling_process/2</c></seealso>.</p>
	<p>If the OS used does not support <c>sendfile</c>, an Erlang fallback
	using
	<seealso marker="#read/2"><c>read/2</c></seealso> and
	<seealso marker="gen_tcp#send/2"><c>gen_tcp:send/2</c></seealso> is used.</p>
	<p>The option list can contain the following options:</p>
	<taglist>
          <tag><c>chunk_size</c></tag>
          <item><p>The chunk size used by the Erlang fallback to send
	  data. If using the fallback, set this to a value
	  that comfortably fits in the systems memory. Default is 20 MB.</p></item>
	  <tag><c>use_threads</c></tag>
          <item><p>Instructs the emulator to use the <c>async</c> thread pool for the
	  <c>sendfile</c> system call. This can be useful if the OS you are running
	  on does not properly support non-blocking <c>sendfile</c> calls. Notice that
	  using <c>async</c> threads potentially makes your system vulnerable to slow
	  client attacks. If set to <c>true</c> and no <c>async</c> threads are available,
	  the <c>sendfile</c> call returns <c>{error,einval}</c>.
	  Introduced in Erlang/OTP 17.0. Default is <c>false</c>.</p></item>
	</taglist>
      </desc>
    </func>
    <func>
      <name name="set_cwd" arity="1"/>
      <fsummary>Set the current working directory.</fsummary>
      <desc>
        <p>Sets the current working directory of the file server to
          <c><anno>Dir</anno></c>. Returns <c>ok</c> if successful.</p>
        <p>The functions in the module <c>file</c> usually treat binaries
          as raw filenames, that is, they are passed "as is" even when the
	  encoding of the binary does not agree with
	  <seealso marker="#native_name_encoding/0"><c>native_name_encoding()</c></seealso>.
          However, this function expects binaries to be encoded according to the
          value returned by <c>native_name_encoding()</c>.</p>
        <p>Typical error reasons are:</p>
        <taglist>
          <tag><c>enoent</c></tag>
          <item>
            <p>The directory does not exist.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of <c><anno>Dir</anno></c> is not a directory.
              On some platforms, <c>enoent</c> is returned.</p>
          </item>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing permission for the directory or one of its
              parents.</p>
          </item>
          <tag><c>badarg</c></tag>
          <item>
            <p><c><anno>Dir</anno></c> has an improper type,
              such as tuple.</p>
          </item>
          <tag><c>no_translation</c></tag>
          <item>
            <p><c><anno>Dir</anno></c> is a <c>binary()</c> with
              characters coded in ISO-latin-1 and the VM is operating
              with unicode filename encoding.</p>
          </item>
        </taglist>
        <warning>
          <p>In a future release, a bad type for argument
            <c><anno>Dir</anno></c>
            will probably generate an exception.</p>
        </warning>
      </desc>
    </func>
    <func>
      <name name="sync" arity="1"/>
      <fsummary>Synchronize the in-memory state of a file with that on the physical medium.</fsummary>
      <desc>
        <p>Ensures that any buffers kept by the operating system
          (not by the Erlang runtime system) are written to disk. On
          some platforms, this function might have no effect.</p>
        <p>A typical error reason is:</p>
        <taglist>
          <tag><c>enospc</c></tag>
          <item>
            <p>Not enough space left to write the file.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="truncate" arity="1"/>
      <fsummary>Truncate a file.</fsummary>
      <desc>
        <p>Truncates the file referenced by <c><anno>IoDevice</anno></c> at
          the current position. Returns <c>ok</c> if successful,
          otherwise <c>{error, <anno>Reason</anno>}</c>.</p>
      </desc>
    </func>
    <func>
      <name name="write" arity="2"/>
      <fsummary>Write to a file.</fsummary>
      <desc>
        <p>Writes <c><anno>Bytes</anno></c> to the file referenced by
          <c><anno>IoDevice</anno></c>. This function is the only way to write to a
          file opened in <c>raw</c> mode (although it works for normally opened
	  files too). Returns <c>ok</c> if successful, and
          <c>{error, <anno>Reason</anno>}</c> otherwise.</p>
	  <p>If the file is opened with <c>encoding</c> set to something else than
	  <c>latin1</c>, each byte written can result in many bytes being written to
	  the file, as the byte range 0..255 can represent anything between one and
	  four bytes depending on value and UTF encoding type.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>ebadf</c></tag>
          <item>
            <p>The file is not opened for writing.</p>
          </item>
          <tag><c>enospc</c></tag>
          <item>
            <p>No space is left on the device.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="write_file" arity="2"/>
      <fsummary>Write a file.</fsummary>
      <desc>
        <p>Writes the contents of the <c>iodata</c> term <c><anno>Bytes</anno></c>
          to file <c><anno>Filename</anno></c>.
          The file is created if it does not exist.
	  If it exists, the previous contents are overwritten.
	  Returns <c>ok</c> if successful, otherwise
	<c>{error, <anno>Reason</anno>}</c>.</p>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>enoent</c></tag>
          <item>
            <p>A component of the filename does not exist.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of the filename is not a directory. On some
              platforms, <c>enoent</c> is returned instead.</p>
          </item>
          <tag><c>enospc</c></tag>
          <item>
            <p>No space is left on the device.</p>
          </item>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing permission for writing the file or searching one
              of the parent directories.</p>
          </item>
          <tag><c>eisdir</c></tag>
          <item>
            <p>The named file is a directory.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="write_file" arity="3"/>
      <fsummary>Write a file.</fsummary>
      <desc>
        <p>Same as <c>write_file/2</c>, but takes a third argument
          <c><anno>Modes</anno></c>, a list of possible modes, see
          <seealso marker="#open/2"><c>open/2</c></seealso>. The mode flags
          <c>binary</c> and <c>write</c> are implicit, so they are
          not to be used.</p>
      </desc>
    </func>
    <func>
      <name name="write_file_info" arity="2"/>
      <name name="write_file_info" arity="3"/>
      <fsummary>Change file information.</fsummary>
      <desc>
        <p>Changes file information. Returns <c>ok</c> if successful,
          otherwise <c>{error, <anno>Reason</anno>}</c>.
          <c><anno>FileInfo</anno></c> is a record
          <c>file_info</c>, defined in the Kernel include file
          <c>file.hrl</c>. Include the following directive in the module
          from which the function is called:</p>
        <code type="none">
 -include_lib("kernel/include/file.hrl").</code>
        <p>The time type set in <c>atime</c>, <c>mtime</c>, and <c>ctime</c>
	  depends on the time type set in <c>Opts :: {time, Type}</c> as
	  follows:</p>
	  <taglist>
	    <tag><c>local</c></tag>
	    <item><p>Interprets the time set as local.</p></item>
	    <tag><c>universal</c></tag>
	    <item><p>Interprets it as universal time.</p></item>
	    <tag><c>posix</c></tag>
	    <item><p>Must be seconds since or before Unix time epoch,
	    which is 1970-01-01 00:00 UTC.</p></item>
	  </taglist>
	<p>Default is <c>{time, local}</c>.</p>
        <p>If the option <c>raw</c> is set, the file server is not called
          and only information about local files is returned.</p>
        <p>The following fields are used from the record, if they are
          specified:</p>
        <taglist>
          <tag><c>atime = </c>
	  <seealso marker="#type-date_time"><c>date_time()</c></seealso><c> |
	  integer() >= 0</c></tag>
          <item>
            <p>The last time the file was read.</p>
          </item>
          <tag><c>mtime = </c>
	  <seealso marker="#type-date_time"><c>date_time()</c></seealso><c> |
	  integer() >= 0</c></tag>
          <item>
            <p>The last time the file was written.</p>
          </item>
          <tag><c>ctime = </c>
	  <seealso marker="#type-date_time"><c>date_time()</c></seealso><c> |
	  integer() >= 0</c></tag>
          <item>
            <p>On Unix, any value specified for this field is ignored
              (the "ctime" for the file is set to the current
              time). On Windows, this field is the new creation time to
              set for the file.</p>
          </item>
          <tag><c>mode = integer() >= 0</c></tag>
          <item>
            <p>The file permissions as the sum of the following bit
              values:</p>
            <taglist>
              <tag><c>8#00400</c></tag>
              <item><p>Read permission: owner</p></item>
              <tag><c>8#00200</c></tag>
              <item><p>Write permission: owner</p></item>
              <tag><c>8#00100</c></tag>
              <item><p>Execute permission: owner</p></item>
              <tag><c>8#00040</c></tag>
              <item><p>Read permission: group</p></item>
              <tag><c>8#00020</c></tag>
              <item><p>Write permission: group</p></item>
              <tag><c>8#00010</c></tag>
              <item><p>Execute permission: group</p></item>
              <tag><c>8#00004</c></tag>
              <item><p>Read permission: other</p></item>
              <tag><c>8#00002</c></tag>
              <item><p>Write permission: other</p></item>
              <tag><c>8#00001</c></tag>
              <item><p>Execute permission: other</p></item>
              <tag><c>16#800</c></tag>
              <item><p>Set user id on execution</p></item>
              <tag><c>16#400</c></tag>
              <item><p>Set group id on execution</p></item>
            </taglist>
            <p>On Unix platforms, other bits than those listed above
		may be set.</p>
          </item>
          <tag><c>uid = integer() >= 0</c></tag>
          <item>
            <p>Indicates the file owner. Ignored for non-Unix
              file systems.</p>
          </item>
          <tag><c>gid = integer() >= 0</c></tag>
          <item>
            <p>Gives the group that the file owner belongs to.
              Ignored for non-Unix file systems.</p>
          </item>
        </taglist>
        <p>Typical error reasons:</p>
        <taglist>
          <tag><c>eacces</c></tag>
          <item>
            <p>Missing search permission for one of the parent
              directories of the file.</p>
          </item>
          <tag><c>enoent</c></tag>
          <item>
            <p>The file does not exist.</p>
          </item>
          <tag><c>enotdir</c></tag>
          <item>
            <p>A component of the filename is not a directory. On some
              platforms, <c>enoent</c> is returned instead.</p>
          </item>
        </taglist>
      </desc>
    </func>
  </funcs>

