file(3erl)
NAME
file - File interface module.
DESCRIPTION
This module provides an interface to the file system. On operating systems with thread support, file operations can be performed in threads of their own, allowing other Erlang processes to continue executing in parallel with the file operations. See command- line flag +A in erl(1). Regarding filename encoding, the Erlang VM can operate in two modes. The current mode can be queried using function native_name_encoding/0. It returns latin1 or utf8. In latin1 mode, the Erlang VM does not change the encoding of filenames. In utf8 mode, filenames can contain Unicode characters greater than 255 and the VM converts filenames back and forth to the native filename encoding (usually UTF-8, but UTF-16 on Windows). The default mode depends on the operating system. Windows and MacOS X enforce consistent filename encoding and therefore the VM uses utf8 mode. On operating systems with transparent naming (for example, all Unix systems except MacOS X), default is utf8 if the terminal supports UTF-8, otherwise latin1. The default can be overridden using +fnl (to force latin1 mode) or +fnu (to force utf8 mode) when starting erts:erl. On operating systems with transparent naming, files can be inconsistently named, for example, some files are encoded in UTF-8 while others are encoded in ISO Latin-1. The concept of raw filenames is introduced to handle file systems with inconsistent naming when running in utf8 mode. A raw filename is a filename specified as a binary. The Erlang VM does not translate a filename specified as a binary on systems with transparent naming. When running in utf8 mode, functions list_dir/1 and read_link/1 never return raw filenames. To return all filenames including raw filenames, use functions list_dir_all/1 and read_link_all/1. See also section Notes About Raw Filenames in the STDLIB User's Guide.
DATA TYPES
deep_list() = [char() | atom() | deep_list()]
fd()
A file descriptor representing a file opened in raw mode.
filename() = string()
filename_all() = string() | binary()
io_device() = pid() | fd()
As returned by open/2; pid() is a process handling I/O-
protocols.
name() = string() | atom() | deep_list()
If VM is in Unicode filename mode, string() and char() are
allowed to be > 255.
name_all() =
string() | atom() | deep_list() | (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 file system
(that is, non-UTF-8 characters although the VM is started in
Unicode filename mode).
posix() =
eacces |
eagain |
ebadf |
ebusy |
edquot |
eexist |
efault |
efbig |
eintr |
einval |
eio |
eisdir |
eloop |
emfile |
emlink |
enametoolong |
enfile |
enodev |
enoent |
enomem |
enospc |
enotblk |
enotdir |
enotsup |
enxio |
eperm |
epipe |
erofs |
espipe |
esrch |
estale |
exdev
An atom that is named from the POSIX error codes used in Unix,
and in the runtime libraries of most C compilers.
date_time() = calendar:datetime()
Must denote a valid date and time.
file_info() =
#file_info{size = integer() >= 0 | undefined,
type =
device |
directory |
other |
regular |
symlink |
undefined,
access =
read | write | read_write | none | undefined,
atime =
file:date_time() |
integer() >= 0 |
undefined,
mtime =
file:date_time() |
integer() >= 0 |
undefined,
ctime =
file:date_time() |
integer() >= 0 |
undefined,
mode = integer() >= 0 | undefined,
links = integer() >= 0 | undefined,
major_device = integer() >= 0 | undefined,
minor_device = integer() >= 0 | undefined,
inode = integer() >= 0 | undefined,
uid = integer() >= 0 | undefined,
gid = integer() >= 0 | undefined}
location() =
integer() |
{bof, Offset :: integer()} |
{cur, Offset :: integer()} |
{eof, Offset :: integer()} |
bof |
cur |
eof
mode() =
read |
write |
append |
exclusive |
raw |
binary |
{delayed_write,
Size :: integer() >= 0,
Delay :: integer() >= 0} |
delayed_write |
{read_ahead, Size :: integer() >= 1} |
read_ahead |
compressed |
{encoding, unicode:encoding()} |
sync
file_info_option() =
{time, local} | {time, universal} | {time, posix} | raw
EXPORTS
advise(IoDevice, Offset, Length, Advise) -> ok | {error, Reason}
Types:
IoDevice = io_device()
Offset = Length = integer()
Advise = posix_file_advise()
Reason = posix() | badarg
posix_file_advise() =
normal |
sequential |
random |
no_reuse |
will_need |
dont_need
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.
allocate(File, Offset, Length) -> ok | {error, posix()}
Types:
File = io_device()
Offset = Length = integer() >= 0
allocate/3 can be used to preallocate space for a file.
