<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">
<erlref>
<header>
<copyright>
<year>1996</year><year>2010</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights 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>The module <c>file</c> provides an interface to the file system.</p>
<p>On operating systems with thread support, it is possible to let
file operations be performed in threads of their own, allowing
other Erlang processes to continue executing in parallel with
the file operations. See the command line flag
<c>+A</c> in <seealso marker="erts:erl">erl(1)</seealso>.</p>
<p>The Erlang VM supports file names in Unicode to a limited
extent. Depending on how the VM is started (with the parameter
<c>+fnu</c> or <c>+fnl</c>), file names given can contain
characters > 255 and the VM system will convert file names
back and forth to the native file name encoding.</p>
<p>The default behavior for Unicode character translation depends
on to what extent the underlying OS/filesystem enforces consistent
naming. On OSes where all file names are ensured to be in one or
another encoding, Unicode is the default (currently this holds for
Windows and MacOSX). On OSes with completely transparent file
naming (i.e. all Unixes except MacOSX), ISO-latin-1 file naming is
the default. The reason for the ISO-latin-1 default is that
file names are not guaranteed to be possible to interpret according to
the Unicode encoding expected (i.e. UTF-8), and file names that
cannot be decoded will only be accessible by using "raw
file names", in other word file names given as binaries.</p>
<p>As file names are traditionally not binaries in Erlang,
applications that need to handle raw file names need to be
converted, why the Unicode mode for file names is not default on
systems having completely transparent file naming.</p>
<note>As of R14B01, the most basic file handling modules
(<c>file</c>, <c>prim_file</c>, <c>filelib</c> and
<c>filename</c>) accept raw file names, but the rest of OTP is not
guaranteed to handle them, why Unicode file naming on systems
where it is not default is still considered experimental.</note>
<p>Raw file names is a new feature in OTP R14B01, which allows the
user to supply completely uninterpreted file names to the
underlying OS/filesystem. They are supplied as binaries, where it
is up to the user to supply a correct encoding for the
environment. The function <c>file:native_name_encoding()</c> can
be used to check what encoding the VM is working in. If the
function returns <c>latin1</c> file names are not in any way
converted to Unicode, if it is <c>utf8</c>, raw file names should
be encoded as UTF-8 if they are to follow the convention of the VM
(and usually the convention of the OS as well). Using raw
file names is useful if you have a filesystem with inconsistent
file naming, where some files are named in UTF-8 encoding while
others are not. A file:list_dir on such mixed file name systems
when the VM is in Unicode file name mode might return file names as
raw binaries as they cannot be interpreted as Unicode
file names. Raw file names can also be used to give UTF-8 encoded
file names even though the VM is not started in Unicode file name
translation mode.</p>
<p>Note that on Windows, <c>file:native_name_encoding()</c>
returns <c>utf8</c> per default, which is the format for raw
file names even on Windows, although the underlying OS specific
code works in a limited version of little endian UTF16. As far as
the Erlang programmer is concerned, Windows native Unicode format
is UTF-8...</p>
</description>
<section>
<title>DATA TYPES</title>
<code type="none">
iodata() = iolist() | binary()
iolist() = [char() | binary() | iolist()]
io_device()
as returned by file:open/2, a process handling IO protocols
name() = string() | atom() | DeepList | RawFilename
DeepList = [char() | atom() | DeepList]
RawFilename = binary()
If VM is in unicode filename mode, string() and char() are allowed to be > 255.
RawFilename is a filename not subject to Unicode translation, meaning that it
can contain characters not conforming to the Unicode encoding expected from the
filesystem (i.e. non-UTF-8 characters although the VM is started in Unicode
filename mode).
posix()
an atom which is named from the POSIX error codes used in
Unix, and in the runtime libraries of most C compilers
ext_posix() = posix() | badarg
time() = {{Year, Month, Day}, {Hour, Minute, Second}}
Year = Month = Day = Hour = Minute = Second = int()
Must denote a valid date and time</code>
</section>
<funcs>
<func>
<name>advise(IoDevice, Offset, Length, Advise) -> ok | {error, Reason}</name>
<fsummary>Predeclare an access pattern for file data</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Offset = int()</v>
<v>Length = int()</v>
<v>Advise = posix_file_advise()</v>
<v>posix_file_advise() = normal | sequential | random | no_reuse
| will_need | dont_need</v>
<v>Reason = ext_posix()</v>
</type>
<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>change_group(Filename, Gid) -> ok | {error, Reason}</name>
<fsummary>Change group of a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Gid = int()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Changes group of a file. See
<seealso marker="#write_file_info/2">write_file_info/2</seealso>.</p>
</desc>
</func>
<func>
<name>change_mode(Filename, Mode) -> ok | {error, Reason}</name>
<fsummary>Change permissions of a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Mode = int()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Changes permissions of a file. See
<seealso marker="#write_file_info/2">write_file_info/2</seealso>.</p>
</desc>
</func>
<func>
<name>change_owner(Filename, Uid) -> ok | {error, Reason}</name>
<fsummary>Change owner of a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Uid = int()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Changes owner of a file. See
<seealso marker="#write_file_info/2">write_file_info/2</seealso>.</p>
</desc>
</func>
<func>
<name>change_owner(Filename, Uid, Gid) -> ok | {error, Reason}</name>
<fsummary>Change owner and group of a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Uid = int()</v>
<v>Gid = int()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Changes owner and group of a file. See
<seealso marker="#write_file_info/2">write_file_info/2</seealso>.</p>
</desc>
</func>
<func>
<name>change_time(Filename, Mtime) -> ok | {error, Reason}</name>
<fsummary>Change the modification time of a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Mtime = time()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Changes the modification and access times of a file. See
<seealso marker="#write_file_info/2">write_file_info/2</seealso>.</p>
</desc>
</func>
<func>
<name>change_time(Filename, Mtime, Atime) -> ok | {error, Reason}</name>
<fsummary>Change the modification and last access time of a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Mtime = Atime = time()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Changes the modification and last access times of a file. See
<seealso marker="#write_file_info/2">write_file_info/2</seealso>.</p>
</desc>
</func>
<func>
<name>close(IoDevice) -> ok | {error, Reason}</name>
<fsummary>Close a file</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Closes the file referenced by <c>IoDevice</c>. It mostly
returns <c>ok</c>, expect for some severe errors such as out
of memory.</p>
<p>Note that if the option <c>delayed_write</c> was
used when opening the file, <c>close/1</c> might return an
old write error and not even try to close the file. See
<seealso marker="#open/2">open/2</seealso>.</p>
</desc>
</func>
<func>
<name>consult(Filename) -> {ok, Terms} | {error, Reason}</name>
<fsummary>Read Erlang terms from a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Terms = [term()]</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see below</v>
</type>
<desc>
<p>Reads Erlang terms, separated by '.', from <c>Filename</c>.