  <section>
    <title>POSIX Error Codes</title>
    <list type="bulleted">
      <item><c>eacces</c> - Permission denied</item>
      <item><c>eagain</c> - Resource temporarily unavailable</item>
      <item><c>ebadf</c> - Bad file number</item>
      <item><c>ebusy</c> - File busy</item>
      <item><c>edquot</c> - Disk quota exceeded</item>
      <item><c>eexist</c> - File already exists</item>
      <item><c>efault</c> - Bad address in system call argument</item>
      <item><c>efbig</c> - File too large</item>
      <item><c>eintr</c> - Interrupted system call</item>
      <item><c>einval</c> - Invalid argument</item>
      <item><c>eio</c> - I/O error</item>
      <item><c>eisdir</c> - Illegal operation on a directory</item>
      <item><c>eloop</c> - Too many levels of symbolic links</item>
      <item><c>emfile</c> - Too many open files</item>
      <item><c>emlink</c> - Too many links</item>
      <item><c>enametoolong</c> - Filename too long</item>
      <item><c>enfile</c> - File table overflow</item>
      <item><c>enodev</c> - No such device</item>
      <item><c>enoent</c> - No such file or directory</item>
      <item><c>enomem</c> - Not enough memory</item>
      <item><c>enospc</c> - No space left on device</item>
      <item><c>enotblk</c> - Block device required</item>
      <item><c>enotdir</c> - Not a directory</item>
      <item><c>enotsup</c> - Operation not supported</item>
      <item><c>enxio</c> - No such device or address</item>
      <item><c>eperm</c> - Not owner</item>
      <item><c>epipe</c> - Broken pipe</item>
      <item><c>erofs</c> - Read-only file system</item>
      <item><c>espipe</c> - Invalid seek</item>
      <item><c>esrch</c> - No such process</item>
      <item><c>estale</c> - Stale remote file handle</item>
      <item><c>exdev</c> - Cross-domain link</item>
    </list>
  </section>

