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file(3erl)			     Erlang Module Definition			       file(3erl)

NAME
       file - 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 per-
       formed 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) .

       The  Erlang VM supports file names in Unicode to a limited extent. Depending on how the VM
       is started (with the parameter +fnu or +fnl ), file names given can contain  characters	>
       255  and  the  VM  system  will	convert file names back and forth to the native file name
       encoding.

       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.

       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.

   Note:As of R14B01, the most basic file handling modules ( file , prim_file , filelib and file-
       name ) 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.
       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
       file:native_name_encoding() can be used to check what encoding the VM is  working  in.  If
       the  function  returns latin1 file names are not in any way converted to Unicode, if it is
       utf8 , 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.

       Note that on Windows, file:native_name_encoding() returns utf8 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...

DATA TYPES
       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

EXPORTS
       advise(IoDevice, Offset, Length, Advise) -> ok | {error, Reason}

	      Types  IoDevice = io_device()
		     Offset = int()
		     Length = int()
		     Advise = posix_file_advise()
		     posix_file_advise()  = normal | sequential | random | no_reuse | will_need |
		     dont_need
		     Reason = ext_posix()

	      advise/4 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.

	      On some platforms, this function might have no effect.

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

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

	      Changes group of a file. See write_file_info/2 .

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

	      Types  Filename = name()
		     Mode = int()
		     Reason = ext_posix()

	      Changes permissions of a file. See write_file_info/2 .

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

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

	      Changes owner of a file. See write_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. See write_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. See write_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. See write_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. See open/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 for-
		  mat_error/1 to convert the three-element tuple to an English description of the
		  error.

	      Example:

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

	      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 to Destination . 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, see open/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  opti-
	      mize the copy.

	      If  Source  refers to a filename, it is opened with read 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 excep-
       tion.

       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), from Filename . The actual result of the evalu-
	      ation  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(3erl)
		     Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
		     Line, Mod, Term -- see eval/1

	      The  same  as eval/1 but the variable bindings Bindings are used in the evaluation.
	      See erl_eval(3erl) 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  descrip-
	      tive 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 speci-
	      fied.

	      This function returns {error, enotsup} on platforms which have no concept  of  cur-
	      rent 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. Other-
	      wise, 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 returns ok 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.

       native_name_encoding() -> latin1 | utf8

	      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.

	      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  +fnu 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.  bina-
	      ries).

	      On  Windows, this function also returns utf8 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.

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

	      Types  Filename = name()
		     Modes = [Mode]
		     Mode  =  read  | write | append | exclusive | raw | binary | {delayed_write,
		     Size, Delay} | delayed_write | {read_ahead, Size} | read_ahead |  compressed
		     | {encoding, Encoding}
		     Size = Delay = int()
		     Encoding  =  latin1  |  unicode  |  utf8 | utf16 | {utf16, Endian} | utf32 |
		     {utf32, Endian}
		     Endian = big | little
		     IoDevice = io_device()
		     Reason = ext_posix() | system_limit

	      Opens the file Filename in the mode determined by Modes , 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.

		exclusive :
		  The file, when opened for writing, is created if it does not exist. If the file
		  exists, open will return {error, eexist} .

   Warning:
       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).

       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 subsequent write/2 calls is buffered until there are
	 at least Size 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  operat-
	 ing  system calls, so the write/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, and close/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  and
	 Delay . (Roughly some 64 KBytes, 2 seconds)

       {read_ahead, Size} :
	 This  option  activates  read data buffering. If read/2 calls are for significantly less
	 than Size 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 compressed option must  be
	 combined with either read or write , but not both. Note that the file size obtained with
	 read_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 (Uni-
	 code) 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(3erl) 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(3erl) 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(3erl) module.

	   Bytes written to the file by any means are translated to UTF-8 encoding  before  actu-
	   ally 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 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(3erl) mod-
	 ule 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(3erl) ).

   Note:
       In previous versions of file , modes were given as one of the atoms  read  ,  write  ,  or
       read_write  instead  of a list. This is still allowed for reasons of backwards compatibil-
       ity, 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 in Path .

		{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 for-
		  mat_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 evalu-
	      ates  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 in Path .

		{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 and FullName is the full name of the file.

		{error, enoent} :
		  The file could not be found in any of the directories in Path .

		{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 evalu-
	      ates 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 in Path .