Returns one of the following:</p>
<taglist>
<tag><c>{ok, Terms}</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.
See <seealso marker="#open/2">open/2</seealso> for a list
of typical error codes.</p>
</item>
<tag><c>{error, {Line, Mod, Term}}</c></tag>
<item>
<p>An error occurred when interpreting the Erlang terms in
the file. Use <c>format_error/1</c> to convert
the three-element tuple to an English description of
the error.</p>
</item>
</taglist>
<p>Example:</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>
</desc>
</func>
<func>
<name>copy(Source, Destination) -></name>
<name>copy(Source, Destination, ByteCount) -> {ok, BytesCopied} | {error, Reason}</name>
<fsummary>Copy file contents</fsummary>
<type>
<v>Source = Destination = io_device() | Filename | {Filename, Modes}</v>
<v> Filename = name()</v>
<v> Modes = [Mode] -- see open/2</v>
<v>ByteCount = int() >= 0 | infinity</v>
<v>BytesCopied = int()</v>
</type>
<desc>
<p>Copies <c>ByteCount</c> bytes from <c>Source</c> to
<c>Destination</c>. <c>Source</c> and <c>Destination</c> refer
to either filenames or IO devices from e.g. <c>open/2</c>.
<c>ByteCount</c> defaults <c>infinity</c>, denoting an
infinite number of bytes.</p>
<p>The argument <c>Modes</c> is a list of possible modes, see
<seealso marker="#open/2">open/2</seealso>, and defaults to
[].</p>
<p>If both <c>Source</c> and <c>Destination</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>Source</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>Destination</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, BytesCopied}</c> where <c>BytesCopied</c> is
the number of bytes that actually was copied, which may be
less than <c>ByteCount</c> if end of file was encountered on
the source. If the operation fails, <c>{error, Reason}</c> is
returned.</p>
<p>Typical error reasons: As for <c>open/2</c> if a file had to
be opened, and as for <c>read/2</c> and <c>write/2</c>.</p>
</desc>
</func>
<func>
<name>del_dir(Dir) -> ok | {error, Reason}</name>
<fsummary>Delete a directory</fsummary>
<type>
<v>Dir = name()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Tries to delete the directory <c>Dir</c>. The directory must
be empty before it can be deleted. Returns <c>ok</c> if
successful.</p>
<p>Typical error reasons are:</p>
<taglist>
<tag><c>eacces</c></tag>
<item>
<p>Missing search or write permissions for the parent
directories of <c>Dir</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>Dir</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>delete(Filename) -> ok | {error, Reason}</name>
<fsummary>Delete a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Tries to delete the file <c>Filename</c>. Returns <c>ok</c>
if successful.</p>
<p>Typical error reasons are:</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 super-user.</p>
</item>
<tag><c>enotdir</c></tag>
<item>
<p>A component of the file name is not a directory. On some
platforms, <c>enoent</c> is returned instead.</p>
</item>
<tag><c>einval</c></tag>
<item>
<p><c>Filename</c> had an improper type, such as tuple.</p>
</item>
</taglist>
<warning>
<p>In a future release, a bad type for the <c>Filename</c>
argument will probably generate an exception.</p>
<p></p>
</warning>
</desc>
</func>
<func>
<name>eval(Filename) -> ok | {error, Reason}</name>
<fsummary>Evaluate Erlang expressions in a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see below</v>
</type>
<desc>
<p>Reads and evaluates Erlang expressions, separated by '.' (or
',', a sequence of expressions is also an expression), from
<c>Filename</c>. The actual 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.
See <c>open/2</c> for a list of typical error codes.</p>
</item>
<tag><c>{error, {Line, Mod, Term}}</c></tag>
<item>
<p>An error occurred when interpreting the Erlang
expressions in the file. Use <c>format_error/1</c> to
convert the three-element tuple to an English description
of the error.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>eval(Filename, Bindings) -> ok | {error, Reason}</name>
<fsummary>Evaluate Erlang expressions in a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Bindings -- see erl_eval(3)</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see eval/1</v>
</type>
<desc>
<p>The same as <c>eval/1</c> but the variable bindings
<c>Bindings</c> are used in the evaluation. See
<seealso marker="stdlib:erl_eval">erl_eval(3)</seealso> about
variable bindings.</p>
</desc>
</func>
<func>
<name>file_info(Filename) -> {ok, FileInfo} | {error, Reason}</name>
<fsummary>Get information about a file (deprecated)</fsummary>
<desc>
<p>This function is obsolete. Use <c>read_file_info/1</c>
instead.</p>
</desc>
</func>
<func>
<name>format_error(Reason) -> Chars</name>
<fsummary>Return a descriptive string for an error reason</fsummary>
<type>
<v>Reason = atom() | {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see eval/1</v>
<v>Chars = [char() | Chars]</v>
</type>
<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>get_cwd() -> {ok, Dir} | {error, Reason}</name>
<fsummary>Get the current working directory</fsummary>
<type>
<v>Dir = string()</v>
<v>Reason = posix()</v>
</type>
<desc>
<p>Returns <c>{ok, Dir}</c>, where <c>Dir</c> is the current
working directory of the file server.</p>
<note>
<p>In rare circumstances, this function can fail on Unix.
