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=======================================
Lib - Kernel-2.13.3 - Doc - Html - File
=======================================

` <http://www.ericsson.com/technology/opensource/erlang>`__


file
====



MODULE
~~~~~~
file


MODULE SUMMARY
~~~~~~~~~~~~~~
File Interface Module


DESCRIPTION
~~~~~~~~~~~

The module file provides an interface to the file system.

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+A in `erl(1)
<javascript:erlhref('../../../../',%20'erts',%20'erl.html');>`__.



DATA TYPES
~~~~~~~~~~


.. sourcecode:: erlang

    
    iodata() = iolist() | binary()
      iolist() = [char() | binary() | iolist()]
    
    io_device()
      as returned by file:open/2, a process handling IO protocols
    
    name() = string() | atom() | DeepList
      DeepList = [char() | atom() | DeepList]
    
    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




EXPORTS
~~~~~~~

change_group(Filename, Gid) -> ok | {error, Reason}


Types:

Filename = name()
Gid = int()
Reason = ext_posix()


Changes group of a file. Seewrite_file_info/2.

change_owner(Filename, Uid) -> ok | {error, Reason}


Types:

Filename = name()
Uid = int()
Reason = ext_posix()


Changes owner of a file. Seewrite_file_info/2.

change_owner(Filename, Uid, Gid) -> ok | {error, Reason}


Types:

Filename = name()
Uid = int()
Gid = int()
Reason = ext_posix()


Changes owner and group of a file. Seewrite_file_info/2.

change_time(Filename, Mtime) -> ok | {error, Reason}


Types:

Filename = name()
Mtime = time()
Reason = ext_posix()


Changes the modification and access times of a file.
Seewrite_file_info/2.

change_time(Filename, Mtime, Atime) -> ok | {error, Reason}


Types:

Filename = name()
Mtime = Atime = time()
Reason = ext_posix()


Changes the modification and last access times of a file.
Seewrite_file_info/2.

close(IoDevice) -> ok | {error, Reason}


Types:

IoDevice = io_device()
Reason = ext_posix() | terminated


Closes the file referenced by IoDevice. It mostly returns ok, expect
for some severe errors such as out of memory.

Note that if the option delayed_write was used when opening the file,
close/1 might return an old write error and not even try to close the
file. Seeopen/2.

consult(Filename) -> {ok, Terms} | {error, Reason}


Types:

Filename = name()
Terms = [term()]
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see below


Reads Erlang terms, separated by '.', from Filename. Returns one of
the following:

:{ok, Terms}: The file was successfully read.
:{error, atom()}: An error occurred when opening the file or reading
  it. See open/2 for a list of typical error codes.
:{error, {Line, Mod, Term}}: An error occurred when interpreting the
  Erlang terms in the file. Use format_error/1 to convert the three-
  element tuple to an English description of the error.


Example:


.. sourcecode:: erlang

    
    f.txt:  {person, "kalle", 25}.
            {person, "pelle", 30}.



.. sourcecode:: erlang

    
    1> file:consult("f.txt").
    {ok,[{person,"kalle",25},{person,"pelle",30}]}


copy(Source, Destination) ->
copy(Source, Destination, ByteCount) -> {ok, BytesCopied} | {error,
Reason}


Types:

Source = Destination = io_device() | Filename | {Filename, Modes}
 Filename = name()
 Modes = [Mode] -- see open/2
ByteCount = int() >= 0 | infinity
BytesCopied = int()


Copies ByteCount bytes from Source toDestination. Source and
Destination refer to either filenames or IO devices from e.g.
open/2.ByteCount defaults infinity, denoting an infinite number of
bytes.

The argument Modes is a list of possible modes, seeopen/2, and
defaults to [].

If both Source and Destination refer to filenames, the files are
opened with [read, binary] and [write, binary] prepended to their mode
lists, respectively, to optimize the copy.

If Source refers to a filename, it is opened withread mode prepended
to the mode list before the copy, and closed when done.

If Destination refers to a filename, it is opened with write mode
prepended to the mode list before the copy, and closed when done.

Returns {ok, BytesCopied} where BytesCopied is the number of bytes
that actually was copied, which may be less than ByteCount if end of
file was encountered on the source. If the operation fails, {error,
Reason} is returned.

Typical error reasons: As for open/2 if a file had to be opened, and
as for read/2 and write/2.

del_dir(Dir) -> ok | {error, Reason}


Types:

Dir = name()
Reason = ext_posix()


Tries to delete the directory Dir. The directory must be empty before
it can be deleted. Returns ok if successful.