		{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(3erl)
		     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 bindings Bindings are used in the evalu-
	      ation. See erl_eval(3erl) about variable bindings.

       pid2name(Pid) -> string() | undefined

	      Types  Pid = pid()

	      If Pid is an IO device, that is, a pid returned from open/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 that Pid 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} . Loca-
	      tion 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 charac-
	      ter. 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 begin-
	      ning/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 call-
	      ing  them  one at a time. Returns {ok, [Data, ...]} or {error, Reason} , where each
	      Data , the result of the corresponding pread , is either a list or a binary depend-
	      ing  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 call-
	      ing  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}} ,	where  N  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 restric-
	      tions 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 by IoDevice . 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(3erl) 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 was opened with another encoding than latin1 and the data in 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 con-
	      tents 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 record file_info , defined in the Kernel include
	      file file.hrl . Include the following directive in the module from which the  func-
	      tion is called:

	      -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 by IoDevice .  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 . If end of file is reached without any LF end-
	      ing 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(3erl) 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 to symlink .

	      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  destina-
	      tion only. The destination file name must also be specified. For example, if bar is
	      a normal file and foo and baz are directories, rename("foo/bar", "baz") returns  an
	      error, but rename("foo/bar", "baz/bar") succeeds. Returns ok 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 either Source or Destination is open.

	 eexist :
	   Destination is not an empty directory. On some platforms, also given when  Source  and
	   Destination 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(3erl)
		     Value = term()
		     Reason = ext_posix() | terminated | system_limit | {Line, Mod, Term}
		     Line, Mod, Term -- see below

	      The same as script/1 but the variable bindings Bindings are used in the evaluation.
	      See erl_eval(3erl) about variable bindings.

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

	      Types  Dir = name()
		     Reason = ext_posix()

	      Sets the current working directory of the file server to Dir . Returns ok  if  suc-
	      cessful.

	      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 excep-
       tion.

       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.

       datasync(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. 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.

	      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.

	      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.

       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 by IoDevice . 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 directo-
		  ries.

		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 argument Modes , a list of possible modes,
	      see open/2 . The mode flags binary 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 record file_info , defined in the  Kernel  include	file  file.hrl	.
	      Include the following directive in the module from which the function is called:

	      -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  cre-
		  ation 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 a sync/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  dur-
       ing 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:

       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 to file:write/2 and so does the same work in  0.52  sec-
       onds, which is 44 times faster.

       create_file(Name, N) when integer(N), N >= 0 ->
	   {ok, FD} = file:open(Name, [raw, write, delayed_write, binary]),
	   ok = create_file(FD, 0, N),
	   ok = ?FILE_MODULE:close(FD),
	   ok.

       create_file(FD, M, M) ->
	   ok;
       create_file(FD, M, N) when M + 1024 =&lt; N ->
	   create_file(FD, M, M + 1024, []),
	   create_file(FD, M + 1024, N);
       create_file(FD, M, N) ->
	   create_file(FD, M, N, []).

       create_file(FD, M, M, R) ->
	   ok = file:write(FD, R);
       create_file(FD, M, N0, R) when M + 8 =&lt; N0 ->
	   N1  = N0-1,	N2  = N0-2,  N3  = N0-3,  N4  = N0-4,
	   N5  = N0-5,	N6  = N0-6,  N7  = N0-7,  N8  = N0-8,
	   create_file(FD, M, N8,
		       [<<N8:32/unsigned,  N7:32/unsigned,
			  N6:32/unsigned,  N5:32/unsigned,
			  N4:32/unsigned,  N3:32/unsigned,
			  N2:32/unsigned,  N1:32/unsigned>> | R]);
       create_file(FD, M, N0, R) ->
	   N1 = N0-1,
	   create_file(FD, M, N1, [<<N1:32/unsigned>> | R]).

   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 pre-
       dict.

       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  pro-
       cesses,	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  com-
       pletely 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,  scat-
       ter 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 system kernel.

       For raw files, pwrite/2 and pread/2 are efficiently implemented. The file driver is called
       only once for the whole operation, and the list iteration is done in the file driver.

       The options delayed_write and read_ahead to file:open/2 makes the file driver  cache  data
       to  reduce the number of operating system calls. The function create_file/2 in the example
       above takes 60 seconds seconds without  the  delayed_write  option,  which  is  2.6  times
       slower.

       And,  as  a  really  bad  example,  create_file_slow/2  above without the raw , binary and
       delayed_write options, that is it calls file:open(Name, [write]) , 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 create_file/2 .

WARNINGS
       If an error occurs when accessing an open file with the io module, the process which  han-
       dles  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.

SEE ALSO
       filename(3erl)

Ericsson AB				  kernel 2.14.3 			       file(3erl)
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