It may happen if read permission does not exist for
the parent directories of the current directory.</p>
</note>
<p>Typical error reasons are:</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>get_cwd(Drive) -> {ok, Dir} | {error, Reason}</name>
<fsummary>Get the current working directory for the drive specified</fsummary>
<type>
<v>Drive = string() -- see below</v>
<v>Dir = string()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p><c>Drive</c> should be of the form "<c>Letter</c><c>:</c>",
for example "c:". Returns <c>{ok, Dir}</c> or
<c>{error, Reason}</c>, where <c>Dir</c> is the current
working directory of the drive specified.</p>
<p>This function returns <c>{error, enotsup}</c> on platforms
which have no concept of current drive (Unix, for example).</p>
<p>Typical error reasons are:</p>
<taglist>
<tag><c>enotsup</c></tag>
<item>
<p>The operating system have 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>Drive</c> is invalid.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>list_dir(Dir) -> {ok, Filenames} | {error, Reason}</name>
<fsummary>List files in a directory</fsummary>
<type>
<v>Dir = name()</v>
<v>Filenames = [Filename]</v>
<v> Filename = string()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Lists all the files in a directory. Returns
<c>{ok, Filenames}</c> if successful. Otherwise, it returns
<c>{error, Reason}</c>. <c>Filenames</c> is a list of
the names of all the files in the directory. The names are
not sorted.</p>
<p>Typical error reasons are:</p>
<taglist>
<tag><c>eacces</c></tag>
<item>
<p>Missing search or write permissions for <c>Dir</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>make_dir(Dir) -> ok | {error, Reason}</name>
<fsummary>Make a directory</fsummary>
<type>
<v>Dir = name()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Tries to create the directory <c>Dir</c>. Missing parent
directories are <em>not</em> created. Returns <c>ok</c> if
successful.</p>
<p>Typical error reasons are:</p>
<taglist>
<tag><c>eacces</c></tag>
<item>
<p>Missing search or write permissions for the parent
directories of <c>Dir</c>.</p>
</item>
<tag><c>eexist</c></tag>
<item>
<p>There is already a file or directory named <c>Dir</c>.</p>
</item>
<tag><c>enoent</c></tag>
<item>
<p>A component of <c>Dir</c> does not exist.</p>
</item>
<tag><c>enospc</c></tag>
<item>
<p>There is a no space left on the device.</p>
</item>
<tag><c>enotdir</c></tag>
<item>
<p>A component of <c>Dir</c> is not a directory. On some
platforms, <c>enoent</c> is returned instead.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>make_link(Existing, New) -> ok | {error, Reason}</name>
<fsummary>Make a hard link to a file</fsummary>
<type>
<v>Existing = New = name()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Makes a hard link from <c>Existing</c> to <c>New</c>, on
platforms that support links (Unix). This function returns
<c>ok</c> if the link was successfully created, or
<c>{error, Reason}</c>. On platforms that do not support
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>Existing</c> or <c>New</c>.</p>
</item>
<tag><c>eexist</c></tag>
<item>
<p><c>New</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>make_symlink(Name1, Name2) -> ok | {error, Reason}</name>
<fsummary>Make a symbolic link to a file or directory</fsummary>
<type>
<v>Name1 = Name2 = name()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>This function creates a symbolic link <c>Name2</c> to
the file or directory <c>Name1</c>, on platforms that support
symbolic links (most Unix systems). <c>Name1</c> need not
exist. This function returns <c>ok</c> if the link was
successfully created, or <c>{error, Reason}</c>. On platforms
that do not support 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>Name1</c> or <c>Name2</c>.</p>
</item>
<tag><c>eexist</c></tag>
<item>
<p><c>Name2</c> already exists.</p>
</item>
<tag><c>enotsup</c></tag>
<item>
<p>Symbolic links are not supported on this platform.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>native_name_encoding() -> latin1 | utf8</name>
<fsummary>Return the VM's configured filename encoding.</fsummary>
<desc>
<p>This function returns the configured default file name encoding to use for raw file names. Generally an application supplying file names raw (as binaries), should obey the character encoding returned by this function.</p>
<p>By default, the VM uses ISO-latin-1 file name encoding on filesystems and/or OSes that use completely transparent file naming. This includes all Unix versions except MacOSX, where the vfs layer enforces UTF-8 file naming. By giving the experimental option <c>+fnu</c> when starting Erlang, UTF-8 translation of file names can be turned on even for those systems. If Unicode file name translation is in effect, the system behaves as usual as long as file names conform to the encoding, but will return file names that are not properly encoded in UTF-8 as raw file names (i.e. binaries).</p>
<p>On Windows, this function also returns <c>utf8</c> by default. The OS uses a pure Unicode naming scheme and file names are always possible to interpret as valid Unicode. The fact that the underlying Windows OS actually encodes file names using little endian UTF-16 can be ignored by the Erlang programmer. Windows and MacOSX are the only operating systems where the VM operates in Unicode file name mode by default.</p>
</desc>
</func>
<func>
<name>open(Filename, Modes) -> {ok, IoDevice} | {error, Reason}</name>
<fsummary>Open a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Modes = [Mode]</v>
<v> Mode = read | write | append | exclusive | raw | binary | {delayed_write, Size, Delay} | delayed_write | {read_ahead, Size} | read_ahead | compressed | {encoding, Encoding}</v>
<v> Size = Delay = int()</v>
<v> Encoding = latin1 | unicode | utf8 | utf16 | {utf16, Endian} | utf32 | {utf32, Endian}</v>
<v> Endian = big | little</v>
<v>IoDevice = io_device()</v>
<v>Reason = ext_posix() | system_limit</v>
</type>
<desc>
<p>Opens the file <c>Filename</c> in the mode determined by
<c>Modes</c>, which may contain one or more of the following
items:</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 if <c>write</c> is not
combined with <c>read</c>, the file will be truncated.</p>
</item>
<tag><c>append</c></tag>
<item>
<p>The file will be opened for writing, and it will be
created if it does not exist. Every write operation to a
file opened with <c>append</c> will take place at
the end of the file.</p>
</item>
<tag><c>exclusive</c></tag>
<item>
<p>The file, when opened for writing, is created if it
does not exist. If the file exists, open will return
<c>{error, eexist}</c>.</p>
<warning><p>This option does not guarantee exclusiveness on
file systems that do not support O_EXCL properly,
such as NFS. Do not depend on this option unless you
know that the file system supports it (in general, local
file systems should be safe).</p></warning>
</item>
<tag><c>raw</c></tag>
<item>
<p>The <c>raw</c> option allows faster access to a file,
because 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>The functions in the <c>io</c> module cannot be used,
because they can only talk to an Erlang process.