Typical error reasons are:

:eacces: Missing search or write permissions for the parent
  directories of Dir.
:eexist: The directory is not empty.
:enoent: The directory does not exist.
:enotdir: A component of Dir is not a directory. On some platforms,
  enoent is returned instead.
:einval: Attempt to delete the current directory. On some platforms,
  eacces is returned instead.


delete(Filename) -> ok | {error, Reason}


Types:

Filename = name()
Reason = ext_posix()


Tries to delete the file Filename. Returns ok if successful.

Typical error reasons are:

:enoent: The file does not exist.
:eacces: Missing permission for the file or one of its parents.
:eperm: The file is a directory and the user is not super-user.
:enotdir: A component of the file name is not a directory. On some
  platforms, enoent is returned instead.
:einval: Filename had an improper type, such as tuple.

Warning
In a future release, a bad type for the Filename argument will
probably generate an exception.



eval(Filename) -> ok | {error, Reason}


Types:

Filename = name()
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see below


Reads and evaluates Erlang expressions, separated by '.' (or ',', a
sequence of expressions is also an expression), fromFilename. 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:

:ok: The file was read and evaluated.
:{error, atom()}: An error occurred when opening the file or reading
  it. See open/2 for a list of typical error codes.
:{error, {Line, Mod, Term}}: An error occurred when interpreting the
  Erlang expressions in the file. Use format_error/1 to convert the
  three-element tuple to an English description of the error.


eval(Filename, Bindings) -> ok | {error, Reason}


Types:

Filename = name()
Bindings -- see erl_eval(3)
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see eval/1


The same as eval/1 but the variable bindingsBindings are used in the
evaluation. See `erl_eval(3) <javascript:erlhref('../../../../',%20'st
dlib',%20'erl_eval.html');>`__ about variable bindings.

file_info(Filename) -> {ok, FileInfo} | {error, Reason}


This function is obsolete. Use read_file_info/1 instead.

format_error(Reason) -> Chars


Types:

Reason = atom() | {Line, Mod, Term}
 Line, Mod, Term -- see eval/1
Chars = [char() | Chars]


Given the error reason returned by any function in this module,
returns a descriptive string of the error in English.

get_cwd() -> {ok, Dir} | {error, Reason}


Types:

Dir = string()
Reason = posix()


Returns {ok, Dir}, where Dir is the current working directory of the
file server.
Note
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.

Typical error reasons are:

:eacces: Missing read permission for one of the parents of the current
  directory.


get_cwd(Drive) -> {ok, Dir} | {error, Reason}


Types:

Drive = string() -- see below
Dir = string()
Reason = ext_posix()


Drive should be of the form "Letter:", for example "c:". Returns {ok,
Dir} or{error, Reason}, where Dir is the current working directory of
the drive specified.

This function returns {error, enotsup} on platforms which have no
concept of current drive (Unix, for example).

Typical error reasons are:

:enotsup: The operating system have no concept of drives.
:eacces: The drive does not exist.
:einval: The format of Drive is invalid.


list_dir(Dir) -> {ok, Filenames} | {error, Reason}


Types:

Dir = name()
Filenames = [Filename]
 Filename = string()
Reason = ext_posix()


Lists all the files in a directory. Returns{ok, Filenames} if
successful. Otherwise, it returns{error, Reason}. Filenames is a list
of the names of all the files in the directory. The names are not
sorted.

Typical error reasons are:

:eacces: Missing search or write permissions for Dir or one of its
  parent directories.
:enoent: The directory does not exist.


make_dir(Dir) -> ok | {error, Reason}


Types:

Dir = name()
Reason = ext_posix()


Tries to create the directory Dir. Missing parent directories are not
created. Returns ok if successful.

Typical error reasons are:

:eacces: Missing search or write permissions for the parent
  directories of Dir.
:eexist: There is already a file or directory named Dir.
:enoent: A component of Dir does not exist.
:enospc: There is a no space left on the device.
:enotdir: A component of Dir is not a directory. On some platforms,
  enoent is returned instead.


make_link(Existing, New) -> ok | {error, Reason}


Types:

Existing = New = name()
Reason = ext_posix()


Makes a hard link from Existing to New, on platforms that support
links (Unix). This function returnsok if the link was successfully
created, or{error, Reason}. On platforms that do not support links,
{error,enotsup} is returned.

Typical error reasons:

:eacces: Missing read or write permissions for the parent directories
  of Existing or New.
:eexist: New already exists.
:enotsup: Hard links are not supported on this platform.