This function only succeeds in platforms that provide this
feature. When it succeeds, space is preallocated for the file
but the file size might not be updated. This behaviour depends
on the preallocation implementation. To guarantee that the file
size is updated, truncate the file to the new size.
change_group(Filename, Gid) -> ok | {error, Reason}
Types:
Filename = name_all()
Gid = integer()
Reason = posix() | badarg
Changes group of a file. See write_file_info/2.
change_mode(Filename, Mode) -> ok | {error, Reason}
Types:
Filename = name_all()
Mode = integer()
Reason = posix() | badarg
Changes permissions of a file. See write_file_info/2.
change_owner(Filename, Uid) -> ok | {error, Reason}
Types:
Filename = name_all()
Uid = integer()
Reason = posix() | badarg
Changes owner of a file. See write_file_info/2.
change_owner(Filename, Uid, Gid) -> ok | {error, Reason}
Types:
Filename = name_all()
Uid = Gid = integer()
Reason = posix() | badarg
Changes owner and group of a file. See write_file_info/2.
change_time(Filename, Mtime) -> ok | {error, Reason}
Types:
Filename = name_all()
Mtime = date_time()
Reason = posix() | badarg
Changes the modification and access times of a file. See
write_file_info/2.
change_time(Filename, Atime, Mtime) -> ok | {error, Reason}
Types:
Filename = name_all()
Atime = Mtime = date_time()
Reason = posix() | badarg
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 = posix() | badarg | terminated
Closes the file referenced by IoDevice. It mostly returns ok,
except for some severe errors such as out of memory.
Notice that if option delayed_write was used when opening the
file, close/1 can 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_all()
Terms = [term()]
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
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. For a
list of typical error codes, see open/2.
{error, {Line, Mod, Term}}:
An error occurred when interpreting the Erlang terms in the
file. To convert the three-element tuple to an English
description of the error, use format_error/1.
Example:
f.txt: {person, "kalle", 25}.
{person, "pelle", 30}.
1> file:consult("f.txt").
{ok,[{person,"kalle",25},{person,"pelle",30}]}
The encoding of Filename can be set by a comment, as described
in epp(3erl).
copy(Source, Destination) -> {ok, BytesCopied} | {error, Reason}
copy(Source, Destination, ByteCount) ->
{ok, BytesCopied} | {error, Reason}
Types:
Source = Destination = io_device() | Filename | {Filename,
Modes}
Filename = name_all()
Modes = [mode()]
ByteCount = integer() >= 0 | infinity
BytesCopied = integer() >= 0
Reason = posix() | badarg | terminated
Copies ByteCount bytes from Source to Destination. Source and
Destination refer to either filenames or IO devices from, for
example, open/2. ByteCount defaults to infinity, denoting an
infinite number of bytes.
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 optimize 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 was copied, which can 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.
datasync(IoDevice) -> ok | {error, Reason}
Types:
IoDevice = io_device()
Reason = posix() | badarg | terminated
Ensures that any buffers kept by the operating system (not by
the Erlang runtime system) are written to disk. In many ways it
resembles fsync but it does not update some of the metadata of
the file, such as the access time. On some platforms this
function has no effect.