Instead, use the <c>read/2</c>, <c>read_line/1</c> and
<c>write/2</c>
functions.</item>
<item>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 the <c>{read_ahead, Size}</c> option as line oriented I/O is inefficient without buffering.</item>
<item>Only the Erlang process which opened the file can use
it.</item>
<item>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).</item>
</list>
</item>
<tag><c>binary</c></tag>
<item>
<p>When this option has been given, read operations on the file
will return binaries rather than lists.</p>
</item>
<tag><c>{delayed_write, Size, Delay}</c></tag>
<item>
<p>If this option is used, the data in subsequent
<c>write/2</c> calls is buffered until there are at least
<c>Size</c> bytes 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, so the
<c>write/2</c> calls should be for sizes significantly
less than <c>Size</c>, and not interspersed by to many
other file operations, for this to happen.</p>
<p>When this option is used, the result of <c>write/2</c>
calls may prematurely be reported as successful, and if
a write error should actually occur 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> might
return <c>{error, enospc}</c> because there was not enough
space on the disc for previously written data, and
<c>close/1</c> should probably be called again since 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 KBytes, 2 seconds)</p>
</item>
<tag><c>{read_ahead, Size}</c></tag>
<item>
<p>This option activates read data buffering. If
<c>read/2</c> calls are for significantly less than
<c>Size</c> bytes, read operations towards 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
since the number of operating system calls is reduced.</p>
<p>The <c>read_ahead</c> buffer is also highly utilized
by the <c>read_line/1</c> function in <c>raw</c> mode,
why 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 KBytes)</p>
</item>
<tag><c>compressed</c></tag>
<item>
<p>Makes it possible to read or write gzip compressed
files. The <c>compressed</c> option must be combined
with either <c>read</c> or <c>write</c>, but not both.
Note that the file size obtained with
<c>read_file_info/1</c> will most 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. Note that the data supplied to file:write or returned by file:read still is byte oriented, this option only denotes how data is actually 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 (the file) module for reading and writing data, as the interfaces provided here work with byte-oriented data, while using other (Unicode) encodings makes the <seealso marker="stdlib:io">io(3)</seealso> module's <c>get_chars</c>, <c>get_line</c> and <c>put_chars</c> functions 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 will be closed.</p>
<p>The allowed values for <c>Encoding</c> are:</p>
<taglist>
<tag><c>latin1</c></tag>
<item>
<p>The default encoding. Bytes supplied to i.e. file:write are written as is on the file, likewise bytes read from the file are returned to i.e. file:read as is. If the <seealso marker="stdlib:io">io(3)</seealso> module is used for writing, the file can only cope with Unicode characters up to codepoint 255 (the ISO-latin-1 range).</p>
</item>
<tag><c>unicode</c> or <c>utf8</c></tag>
<item>
<p>Characters are translated to and from the UTF-8 encoding before being written to or read from the file. A file opened in this way might be readable using the file:read function, as long as no data stored on the file lies beyond the ISO-latin-1 range (0..255), but failure will occur if the data contains Unicode codepoints beyond that range. The file is best read with the functions in the Unicode aware <seealso marker="stdlib:io">io(3)</seealso> module.</p>
<p>Bytes written to the file by any means are translated to UTF-8 encoding before actually being stored on the disk file.</p>
</item>
<tag><c>utf16</c> or <c>{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</c> or <c>{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 the <seealso marker="stdlib:io#setopts/2">io:setopts/2</seealso> function, why a file can be analyzed in latin1 encoding for i.e. a BOM, positioned beyond the BOM and then be set for the right encoding before further reading.See the <seealso marker="stdlib:unicode">unicode(3)</seealso> module for functions identifying BOM's.</p>
<p>This option is not allowed on <c>raw</c> files.</p>
</item>
</taglist>
<p>Returns:</p>
<taglist>
<tag><c>{ok, IoDevice}</c></tag>
<item>
<p>The file has been opened in the requested mode.
<c>IoDevice</c> is a reference to the file.</p>
</item>
<tag><c>{error, Reason}</c></tag>
<item>
<p>The file could not be opened.</p>
</item>
</taglist>
<p><c>IoDevice</c> is really the pid of the process which
handles the file. This process is linked to the process
which originally opened the file. If any process to which
the <c>IoDevice</c> is linked terminates, the file will be
closed and the process itself will be terminated.
An <c>IoDevice</c> returned from this call can be used as an
argument to the IO functions (see
<seealso marker="stdlib:io">io(3)</seealso>).</p>
<note>
<p>In previous versions of <c>file</c>, modes were given
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 should not 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 may be a
directory, a fifo, or a device.</p>
</item>
<tag><c>enotdir</c></tag>
<item>
<p>A component of the file name is not a directory. On some
platforms, <c>enoent</c> is returned instead.</p>
</item>
<tag><c>enospc</c></tag>
<item>
<p>There is a no space left on the device (if <c>write</c>
access was specified).</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>path_consult(Path, Filename) -> {ok, Terms, FullName} | {error, Reason}</name>
<fsummary>Read Erlang terms from a file</fsummary>
<type>
<v>Path = [Dir]</v>
<v> Dir = name()</v>
<v>Filename = name()</v>
<v>Terms = [term()]</v>
<v>FullName = string()</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see below</v>
</type>
<desc>
<p>Searches the path <c>Path</c> (a list of directory names)
until the file <c>Filename</c> is found. If <c>Filename</c>
is an absolute filename, <c>Path</c> is ignored.
Then reads Erlang terms, separated by '.', from the file.
Returns one of the following:</p>
<taglist>
<tag><c>{ok, Terms, FullName}</c></tag>
<item>
<p>The file was successfully read. <c>FullName</c> is
the full name of the file.</p>
</item>
<tag><c>{error, enoent}</c></tag>
<item>
<p>The file could not be found in any of the directories in
<c>Path</c>.</p>
</item>
<tag><c>{error, atom()}</c></tag>
<item>
<p>An error occurred when opening the file or reading it.
See <seealso marker="#open/2">open/2</seealso> for a list
of typical error codes.</p>
</item>
<tag><c>{error, {Line, Mod, Term}}</c></tag>
<item>
<p>An error occurred when interpreting the Erlang terms in
the file. Use <c>format_error/1</c> to convert
the three-element tuple to an English description of
the error.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>path_eval(Path, Filename) -> {ok, FullName} | {error, Reason}</name>
<fsummary>Evaluate Erlang expressions in a file</fsummary>
<type>
<v>Path = [Dir]</v>
<v> Dir = name()</v>
<v>Filename = name()</v>
<v>FullName = string()</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see below</v>
</type>
<desc>
<p>Searches the path <c>Path</c> (a list of directory names)
until the file <c>Filename</c> is found. If <c>Filename</c>
is an absolute file name, <c>Path</c> is ignored. Then reads
and evaluates Erlang expressions, separated by '.' (or ',', a
sequence of expressions is also an expression), from the file.