make_symlink(Name1, Name2) -> ok | {error, Reason}


Types:

Name1 = Name2 = name()
Reason = ext_posix()


This function creates a symbolic link Name2 to the file or directory
Name1, on platforms that support symbolic links (most Unix systems).
Name1 need not exist. This function returns ok if the link was
successfully created, or {error, Reason}. On platforms that do not
support symbolic links, {error, enotsup} is returned.

Typical error reasons:

:eacces: Missing read or write permissions for the parent directories
  of Name1 or Name2.
:eexist: Name2 already exists.
:enotsup: Symbolic links are not supported on this platform.


open(Filename, Modes) -> {ok, IoDevice} | {error, Reason}


Types:

Filename = name()
Modes = [Mode]
 Mode = read | write | append | raw | binary | {delayed_write, Size,
Delay} | delayed_write | {read_ahead, Size} | read_ahead | compressed
  Size = Delay = int()
IoDevice = io_device()
Reason = ext_posix() | system_limit


Opens the file Filename in the mode determined byModes, which may
contain one or more of the following items:

:read: The file, which must exist, is opened for reading.
:write: The file is opened for writing. It is created if it does not
  exist. If the file exists, and if write is not combined with read, the
  file will be truncated.
:append: 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
  append will take place at the end of the file.
:raw: The raw 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:

    + The functions in the io module cannot be used, because they can only
      talk to an Erlang process. Instead, use the read/2, read_line/1 and
      write/2 functions.
    + Especially if read_line/1 is to be used on a raw file, it is
      recommended to combine this option with the {read_ahead, Size} option
      as line oriented I/O is inefficient without buffering.
    + Only the Erlang process which opened the file can use it.
    + 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).

:binary: When this option has been given, read operations on the file
  will return binaries rather than lists.
:{delayed_write, Size, Delay}: If this option is used, the data in
  subsequentwrite/2 calls is buffered until there are at leastSize bytes
  buffered, or until the oldest buffered data is Delay 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
  write/2 is executed. The purpose of this option is to increase
  performance by reducing the number of operating system calls, so
  thewrite/2 calls should be for sizes significantly less than Size, and
  not interspersed by to many other file operations, for this to happen.
  When this option is used, the result of write/2 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. For example, when delayed_write is used, after a
  number of write/2 calls, close/1 might return {error, enospc} because
  there was not enough space on the disc for previously written data,
  andclose/1 should probably be called again since the file is still
  open.
:delayed_write: The same as {delayed_write, Size, Delay} with
  reasonable default values for Size andDelay. (Roughly some 64 KBytes,
  2 seconds)
:{read_ahead, Size}: This option activates read data buffering.
  Ifread/2 calls are for significantly less thanSize bytes, read
  operations towards the operating system are still performed for blocks
  of Size bytes. The extra data is buffered and returned in subsequent
  read/2 calls, giving a performance gain since the number of operating
  system calls is reduced. The read_ahead buffer is also highly utilized
  by the read_line/1 function in raw mode, why this option is
  recommended (for performance reasons) when accessing raw files using
  that function. If read/2 calls are for sizes not significantly less
  than, or even greater than Size bytes, no performance gain can be
  expected.
:read_ahead: The same as {read_ahead, Size} with a reasonable default
  value for Size. (Roughly some 64 KBytes)
:compressed: Makes it possible to read or write gzip compressed files.
  The compressesed option must be combined with either read or write,
  but not both. Note that the file size obtained withread_file_info/1
  will most probably not match the number of bytes that can be read from
  a compressed file.
:{encoding, Encoding}: 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. Depending on the encoding, different methods of reading and
writing data is preferred. The default encoding of latin1 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 `io(3)
<javascript:erlhref('../../../../',%20'stdlib',%20'io.html');>`__
module's get_chars, get_line and put_chars functions more suitable, as
they can work with the full Unicode range. If data is sent to an
io_device() 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. The allowed values for Encoding
are:
    :latin1: 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 `io(3)
      <javascript:erlhref('../../../../',%20'stdlib',%20'io.html');>`__
      module is used for writing, the file can only cope with Unicode
      characters up to codepoint 255 (the ISO-latin-1 range).
    :unicode or utf8: 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 `io(3)
      <javascript:erlhref('../../../../',%20'stdlib',%20'io.html');>`__
      module. Bytes written to the file by any means are translated to UTF-8
      encoding before actually being stored on the disk file.
    :utf16 or {utf16,big}: Works like unicode, but translation is done to
      and from big endian UTF-16 instead of UTF-8.
    :{utf16,little}: Works like unicode, but translation is done to and
      from little endian UTF-16 instead of UTF-8.
    :utf32 or {utf32,big}: Works like unicode, but translation is done to
      and from big endian UTF-32 instead of UTF-8.
    :{utf32,little}: Works like unicode, but translation is done to and
      from little endian UTF-32 instead of UTF-8.