Applications that access databases or log files often write a
tiny data fragment (for example, one line in a log file) and
then call fsync() immediately to ensure that the written data is
physically stored on the hard disk. Unfortunately, fsync()
always initiates two write operations: one for the newly written
data and another one 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 providing the
fdatasync() syscall.
del_dir(Dir) -> ok | {error, Reason}
Types:
Dir = name_all()
Reason = posix() | badarg
Tries to delete directory Dir. The directory must be empty
before it can be deleted. Returns ok if successful.
Typical error reasons:
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_all()
Reason = posix() | badarg
Tries to delete file Filename. Returns ok if successful.
Typical error reasons:
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 superuser.
enotdir:
A component of the filename is not a directory. On some
platforms, enoent is returned instead.
einval:
Filename has an improper type, such as tuple.
Warning:
In a future release, a bad type for argument Filename will
probably generate an exception.
eval(Filename) -> ok | {error, Reason}
Types:
Filename = name_all()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
Reads and evaluates Erlang expressions, separated by '.' (or
',', a sequence of expressions is also an expression) from
Filename. The result of the evaluation is not returned; any
expression sequence in the file must be there for its side
effect. Returns one of the following:
ok:
The file was read and evaluated.
{error, atom()}:
An error occurred when opening the file or reading it. For a
list of typical error codes, see open/2.
{error, {Line, Mod, Term}}:
An error occurred when interpreting the Erlang expressions
in the file. To convert the three-element tuple to an
English description of the error, use format_error/1.
The encoding of Filename can be set by a comment, as described
in epp(3erl).
eval(Filename, Bindings) -> ok | {error, Reason}
Types:
Filename = name_all()
Bindings = erl_eval:binding_struct()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
The same as eval/1, but the variable bindings Bindings are used
in the evaluation. For information about the variable bindings,
see erl_eval(3erl).
format_error(Reason) -> Chars
Types:
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
Chars = string()
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 = filename()
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 can
occur if read permission does not exist for the parent
directories of the current directory.
A typical error reason:
eacces:
Missing read permission for one of the parents of the
current directory.
get_cwd(Drive) -> {ok, Dir} | {error, Reason}
Types:
Drive = string()
Dir = filename()
Reason = posix() | badarg
Returns {ok, Dir} or {error, Reason}, where Dir is the current
working directory of the specified drive.
Drive is to be of the form "Letter:", for example, "c:".
Returns {error, enotsup} on platforms that have no concept of
current drive (Unix, for example).
Typical error reasons:
enotsup:
The operating system has 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_all()
Filenames = [filename()]
Reason =
posix() |
badarg |
{no_translation, Filename :: unicode:latin1_binary()}
Lists all files in a directory, except files with raw filenames.
Returns {ok, Filenames} if successful, otherwise {error,
Reason}. Filenames is a list of the names of all the files in
the directory. The names are not sorted.
Typical error reasons:
eacces:
Missing search or write permissions for Dir or one of its
parent directories.
enoent:
The directory does not exist.
{no_translation, Filename}:
Filename is a binary() with characters coded in ISO Latin-1
and the VM was started with parameter +fnue.
list_dir_all(Dir) -> {ok, Filenames} | {error, Reason}
Types:
Dir = name_all()
Filenames = [filename_all()]
Reason = posix() | badarg
Lists all the files in a directory, including files with raw
filenames. Returns {ok, Filenames} if successful, otherwise
{error, Reason}. Filenames is a list of the names of all the
files in the directory. The names are not sorted.
Typical error reasons:
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_all()
Reason = posix() | badarg
Tries to create directory Dir. Missing parent directories are
not created. Returns ok if successful.
Typical error reasons:
eacces:
Missing search or write permissions for the parent
directories of Dir.
eexist:
A file or directory named Dir exists already.
enoent:
A component of Dir does not exist.
enospc:
No space is 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_all()
Reason = posix() | badarg
Makes a hard link from Existing to New on platforms supporting
links (Unix and Windows). This function returns ok if the link
was successfully created, otherwise {error, Reason}. On
platforms not supporting 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(Existing, New) -> ok | {error, Reason}
Types:
Existing = New = name_all()
Reason = posix() | badarg
Creates a symbolic link New to the file or directory Existing on
platforms supporting symbolic links (most Unix systems and
Windows, beginning with Vista). Existing does not need to exist.