The actual result of 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, FullName}</c></tag>
<item>
<p>The file was read and evaluated. <c>FullName</c> is
the full name of the file.</p>
</item>
<tag><c>{error, enoent}</c></tag>
<item>
<p>The file could not be found in any of the directories in
<c>Path</c>.</p>
</item>
<tag><c>{error, atom()}</c></tag>
<item>
<p>An error occurred when opening the file or reading it.
See <seealso marker="#open/2">open/2</seealso> for a list
of typical error codes.</p>
</item>
<tag><c>{error, {Line, Mod, Term}}</c></tag>
<item>
<p>An error occurred when interpreting the Erlang
expressions in the file. Use <c>format_error/1</c> to
convert the three-element tuple to an English description
of the error.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>path_open(Path, Filename, Modes) -> {ok, IoDevice, FullName} | {error, Reason}</name>
<fsummary>Open a file</fsummary>
<type>
<v>Path = [Dir]</v>
<v> Dir = name()</v>
<v>Filename = name()</v>
<v>Modes = [Mode] -- see open/2</v>
<v>IoDevice = io_device()</v>
<v>FullName = string()</v>
<v>Reason = ext_posix() | system_limit</v>
</type>
<desc>
<p>Searches the path <c>Path</c> (a list of directory names)
until the file <c>Filename</c> is found. If <c>Filename</c>
is an absolute file name, <c>Path</c> is ignored.
Then opens the file in the mode determined by <c>Modes</c>.
Returns one of the following:</p>
<taglist>
<tag><c>{ok, IoDevice, FullName}</c></tag>
<item>
<p>The file has been opened in the requested mode.
<c>IoDevice</c> is a reference to the file and
<c>FullName</c> is the full name of the file.</p>
</item>
<tag><c>{error, enoent}</c></tag>
<item>
<p>The file could not be found in any of the directories in
<c>Path</c>.</p>
</item>
<tag><c>{error, atom()}</c></tag>
<item>
<p>The file could not be opened.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>path_script(Path, Filename) -> {ok, Value, FullName} | {error, Reason}</name>
<fsummary>Evaluate and return the value of Erlang expressions in a file</fsummary>
<type>
<v>Path = [Dir]</v>
<v> Dir = name()</v>
<v>Filename = name()</v>
<v>Value = term()</v>
<v>FullName = string()</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see below</v>
</type>
<desc>
<p>Searches the path <c>Path</c> (a list of directory names)
until the file <c>Filename</c> is found. If <c>Filename</c>
is an absolute file name, <c>Path</c> is ignored. Then reads
and evaluates Erlang expressions, separated by '.' (or ',', a
sequence of expressions is also an expression), from the file.
Returns one of the following:</p>
<taglist>
<tag><c>{ok, Value, FullName}</c></tag>
<item>
<p>The file was read and evaluated. <c>FullName</c> is
the full name of the file and <c>Value</c> the value of
the last expression.</p>
</item>
<tag><c>{error, enoent}</c></tag>
<item>
<p>The file could not be found in any of the directories in
<c>Path</c>.</p>
</item>
<tag><c>{error, atom()}</c></tag>
<item>
<p>An error occurred when opening the file or reading it.
See <seealso marker="#open/2">open/2</seealso> for a list
of typical error codes.</p>
</item>
<tag><c>{error, {Line, Mod, Term}}</c></tag>
<item>
<p>An error occurred when interpreting the Erlang
expressions in the file. Use <c>format_error/1</c> to
convert the three-element tuple to an English description
of the error.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>path_script(Path, Filename, Bindings) -> {ok, Value, FullName} | {error, Reason}</name>
<fsummary>Evaluate and return the value of Erlang expressions in a file</fsummary>
<type>
<v>Path = [Dir]</v>
<v> Dir = name()</v>
<v>Filename = name()</v>
<v>Bindings -- see erl_eval(3)</v>
<v>Value = term()</v>
<v>FullName = string()</v>
<v>Reason = posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see path_script/2</v>
</type>
<desc>
<p>The same as <c>path_script/2</c> but the variable bindings
<c>Bindings</c> are used in the evaluation. See
<seealso marker="stdlib:erl_eval">erl_eval(3)</seealso> about
variable bindings.</p>
</desc>
</func>
<func>
<name>pid2name(Pid) -> string() | undefined</name>
<fsummary>Return the name of the file handled by a pid</fsummary>
<type>
<v>Pid = pid()</v>
</type>
<desc>
<p>If <c>Pid</c> is an IO device, that is, a pid returned from
<c>open/2</c>, this function returns the filename, or rather:</p>
<taglist>
<tag><c>{ok, Filename}</c></tag>
<item>
<p>If this node's file server is not a slave, the file was
opened by this node's file server, (this implies that
<c>Pid</c> must be a local pid) and the file is not
closed. <c>Filename</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>position(IoDevice, Location) -> {ok, NewPosition} | {error, Reason}</name>
<fsummary>Set position in a file</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Location = Offset | {bof, Offset} | {cur, Offset} | {eof, Offset} | bof | cur | eof</v>
<v> Offset = int()</v>
<v>NewPosition = int()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Sets the position of the file referenced by <c>IoDevice</c>
to <c>Location</c>. Returns <c>{ok, NewPosition}</c> (as
absolute offset) if successful, otherwise
<c>{error, Reason}</c>. <c>Location</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>Note 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, i.e. 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>Typical error reasons are:</p>
<taglist>
<tag><c>einval</c></tag>
<item>
<p>Either <c>Location</c> was illegal, or it evaluated to a
negative offset in the file. Note 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>pread(IoDevice, LocNums) -> {ok, DataL} | eof | {error, Reason}</name>
<fsummary>Read from a file at certain positions</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>LocNums = [{Location, Number}]</v>
<v> Location -- see position/2</v>
<v> Number = int()</v>
<v>DataL = [Data]</v>
<v> Data = [char()] | binary()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<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, [Data, ...]}</c> or <c>{error, Reason}</c>,
where each <c>Data</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
was beyond end of file.</p>
<p>As the position is given as a byte-offset, special caution has to be taken when working with files where <c>encoding</c> is set to something else than <c>latin1</c>, as not every byte position will be a valid character boundary on such a file.</p>
</desc>
</func>
<func>
<name>pread(IoDevice, Location, Number) -> {ok, Data} | eof | {error, Reason}</name>