The Encoding can be changed for a file "on the fly" by using the
  `io:setopts/2 <javascript:erlhref('../../../../',%20'stdlib',%20'io.ht
  ml#setopts-2');>`__ 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 `unicode(3)
  <javascript:erlhref('../../../../',%20'stdlib',%20'unicode.html');>`__
  module for functions identifying BOM's. This option is not allowed on
  raw files.


Returns:

:{ok, IoDevice}: The file has been opened in the requested
  mode.IoDevice is a reference to the file.
:{error, Reason}: The file could not be opened.


IoDevice 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 IoDevice is linked terminates, the file will
be closed and the process itself will be terminated. An IoDevice
returned from this call can be used as an argument to the IO functions
(see `io(3)
<javascript:erlhref('../../../../',%20'stdlib',%20'io.html');>`__).
Note
In previous versions of file, modes were given as one of the atoms
read, write, orread_write instead of a list. This is still allowed for
reasons of backwards compatibility, but should not be used for new
code. Also note that read_write is not allowed in a mode list.

Typical error reasons:

:enoent: The file does not exist.
:eacces: Missing permission for reading the file or searching one of
  the parent directories.
:eisdir: The named file is not a regular file. It may be a directory,
  a fifo, or a device.
:enotdir: A component of the file name is not a directory. On some
  platforms, enoent is returned instead.
:enospc: There is a no space left on the device (if write access was
  specified).


path_consult(Path, Filename) -> {ok, Terms, FullName} | {error,
Reason}


Types:

Path = [Dir]
 Dir = name()
Filename = name()
Terms = [term()]
FullName = string()
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see below


Searches the path Path (a list of directory names) until the file
Filename is found. If Filename is an absolute filename, Path is
ignored. Then reads Erlang terms, separated by '.', from the file.
Returns one of the following:

:{ok, Terms, FullName}: The file was successfully read. FullName is
  the full name of the file.
:{error, enoent}: The file could not be found in any of the
  directories inPath.
:{error, atom()}: An error occurred when opening the file or reading
  it. See open/2 for a list of typical error codes.
:{error, {Line, Mod, Term}}: An error occurred when interpreting the
  Erlang terms in the file. Use format_error/1 to convert the three-
  element tuple to an English description of the error.


path_eval(Path, Filename) -> {ok, FullName} | {error, Reason}


Types:

Path = [Dir]
 Dir = name()
Filename = name()
FullName = string()
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see below


Searches the path Path (a list of directory names) until the file
Filename is found. If Filename is an absolute file name, Path 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:

:{ok, FullName}: The file was read and evaluated. FullName is the full
  name of the file.
:{error, enoent}: The file could not be found in any of the
  directories inPath.
:{error, atom()}: An error occurred when opening the file or reading
  it. See open/2 for a list of typical error codes.
:{error, {Line, Mod, Term}}: An error occurred when interpreting the
  Erlang expressions in the file. Use format_error/1 to convert the
  three-element tuple to an English description of the error.


path_open(Path, Filename, Modes) -> {ok, IoDevice, FullName} | {error,
Reason}


Types:

Path = [Dir]
 Dir = name()
Filename = name()
Modes = [Mode] -- see open/2
IoDevice = io_device()
FullName = string()
Reason = ext_posix() | system_limit


Searches the path Path (a list of directory names) until the file
Filename is found. If Filename is an absolute file name, Path is
ignored. Then opens the file in the mode determined by Modes. Returns
one of the following:

:{ok, IoDevice, FullName}: The file has been opened in the requested
  mode.IoDevice is a reference to the file andFullName is the full name
  of the file.
:{error, enoent}: The file could not be found in any of the
  directories inPath.
:{error, atom()}: The file could not be opened.


path_script(Path, Filename) -> {ok, Value, FullName} | {error, Reason}


Types:

Path = [Dir]
 Dir = name()
Filename = name()
Value = term()
FullName = string()
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see below


Searches the path Path (a list of directory names) until the file
Filename is found. If Filename is an absolute file name, Path 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:

:{ok, Value, FullName}: The file was read and evaluated. FullName is
  the full name of the file and Value the value of the last expression.
:{error, enoent}: The file could not be found in any of the
  directories inPath.
:{error, atom()}: An error occurred when opening the file or reading
  it. See open/2 for a list of typical error codes.
:{error, {Line, Mod, Term}}: An error occurred when interpreting the
  Erlang expressions in the file. Use format_error/1 to convert the
  three-element tuple to an English description of the error.