  <section>
    <title>Performance</title>
    <p>Some operating system file operations, for example, a
      <c>sync/1</c> or <c>close/1</c> on a huge file, can block their
      calling thread for seconds. If this affects the emulator main
      thread, the response time is no longer in the order of
      milliseconds, depending on the definition of "soft" in soft
      real-time system.</p>
    <p>If the device driver thread pool is active, file operations are
      done through those threads instead, so the emulator can go on
      executing Erlang processes. Unfortunately, the time for serving a
      file operation increases because of the extra scheduling required
      from the operating system.</p>
    <p>If the device driver thread pool is disabled or of size 0, large
      file reads and writes are segmented into many smaller, which
      enable the emulator to serve other processes during the file
      operation. This has the same effect as when using the thread
      pool, but with larger overhead. Other file operations, for
      example, <c>sync/1</c> or <c>close/1</c> on a huge file, still are
      a problem.</p>
    <p>For increased performance, raw files are recommended. Raw files
      use the file system of the host machine of the node.</p>
    <note>
      <p>
	For normal files (non-raw), the file server is used to find the files,
	and if the node is running its file server as slave to the file server
	of another node, and the other node runs on some other host machine,
	they can have different file systems.
        However, this is seldom a problem.</p>
    </note>
    <p>A normal file is really a process so it can be used as an I/O
      device (see
      <seealso marker="stdlib:io"><c>io</c></seealso>).
      Therefore, when data is written to a
      normal file, the sending of the data to the file process, copies 
      all data that are not binaries. Opening the file in binary mode
      and writing binaries is therefore recommended. If the file is
      opened on another node, or if the file server runs as slave to
      the file server of another node, also binaries are copied.</p>
    <p>Caching data to reduce the number of file operations, or rather
      the number of calls to the file driver, generally increases
      performance. The following function writes 4 MBytes in 23
      seconds when tested:</p>
    <code type="none"><![CDATA[
create_file_slow(Name, N) when integer(N), N >= 0 ->
    {ok, FD} = file:open(Name, [raw, write, delayed_write, binary]),
    ok = create_file_slow(FD, 0, N),
    ok = ?FILE_MODULE:close(FD),
    ok.
      