<fsummary>Read from a file at a certain position</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Location -- see position/2</v>
<v>Number = int()</v>
<v>Data = [char()] | binary()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<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>IoDevice</c> has been opened in raw mode, some
restrictions apply: <c>Location</c> is only allowed to be an
integer; and the current position of the file is undefined
after the operation.</p>
<p>As the position is given as a byte-offset, special caution has to be taken when working with files where <c>encoding</c> is set to something else than <c>latin1</c>, as not every byte position will be a valid character boundary on such a file.</p>
</desc>
</func>
<func>
<name>pwrite(IoDevice, LocBytes) -> ok | {error, {N, Reason}}</name>
<fsummary>Write to a file at certain positions</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>LocBytes = [{Location, Bytes}]</v>
<v> Location -- see position/2</v>
<v> Bytes = iodata()</v>
<v>N = int()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<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, {N, Reason}}</c>, where
<c>N</c> is the number of successful writes that was 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, see <seealso marker="#position/2">position/2</seealso> for details.</p>
</desc>
</func>
<func>
<name>pwrite(IoDevice, Location, Bytes) -> ok | {error, Reason}</name>
<fsummary>Write to a file at a certain position</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Location -- see position/2</v>
<v>Bytes = iodata()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<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>IoDevice</c> has been opened in raw mode, some
restrictions apply: <c>Location</c> is only allowed to be an
integer; and the current position of the file is undefined
after the operation.</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, see <seealso marker="#position/2">position/2</seealso> for details.</p>
</desc>
</func>
<func>
<name>read(IoDevice, Number) -> {ok, Data} | eof | {error, Reason}</name>
<fsummary>Read from a file</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Number = int()</v>
<v>Data = [char()] | binary()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Reads <c>Number</c> bytes/characters from the file referenced by
<c>IoDevice</c>. The functions <c>read/2</c>, <c>pread/3</c>
and <c>read_line/1</c> are the only ways to read from a file
opened in raw 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 might 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, why the position in the file might be moved a lot 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 will fail if the data contains characters larger than 255, why the <seealso marker="stdlib:io">io(3)</seealso> module is to be preferred when reading such a file.</p>
<p>The function returns:</p>
<taglist>
<tag><c>{ok, Data}</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 will be 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>Number>0</c> and end of file was reached
before anything at all could be read.</p>
</item>
<tag><c>{error, Reason}</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 was opened with another <c>encoding</c> than <c>latin1</c> and the data in the file can not be translated to the byte-oriented data that this function returns.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>read_file(Filename) -> {ok, Binary} | {error, Reason}</name>
<fsummary>Read a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Binary = binary()</v>
<v>Reason = ext_posix() | terminated | system_limit</v>
</type>
<desc>
<p>Returns <c>{ok, Binary}</c>, where <c>Binary</c> is a binary
data object that contains the contents of <c>Filename</c>, or
<c>{error, Reason}</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 file name 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>read_file_info(Filename) -> {ok, FileInfo} | {error, Reason}</name>
<fsummary>Get information about a file</fsummary>
<type>
<v>Filename = name()</v>
<v>FileInfo = #file_info{}</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Retrieves information about a file. Returns
<c>{ok, FileInfo}</c> if successful, otherwise
<c>{error, Reason}</c>. <c>FileInfo</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 record <c>file_info</c> contains the following fields.</p>
<taglist>
<tag><c>size = int()</c></tag>
<item>
<p>Size of file in bytes.</p>
</item>
<tag><c>type = device | directory | regular | other</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 = time()</c></tag>
<item>
<p>The last (local) time the file was read.</p>
</item>
<tag><c>mtime = time()</c></tag>
<item>
<p>The last (local) time the file was written.</p>
</item>
<tag><c>ctime = time()</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 inode was changed. In Windows, it is
the create time.</p>
</item>
<tag><c>mode = int()</c></tag>
<item>
<p>The file permissions as the sum of the following bit
values:</p>
<taglist>
<tag>8#00400</tag>
<item>read permission: owner</item>
<tag>8#00200</tag>
<item>write permission: owner</item>
<tag>8#00100</tag>
<item>execute permission: owner</item>
<tag>8#00040</tag>
<item>read permission: group</item>
<tag>8#00020</tag>
<item>write permission: group</item>
<tag>8#00010</tag>
<item>execute permission: group</item>
<tag>8#00004</tag>
<item>read permission: other</item>
<tag>8#00002</tag>
<item>write permission: other</item>
<tag>8#00001</tag>
<item>execute permission: other</item>
<tag>16#800</tag>
<item>set user id on execution</item>
<tag>16#400</tag>
<item>set group id on execution</item>
</taglist>
<p>On Unix platforms, other bits than those listed above
may be set.</p>
</item>
<tag><c>links = int()</c></tag>
<item>
<p>Number of links to the file (this will always be 1 for
file systems which have no concept of links).</p>
</item>
<tag><c>major_device = int()</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 = int()</c></tag>
<item>
<p>Only valid for character devices on Unix. In all other
cases, this field is zero.</p>
</item>
<tag><c>inode = int()</c></tag>
<item>
<p>Gives the <c>inode</c> number. On non-Unix file systems,
this field will be zero.</p>
</item>
<tag><c>uid = int()</c></tag>
<item>
<p>Indicates the owner of the file. Will be zero for
non-Unix file systems.</p>
</item>
<tag><c>gid = int()</c></tag>
<item>
<p>Gives the group that the owner of the file belongs to.