path_script(Path, Filename, Bindings) -> {ok, Value, FullName} |
{error, Reason}


Types:

Path = [Dir]
 Dir = name()
Filename = name()
Bindings -- see erl_eval(3)
Value = term()
FullName = string()
Reason = posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see path_script/2


The same as path_script/2 but the variable bindingsBindings are used
in the evaluation. See `erl_eval(3) <javascript:erlhref('../../../../'
,%20'stdlib',%20'erl_eval.html');>`__ about variable bindings.

pid2name(Pid) -> string() | undefined


Types:

Pid = pid()


If Pid is an IO device, that is, a pid returned fromopen/2, this
function returns the filename, or rather:

:{ok, Filename}: If this node's file server is not a slave, the file
  was opened by this node's file server, (this implies thatPid must be a
  local pid) and the file is not closed. Filename is the filename in
  flat string format.
:undefined: In all other cases.

Warning
This function is intended for debugging only.

position(IoDevice, Location) -> {ok, NewPosition} | {error, Reason}


Types:

IoDevice = io_device()
Location = Offset | {bof, Offset} | {cur, Offset} | {eof, Offset} |
bof | cur | eof
 Offset = int()
NewPosition = int()
Reason = ext_posix() | terminated


Sets the position of the file referenced by IoDevice to Location.
Returns {ok, NewPosition} (as absolute offset) if successful,
otherwise{error, Reason}. Location is one of the following:

:Offset: The same as {bof, Offset}.
:{bof, Offset}: Absolute offset.
:{cur, Offset}: Offset from the current position.
:{eof, Offset}: Offset from the end of file.
:bof | cur | eof: The same as above with Offset 0.


Note that offsets are counted in bytes, not in characters. If the file
is opened using some other encoding than latin1, 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 known 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).

Typical error reasons are:

:einval: Either Location 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.


pread(IoDevice, LocNums) -> {ok, DataL} | eof | {error, Reason}


Types:

IoDevice = io_device()
LocNums = [{Location, Number}]
 Location -- see position/2
 Number = int()
DataL = [Data]
 Data = [char()] | binary()
Reason = ext_posix() | terminated


Performs a sequence of pread/3 in one operation, which is more
efficient than calling them one at a time. Returns {ok, [Data, ...]}
or {error, Reason}, where each Data, the result of the
correspondingpread, is either a list or a binary depending on the mode
of the file, or eof if the requested position was beyond end of file.

As the position is given as a byte-offset, special caution has to be
taken when working with files where encoding is set to something else
than latin1, as not every byte position will be a valid character
boundary on such a file.

pread(IoDevice, Location, Number) -> {ok, Data} | eof | {error,
Reason}


Types:

IoDevice = io_device()
Location -- see position/2
Number = int()
Data = [char()] | binary()
Reason = ext_posix() | terminated


Combines position/2 and read/2 in one operation, which is more
efficient than calling them one at a time. If IoDevice has been opened
in raw mode, some restrictions apply: Location is only allowed to be
an integer; and the current position of the file is undefined after
the operation.

As the position is given as a byte-offset, special caution has to be
taken when working with files where encoding is set to something else
than latin1, as not every byte position will be a valid character
boundary on such a file.

pwrite(IoDevice, LocBytes) -> ok | {error, {N, Reason}}


Types:

IoDevice = io_device()
LocBytes = [{Location, Bytes}]
 Location -- see position/2
 Bytes = iodata()
N = int()
Reason = ext_posix() | terminated


Performs a sequence of pwrite/3 in one operation, which is more
efficient than calling them one at a time. Returns ok or {error, {N,
Reason}}, whereN is the number of successful writes that was done
before the failure.

When positioning in a file with other encoding than latin1, caution
must be taken to set the position on a correct character boundary, see
position/2 for details.

pwrite(IoDevice, Location, Bytes) -> ok | {error, Reason}


Types:

IoDevice = io_device()
Location -- see position/2
Bytes = iodata()
Reason = ext_posix() | terminated


Combines position/2 and write/2 in one operation, which is more
efficient than calling them one at a time. If IoDevice has been opened
in raw mode, some restrictions apply: Location is only allowed to be
an integer; and the current position of the file is undefined after
the operation.

When positioning in a file with other encoding than latin1, caution
must be taken to set the position on a correct character boundary, see
position/2 for details.

read(IoDevice, Number) -> {ok, Data} | eof | {error, Reason}


Types:

IoDevice = io_device()
Number = int()
Data = [char()] | binary()
Reason = ext_posix() | terminated


Reads Number bytes/characters from the file referenced byIoDevice. The
functions read/2, pread/3 and read_line/1 are the only ways to read
from a file opened in raw mode (although they work for normally opened
files, too).

For files where encoding is set to something else than latin1, one
character might be represented by more than one byte on the file. The
parameter Number always denotes the number of characters read from the
file, why the position in the file might be moved a lot more than this
number when reading a Unicode file.