create_file_slow(FD, M, M) ->
    ok;
create_file_slow(FD, M, N) ->
    ok = file:write(FD, <<M:32/unsigned>>),
    create_file_slow(FD, M+1, N).]]></code>

    <p>The following, functionally equivalent, function collects 1024
      entries into a list of 128 32-byte binaries before each call to
      <seealso marker="#write/2"><c>write/2</c></seealso> and so
      does the same work in 0.52 seconds,
      which is 44 times faster:</p>
    <code type="none"><![CDATA[
create_file(Name, N) when integer(N), N >= 0 ->
    {ok, FD} = file:open(Name, [raw, write, delayed_write, binary]),
    ok = create_file(FD, 0, N),
    ok = ?FILE_MODULE:close(FD),
    ok.
      
create_file(FD, M, M) ->
    ok;
create_file(FD, M, N) when M + 1024 =&lt; N ->
    create_file(FD, M, M + 1024, []),
    create_file(FD, M + 1024, N);
create_file(FD, M, N) ->
    create_file(FD, M, N, []).
      
create_file(FD, M, M, R) ->
    ok = file:write(FD, R);
create_file(FD, M, N0, R) when M + 8 =&lt; N0 ->
    N1  = N0-1,  N2  = N0-2,  N3  = N0-3,  N4  = N0-4, 
    N5  = N0-5,  N6  = N0-6,  N7  = N0-7,  N8  = N0-8, 
    create_file(FD, M, N8, 
                [<<N8:32/unsigned,  N7:32/unsigned, 
                   N6:32/unsigned,  N5:32/unsigned, 
                   N4:32/unsigned,  N3:32/unsigned, 
                   N2:32/unsigned,  N1:32/unsigned>> | R]);
create_file(FD, M, N0, R) ->
    N1 = N0-1,
    create_file(FD, M, N1, [<<N1:32/unsigned>> | R]).]]></code>

    <note>
      <p>Trust only your own benchmarks. If the list length in 
        <c>create_file/2</c> above is increased, it runs slightly
        faster, but consumes more memory and causes more memory
        fragmentation. How much this affects your application is
        something that this simple benchmark cannot predict.</p>
      <p>If the size of each binary is increased to 64 bytes, it
        also runs slightly faster, but the code is then twice as clumsy.
        In the current implementation, binaries larger than 64 bytes are
        stored in memory common to all processes and not copied when
        sent between processes, while these smaller binaries are stored
        on the process heap and copied when sent like any other term.</p>
      <p>So, with a binary size of 68 bytes, <c>create_file/2</c> runs
        30 percent slower than with 64 bytes, and causes much more
        memory fragmentation. Notice that if the binaries were to be sent
        between processes (for example, a non-raw file), the results
        would probably be completely different.</p>
    </note>
    <p>A raw file is really a port. When writing data to a port, it is
      efficient to write a list of binaries. It is not needed to
      flatten a deep list before writing. On Unix hosts, scatter output,
      which writes a set of buffers in one operation, is used when
      possible. In this way <c>write(FD, [Bin1, Bin2 | Bin3])</c>
      writes the contents of the binaries without copying the data
      at all, except for perhaps deep down in the operating system
      kernel.</p>
    <p>For raw files, <c>pwrite/2</c> and <c>pread/2</c> are
      efficiently implemented. The file driver is called only once for
      the whole operation, and the list iteration is done in the file
      driver.</p>
    <p>The options <c>delayed_write</c> and <c>read_ahead</c> to 
      <seealso marker="#open/2"><c>open/2</c></seealso>
      make the file driver cache data to reduce
      the number of operating system calls. The function
      <c>create_file/2</c> in the recent example takes 60 seconds
      without option <c>delayed_write</c>, which is 2.6
      times slower.</p>
    <p>As a bad example, <c>create_file_slow/2</c>
      without options <c>raw</c>, <c>binary</c>, and <c>delayed_write</c>,
      meaning it calls <c>open(Name, [write])</c>, needs
      1 min 20 seconds for the job, which is 3.5 times slower than
      the first example, and 150 times slower than the optimized
      <c>create_file/2</c>.</p>
      <warning>
	<p>If an error occurs when accessing an open file with module
      <seealso marker="stdlib:io"><c>io</c></seealso>,
      the process handling the file exits. The dead
      file process can hang if a process tries to access it later.
      This will be fixed in a future release.</p>
      </warning>
  </section>

  <section>
    <title>See Also</title>
    <p><seealso marker="stdlib:filename"><c>filename(3)</c></seealso></p>
  </section>
</erlref>