Will be zero 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 file name is not a directory. On some
platforms, <c>enoent</c> is returned instead.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>read_line(IoDevice) -> {ok, Data} | eof | {error, Reason}</name>
<fsummary>Read a line from a file</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Data = [char()] | binary()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Reads a line of bytes/characters from the file referenced by
<c>IoDevice</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 a LF. This behaviour is consistent with the behaviour of <seealso marker="stdlib:io#get_line/2">io:get_line/2</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. It is however inefficient to use it on <c>raw</c> files if the file is not opened with the option <c>{read_ahead, Size}</c> specified, why 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 will fail if the data contains characters larger than 255, why the <seealso marker="stdlib:io">io(3)</seealso> module is to be preferred when reading such a file.</p>
<p>The function returns:</p>
<taglist>
<tag><c>{ok, Data}</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 was 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, Reason}</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 was opened with another <c>encoding</c> than <c>latin1</c> and the data on the file can not be translated to the byte-oriented data that this function returns.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>read_link(Name) -> {ok, Filename} | {error, Reason}</name>
<fsummary>See what a link is pointing to</fsummary>
<type>
<v>Name = name()</v>
<v>Filename = string()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>This function returns <c>{ok, Filename}</c> if <c>Name</c>
refers to a symbolic link or <c>{error, Reason}</c> otherwise.
On platforms that do not support symbolic links, the return
value will be <c>{error,enotsup}</c>.</p>
<p>Typical error reasons:</p>
<taglist>
<tag><c>einval</c></tag>
<item>
<p><c>Linkname</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>read_link_info(Name) -> {ok, FileInfo} | {error, Reason}</name>
<fsummary>Get information about a link or file</fsummary>
<type>
<v>Name = name()</v>
<v>FileInfo = #file_info{}, see read_file_info/1</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>This function works like <c>read_file_info/1</c>, except that
if <c>Name</c> is a symbolic link, information about the link
will be returned in the <c>file_info</c> record and
the <c>type</c> field of the record will be set to
<c>symlink</c>.</p>
<p>If <c>Name</c> is not a symbolic link, this function returns
exactly 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>rename(Source, Destination) -> ok | {error, Reason}</name>
<fsummary>Rename a file</fsummary>
<type>
<v>Source = Destination = name()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Tries to rename the file <c>Source</c> to <c>Destination</c>.
It can be used to move files (and directories) between
directories, but it is not sufficient to specify
the destination only. The destination file name 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>Source</c> or <c>Destination</c>. On
some platforms, this error is given if either
<c>Source</c> or <c>Destination</c> is open.</p>
</item>
<tag><c>eexist</c></tag>
<item>
<p><c>Destination</c> is not an empty directory. On some
platforms, also given when <c>Source</c> and
<c>Destination</c> are not of the same type.</p>
</item>
<tag><c>einval</c></tag>
<item>
<p><c>Source</c> is a root directory, or <c>Destination</c>
is a sub-directory of <c>Source</c>.</p>
</item>
<tag><c>eisdir</c></tag>
<item>
<p><c>Destination</c> is a directory, but <c>Source</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>Source</c> is a directory, but <c>Destination</c> is
not.</p>
</item>
<tag><c>exdev</c></tag>
<item>
<p><c>Source</c> and <c>Destination</c> are on different
file systems.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>script(Filename) -> {ok, Value} | {error, Reason}</name>
<fsummary>Evaluate and return the value of Erlang expressions in a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Value = term()</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see below</v>
</type>
<desc>
<p>Reads and evaluates Erlang expressions, separated by '.' (or
',', a sequence of expressions is also an expression), from
the file. Returns one of the following:</p>
<taglist>
<tag><c>{ok, Value}</c></tag>
<item>
<p>The file was read and evaluated. <c>Value</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.
See <seealso marker="#open/2">open/2</seealso> for a list
of typical error codes.</p>
</item>
<tag><c>{error, {Line, Mod, Term}}</c></tag>
<item>
<p>An error occurred when interpreting the Erlang
expressions in the file. Use <c>format_error/1</c> to
convert the three-element tuple to an English description
of the error.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>script(Filename, Bindings) -> {ok, Value} | {error, Reason}</name>
<fsummary>Evaluate and return the value of Erlang expressions in a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Bindings -- see erl_eval(3)</v>
<v>Value = term()</v>
<v>Reason = ext_posix() | terminated | system_limit
| {Line, Mod, Term}</v>
<v> Line, Mod, Term -- see below</v>
</type>
<desc>
<p>The same as <c>script/1</c> but the variable bindings
<c>Bindings</c> are used in the evaluation. See
<seealso marker="stdlib:erl_eval">erl_eval(3)</seealso> about
variable bindings.</p>
</desc>
</func>
<func>
<name>set_cwd(Dir) -> ok | {error,Reason}</name>
<fsummary>Set the current working directory</fsummary>
<type>
<v>Dir = name()</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Sets the current working directory of the file server to
<c>Dir</c>. Returns <c>ok</c> if successful.</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>Dir</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>Filename</c> had an improper type, such as tuple.</p>
</item>
</taglist>
<warning>
<p>In a future release, a bad type for the <c>Filename</c>
argument will probably generate an exception.</p>
<p></p>
</warning>
</desc>
</func>
<func>
<name>sync(IoDevice) -> ok | {error, Reason}</name>
<fsummary>Synchronizes the in-memory state of a file with that on the physical medium</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Makes sure 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>Typical error reasons are:</p>
<taglist>
<tag><c>enospc</c></tag>
<item>
<p>Not enough space left to write the file.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>datasync(IoDevice) -> ok | {error, Reason}</name>
<fsummary>Synchronizes the in-memory data of a file, ignoring most of its metadata, with that on the physical medium</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Makes sure that any buffers kept by the operating system
(not by the Erlang runtime system) are written to disk. In
many ways it's resembles fsync but it not requires to update
some of file's metadata such as the access time. On
some platforms, this function might have no effect.</p>
<p>Applications that access databases or log files often write
a tiny data fragment (e.g., one line in a log file) and then
call fsync() immediately in order to ensure that the written
data is physically stored on the harddisk. Unfortunately, fsync()
will always initiate two write operations: one for the newly
written data and another one in order to update the modification
time stored in the inode. If the modification time is not a part
of the transaction concept fdatasync() can be used to avoid
unnecessary inode disk write operations.</p>