Also if encoding is set to something else than latin1, the read/3 call
will fail if the data contains characters larger than 255, why the
`io(3)
<javascript:erlhref('../../../../',%20'stdlib',%20'io.html');>`__
module is to be preferred when reading such a file.

The function returns:

:{ok, Data}: 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.
:eof: Returned if Number>0 and end of file was reached before anything
  at all could be read.
:{error, Reason}: An error occurred.


Typical error reasons:

:ebadf: The file is not opened for reading.
:{no_translation, unicode, latin1}: The file is was opened with
  another encoding than latin1 and the data on the file can not be
  translated to the byte-oriented data that this function returns.


read_file(Filename) -> {ok, Binary} | {error, Reason}


Types:

Filename = name()
Binary = binary()
Reason = ext_posix() | terminated | system_limit


Returns {ok, Binary}, where Binary is a binary data object that
contains the contents of Filename, or{error, Reason} if an error
occurs.

Typical error reasons:

:enoent: The file does not exist.
:eacces: Missing permission for reading the file, or for searching one
  of the parent directories.
:eisdir: The named file is a directory.
:enotdir: A component of the file name is not a directory. On some
  platforms, enoent is returned instead.
:enomem: There is not enough memory for the contents of the file.


read_file_info(Filename) -> {ok, FileInfo} | {error, Reason}


Types:

Filename = name()
FileInfo = #file_info{}
Reason = ext_posix()


Retrieves information about a file. Returns{ok, FileInfo} if
successful, otherwise{error, Reason}. FileInfo is a recordfile_info,
defined in the Kernel include filefile.hrl. Include the following
directive in the module from which the function is called:


.. sourcecode:: erlang

    
    -include_lib("kernel/include/file.hrl").


The record file_info contains the following fields.

:size = int(): Size of file in bytes.
:type = device | directory | regular | other: The type of the file.
:access = read | write | read_write | none: The current system access
  to the file.
:atime = time(): The last (local) time the file was read.
:mtime = time(): The last (local) time the file was written.
:ctime = time(): 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.
:mode = int(): The file permissions as the sum of the following bit
values:
    : 8#00400: read permission: owner
    : 8#00200: write permission: owner
    : 8#00100: execute permission: owner
    : 8#00040: read permission: group
    : 8#00020: write permission: group
    : 8#00010: execute permission: group
    : 8#00004: read permission: other
    : 8#00002: write permission: other
    : 8#00001: execute permission: other
    : 16#800: set user id on execution
    : 16#400: set group id on execution

On Unix platforms, other bits than those listed above may be set.
:links = int(): Number of links to the file (this will always be 1 for
  file systems which have no concept of links).
:major_device = int(): 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.
:minor_device = int(): Only valid for character devices on Unix. In
  all other cases, this field is zero.
:inode = int(): Gives the inode number. On non-Unix file systems, this
  field will be zero.
:uid = int(): Indicates the owner of the file. Will be zero for non-
  Unix file systems.
:gid = int(): Gives the group that the owner of the file belongs to.
  Will be zero for non-Unix file systems.


Typical error reasons:

:eacces: Missing search permission for one of the parent directories
  of the file.
:enoent: The file does not exist.
:enotdir: A component of the file name is not a directory. On some
  platforms, enoent is returned instead.


read_line(IoDevice) -> {ok, Data} | eof | {error, Reason}


Types:

IoDevice = io_device()
Data = [char()] | binary()
Reason = ext_posix() | terminated


Reads a line of bytes/characters from the file referenced byIoDevice.
Lines are defined to be delimited by the linefeed (LF, \n) character,
but any carriage return (CR, \r) followed by a newline is also treated
as a single LF character (the carriage return is silently ignored).
The line is returned including the LF, but excluding any CR
immediately followed by a LF. This behaviour is consistent with the
behaviour of `io:get_line/2 <javascript:erlhref('../../../../',%20'std
lib',%20'io.html#get_line-2');>`__. If end of file is reached without
any LF ending the last line, a line with no trailing LF is returned.

The function can be used on files opened in raw mode. It is however
inefficient to use it on raw files if the file is not opened with the
option {read_ahead, Size} specified, why combining raw and
{read_ahead, Size} is highly recommended when opening a text file for
raw line oriented reading.

If encoding is set to something else than latin1, the read_line/1 call
will fail if the data contains characters larger than 255, why the
`io(3)
<javascript:erlhref('../../../../',%20'stdlib',%20'io.html');>`__
module is to be preferred when reading such a file.

The function returns:

:{ok, Data}: One line from the file is returned, including the
  trailing LF, but with CRLF sequences replaced by a single LF (see
  above). If the file was opened in binary mode, the read bytes are
  returned in a binary, otherwise in a list.
:eof: Returned if end of file was reached before anything at all could
  be read.
:{error, Reason}: An error occurred.


Typical error reasons:

:ebadf: The file is not opened for reading.
:{no_translation, unicode, latin1}: The file is was opened with
  another encoding than latin1 and the data on the file can not be
  translated to the byte-oriented data that this function returns.


read_link(Name) -> {ok, Filename} | {error, Reason}


Types:

Name = name()
Filename = string()
Reason = ext_posix()


This function returns {ok, Filename} if Name refers to a symbolic link
or {error, Reason} otherwise. On platforms that do not support
symbolic links, the return value will be {error,enotsup}.

Typical error reasons:

:einval: Linkname does not refer to a symbolic link.
:enoent: The file does not exist.
:enotsup: Symbolic links are not supported on this platform.


read_link_info(Name) -> {ok, FileInfo} | {error, Reason}


Types:

Name = name()
FileInfo = #file_info{}, see read_file_info/1
Reason = ext_posix()


This function works like read_file_info/1, except that if Name is a
symbolic link, information about the link will be returned in the
file_info record and the type field of the record will be set
tosymlink.

If Name is not a symbolic link, this function returns exactly the same
result as read_file_info/1. On platforms that do not support symbolic
links, this function is always equivalent to read_file_info/1.

rename(Source, Destination) -> ok | {error, Reason}


Types:

Source = Destination = name()
Reason = ext_posix()


Tries to rename the file Source to Destination. 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 bar is a normal file andfoo and baz are
directories,rename("foo/bar", "baz") returns an error,
butrename("foo/bar", "baz/bar") succeeds. Returnsok if it is
successful.
Note
Renaming of open files is not allowed on most platforms (see eacces
below).

Typical error reasons:

:eacces: Missing read or write permissions for the parent directories
  of Source or Destination. On some platforms, this error is given if
  eitherSource or Destination is open.
:eexist: Destination is not an empty directory. On some platforms,
  also given when Source andDestination are not of the same type.
:einval: Source is a root directory, or Destination is a sub-directory
  of Source.
:eisdir: Destination is a directory, but Source is not.
:enoent: Source does not exist.
:enotdir: Source is a directory, but Destination is not.
:exdev: Source and Destination are on different file systems.


script(Filename) -> {ok, Value} | {error, Reason}


Types:

Filename = name()
Value = term()
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see below


Reads and evaluates Erlang expressions, separated by '.' (or ',', a
sequence of expressions is also an expression), from the file. Returns
one of the following:

:{ok, Value}: The file was read and evaluated. Value is the value of
  the last expression.
:{error, atom()}: An error occurred when opening the file or reading
  it. See open/2 for a list of typical error codes.
:{error, {Line, Mod, Term}}: An error occurred when interpreting the
  Erlang expressions in the file. Use format_error/1 to convert the
  three-element tuple to an English description of the error.


script(Filename, Bindings) -> {ok, Value} | {error, Reason}


Types:

Filename = name()
Bindings -- see erl_eval(3)
Value = term()
Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
 Line, Mod, Term -- see below


The same as script/1 but the variable bindingsBindings are used in the
evaluation. See `erl_eval(3) <javascript:erlhref('../../../../',%20'st
dlib',%20'erl_eval.html');>`__ about variable bindings.

set_cwd(Dir) -> ok | {error,Reason}


Types:

Dir = name()
Reason = ext_posix()


Sets the current working directory of the file server toDir. Returns
ok if successful.

Typical error reasons are:

:enoent: The directory does not exist.
:enotdir: A component of Dir is not a directory. On some platforms,
  enoent is returned.
:eacces: Missing permission for the directory or one of its parents.
:badarg: Filename had an improper type, such as tuple.

Warning
In a future release, a bad type for the Filename argument will
probably generate an exception.



sync(IoDevice) -> ok | {error, Reason}


Types:

IoDevice = io_device()
Reason = ext_posix() | terminated


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.

Typical error reasons are:

:enospc: Not enough space left to write the file.


truncate(IoDevice) -> ok | {error, Reason}


Types:

IoDevice = io_device()
Reason = ext_posix() | terminated


Truncates the file referenced by IoDevice at the current position.
Returns ok if successful, otherwise {error, Reason}.

write(IoDevice, Bytes) -> ok | {error, Reason}


Types:

IoDevice = io_device()
Bytes = iodata()
Reason = ext_posix() | terminated


Writes Bytes to the file referenced byIoDevice. This function is the
only way to write to a file opened in raw mode (although it works for
normally opened files, too). Returns ok if successful, and{error,
Reason} otherwise.