<p>Available only in some POSIX systems. This call results in a
call to fsync(), or has no effect, in systems not implementing
the fdatasync syscall.</p>
</desc>
</func>
<func>
<name>truncate(IoDevice) -> ok | {error, Reason}</name>
<fsummary>Truncate a file</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Truncates the file referenced by <c>IoDevice</c> at
the current position. Returns <c>ok</c> if successful,
otherwise <c>{error, Reason}</c>.</p>
</desc>
</func>
<func>
<name>write(IoDevice, Bytes) -> ok | {error, Reason}</name>
<fsummary>Write to a file</fsummary>
<type>
<v>IoDevice = io_device()</v>
<v>Bytes = iodata()</v>
<v>Reason = ext_posix() | terminated</v>
</type>
<desc>
<p>Writes <c>Bytes</c> to the file referenced by
<c>IoDevice</c>. This function is the only way to write to a
file opened in raw mode (although it works for normally
opened files, too). Returns <c>ok</c> if successful, and
<c>{error, Reason}</c> otherwise.</p>
<p>If the file is opened with <c>encoding</c> set to something else than <c>latin1</c>, each byte written might result in several bytes actually being written to the file, as the byte range 0..255 might represent anything between one and four bytes depending on value and UTF encoding type.</p>
<p>Typical error reasons are:</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>There is a no space left on the device.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>write_file(Filename, Bytes) -> ok | {error, Reason}</name>
<fsummary>Write a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Bytes = iodata()</v>
<v>Reason = ext_posix() | terminated | system_limit</v>
</type>
<desc>
<p>Writes the contents of the iodata term <c>Bytes</c> to the
file <c>Filename</c>. The file is created if it does not
exist. If it exists, the previous contents are
overwritten. Returns <c>ok</c>, or <c>{error, Reason}</c>.</p>
<p>Typical error reasons are:</p>
<taglist>
<tag><c>enoent</c></tag>
<item>
<p>A component of the file name does not exist.</p>
</item>
<tag><c>enotdir</c></tag>
<item>
<p>A component of the file name is not a directory. On some
platforms, <c>enoent</c> is returned instead.</p>
</item>
<tag><c>enospc</c></tag>
<item>
<p>There is a no space 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>write_file(Filename, Bytes, Modes) -> ok | {error, Reason}</name>
<fsummary>Write a file</fsummary>
<type>
<v>Filename = name()</v>
<v>Bytes = iodata()</v>
<v>Modes = [Mode] -- see open/2</v>
<v>Reason = ext_posix() | terminated | system_limit</v>
</type>
<desc>
<p>Same as <c>write_file/2</c>, but takes a third argument
<c>Modes</c>, a list of possible modes, see
<seealso marker="#open/2">open/2</seealso>. The mode flags
<c>binary</c> and <c>write</c> are implicit, so they should
not be used.</p>
</desc>
</func>
<func>
<name>write_file_info(Filename, FileInfo) -> ok | {error, Reason}</name>
<fsummary>Change information about a file</fsummary>
<type>
<v>Filename = name()</v>
<v>FileInfo = #file_info{} -- see also read_file_info/1</v>
<v>Reason = ext_posix()</v>
</type>
<desc>
<p>Change file information. Returns <c>ok</c> if successful,
otherwise <c>{error, Reason}</c>. <c>FileInfo</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 following fields are used from the record, if they are
given.</p>
<taglist>
<tag><c>atime = time()</c></tag>
<item>
<p>The last (local) time the file was read.</p>
</item>
<tag><c>mtime = time()</c></tag>
<item>
<p>The last (local) time the file was written.</p>
</item>
<tag><c>ctime = time()</c></tag>
<item>
<p>On Unix, any value give for this field will be ignored
(the "ctime" for the file will be set to the current
time). On Windows, this field is the new creation time to
set for the file.</p>
</item>
<tag><c>mode = int()</c></tag>
<item>
<p>The file permissions as the sum of the following bit
values:</p>
<taglist>
<tag>8#00400</tag>
<item>read permission: owner</item>
<tag>8#00200</tag>
<item>write permission: owner</item>
<tag>8#00100</tag>
<item>execute permission: owner</item>
<tag>8#00040</tag>
<item>read permission: group</item>
<tag>8#00020</tag>
<item>write permission: group</item>
<tag>8#00010</tag>
<item>execute permission: group</item>
<tag>8#00004</tag>
<item>read permission: other</item>
<tag>8#00002</tag>
<item>write permission: other</item>
<tag>8#00001</tag>
<item>execute permission: other</item>
<tag>16#800</tag>
<item>set user id on execution</item>
<tag>16#400</tag>
<item>set group id on execution</item>
</taglist>
<p>On Unix platforms, other bits than those listed above
may be set.</p>
</item>
<tag><c>uid = int()</c></tag>
<item>
<p>Indicates the owner of the file. Ignored for non-Unix
file systems.</p>
</item>
<tag><c>gid = int()</c></tag>
<item>
<p>Gives the group that the owner of the file belongs to.
Ignored 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 file name 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> - IO 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> - file name 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, may block their
calling thread for seconds. If this befalls 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 due to 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 several smaller, which
enables the emulator so server other processes during the file
operation. This gives 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
uses the file system of the node's host machine. 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 another node's,
and the other node runs on some other host machine, they may have
different file systems. This is seldom a problem, but you have
now been warned.</p>
<p>A normal file is really a process so it can be used as an IO
device (see <c>io</c>). 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
another node's, 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, will generally increase
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
<c>file:write/2</c> 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 =< 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 =< 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 will run slightly
faster, but consume more memory and cause more memory
fragmentation. How much this affects your application is
something that this simple benchmark can not predict.</p>
<p>If the size of each binary is increased to 64 bytes, it will
also run slightly faster, but the code will be twice as clumsy.
In the current implementation are binaries larger than 64 bytes
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 then with 64 bytes, and will cause much more
memory fragmentation. Note 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. There is no need 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>file:write(FD, [Bin1, Bin2 | Bin3])</c>
will write 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
<c>file:open/2</c> makes the file driver cache data to reduce
the number of operating system calls. The function
<c>create_file/2</c> in the example above takes 60 seconds
seconds without the <c>delayed_write</c> option, which is 2.6
times slower.</p>
<p>And, as a really bad example, <c>create_file_slow/2</c> above
without the <c>raw</c>, <c>binary</c> and <c>delayed_write</c>
options, that is it calls <c>file: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>
</section>
<section>
<title>Warnings</title>
<p>If an error occurs when accessing an open file with the <c>io</c>
module, the process which handles the file will exit. The dead
file process might hang if a process tries to access it later.
This will be fixed in a future release.</p>
</section>
<section>
<title>SEE ALSO</title>
<p><seealso marker="stdlib:filename">filename(3)</seealso></p>
</section>
</erlref>