If the file is opened with encoding set to something else than latin1,
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.

Typical error reasons are:

:ebadf: The file is not opened for writing.
:enospc: There is a no space left on the device.


write_file(Filename, Bytes) -> ok | {error, Reason}


Types:

Filename = name()
Bytes = iodata()
Reason = ext_posix() | terminated | system_limit


Writes the contents of the iodata term Bytes to the file Filename. The
file is created if it does not exist. If it exists, the previous
contents are overwritten. Returns ok, or {error, Reason}.

Typical error reasons are:

:enoent: A component of the file name does not exist.
:enotdir: A component of the file name is not a directory. On some
  platforms, enoent is returned instead.
:enospc: There is a no space left on the device.
:eacces: Missing permission for writing the file or searching one of
  the parent directories.
:eisdir: The named file is a directory.


write_file(Filename, Bytes, Modes) -> ok | {error, Reason}


Types:

Filename = name()
Bytes = iodata()
Modes = [Mode] -- see open/2
Reason = ext_posix() | terminated | system_limit


Same as write_file/2, but takes a third argumentModes, a list of
possible modes, seeopen/2. The mode flagsbinary and write are
implicit, so they should not be used.

write_file_info(Filename, FileInfo) -> ok | {error, Reason}


Types:

Filename = name()
FileInfo = #file_info{} -- see also read_file_info/1
Reason = ext_posix()


Change file information. Returns ok if successful, otherwise {error,
Reason}. FileInfo is a recordfile_info, defined in the Kernel include
filefile.hrl. Include the following directive in the module from which
the function is called:


.. sourcecode:: erlang

    
    -include_lib("kernel/include/file.hrl").


The following fields are used from the record, if they are given.

:atime = time(): The last (local) time the file was read.
:mtime = time(): The last (local) time the file was written.
:ctime = time(): 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.
:mode = int(): The file permissions as the sum of the following bit
values:
    : 8#00400: read permission: owner
    : 8#00200: write permission: owner
    : 8#00100: execute permission: owner
    : 8#00040: read permission: group
    : 8#00020: write permission: group
    : 8#00010: execute permission: group
    : 8#00004: read permission: other
    : 8#00002: write permission: other
    : 8#00001: execute permission: other
    : 16#800: set user id on execution
    : 16#400: set group id on execution

On Unix platforms, other bits than those listed above may be set.
:uid = int(): Indicates the owner of the file. Ignored for non-Unix
  file systems.
:gid = int(): Gives the group that the owner of the file belongs to.
  Ignored non-Unix file systems.


Typical error reasons:

:eacces: Missing search permission for one of the parent directories
  of the file.
:enoent: The file does not exist.
:enotdir: A component of the file name is not a directory. On some
  platforms, enoent is returned instead.




POSIX Error Codes
~~~~~~~~~~~~~~~~~


+ eacces - permission denied
+ eagain - resource temporarily unavailable
+ ebadf - bad file number
+ ebusy - file busy
+ edquot - disk quota exceeded
+ eexist - file already exists
+ efault - bad address in system call argument
+ efbig - file too large
+ eintr - interrupted system call
+ einval - invalid argument
+ eio - IO error
+ eisdir - illegal operation on a directory
+ eloop - too many levels of symbolic links
+ emfile - too many open files
+ emlink - too many links
+ enametoolong - file name too long
+ enfile - file table overflow
+ enodev - no such device
+ enoent - no such file or directory
+ enomem - not enough memory
+ enospc - no space left on device
+ enotblk - block device required
+ enotdir - not a directory
+ enotsup - operation not supported
+ enxio - no such device or address
+ eperm - not owner
+ epipe - broken pipe
+ erofs - read-only file system
+ espipe - invalid seek
+ esrch - no such process
+ estale - stale remote file handle
+ exdev - cross-domain link




Performance
~~~~~~~~~~~

Some operating system file operations, for example async/1 or close/1
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.

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.

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 sync/1 or close/1 on a
huge file, still are a problem.

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.

A normal file is really a process so it can be used as an IO device
(see io). 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.

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:


.. sourcecode:: erlang

    
    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).


The following, functionally equivalent, function collects 1024 entries
into a list of 128 32-byte binaries before each call tofile:write/2
and so does the same work in 0.52 seconds, which is 44 times faster.


.. sourcecode:: erlang

    
    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]).

Note
Trust only your own benchmarks. If the list length in create_file/2
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.

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.

So, with a binary size of 68 bytes create_file/2 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.

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
file:write(FD, [Bin1, Bin2 | Bin3]) will write the contents of the
binaries without copying the data at all except for perhaps deep down
in the operating syste…