Returns ok if the link is successfully created, otherwise
{error, Reason}. On platforms not supporting symbolic 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:
Symbolic links are not supported on this platform.
eperm:
User does not have privileges to create symbolic links
(SeCreateSymbolicLinkPrivilege on Windows).
native_name_encoding() -> latin1 | utf8
Returns the filename encoding mode. If it is latin1, the system
translates no filenames. If it is utf8, filenames are converted
back and forth to the native filename encoding (usually UTF-8,
but UTF-16 on Windows).
open(File, Modes) -> {ok, IoDevice} | {error, Reason}
Types:
File = Filename | iodata()
Filename = name_all()
Modes = [mode() | ram]
IoDevice = io_device()
Reason = posix() | badarg | system_limit
Opens file File in the mode determined by Modes, which can
contain one or more of the following options:
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 write is not combined with
read, the file is truncated.
append:
The file is opened for writing. It is created if it does not
exist. Every write operation to a file opened with append
takes place at the end of the file.
exclusive:
The file is opened for writing. It is created if it does not
exist. If the file exists, {error, eexist} is returned.
Warning:
This option does not guarantee exclusiveness on file systems
not supporting 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 are safe).
raw:
Allows faster access to a file, as no Erlang process is
needed to handle the file. However, a file opened in this
way has the following limitations:
* The functions in the io module cannot be used, as they can
only talk to an Erlang process. Instead, use functions
read/2, read_line/1, and write/2.
* Especially if read_line/1 is to be used on a raw file, it
is recommended to combine this option with option
{read_ahead, Size} as line-oriented I/O is inefficient
without buffering.
* Only the Erlang process that 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:
Read operations on the file return binaries rather than
lists.
{delayed_write, Size, Delay}:
Data in subsequent write/2 calls is buffered until at least
Size bytes are 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. Thus, the
write/2 calls must be for sizes significantly less than
Size, and not interspersed by too many other file
operations.
When this option is used, the result of write/2 calls can
prematurely be reported as successful, and if a write error
occurs, the error is reported as the result of the next file
operation, which is not executed.
For example, when delayed_write is used, after a number of
write/2 calls, close/1 can return {error, enospc}, as there
is not enough space on the disc for previously written data.
close/1 must probably be called again, as 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 KB, 2
seconds).
{read_ahead, Size}:
Activates read data buffering. If read/2 calls are for
significantly less than Size bytes, read operations to 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 as the number of
operating system calls is reduced.
The read_ahead buffer is also highly used by function
read_line/1 in raw mode, therefore 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 KB).
compressed:
Makes it possible to read or write gzip compressed files.
Option compressed must be combined with read or write, but
not both. Notice that the file size obtained with
read_file_info/1 does 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. Notice that the
data supplied to write/2 or returned by read/2 still is
byte-oriented; this option denotes only how data is 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 module (file) for reading and writing
data as the interfaces provided here work with byte-oriented
data. Using other (Unicode) encodings makes the io(3erl)
functions get_chars, get_line, and put_chars 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 is closed.
Allowed values for Encoding:
latin1:
The default encoding. Bytes supplied to the file, that is,
write/2 are written "as is" on the file. Likewise, bytes
read from the file, that is, read/2 are returned "as is".
If module io(3erl) is used for writing, the file can only
cope with Unicode characters up to code point 255 (the ISO
Latin-1 range).
unicode or utf8:
Characters are translated to and from UTF-8 encoding
before they are written to or read from the file. A file
opened in this way can be readable using function read/2,
as long as no data stored on the file lies beyond the ISO
Latin-1 range (0..255), but failure occurs if the data
contains Unicode code points beyond that range. The file
is best read with the functions in the Unicode aware
module io(3erl).
Bytes written to the file by any means are translated to
UTF-8 encoding before 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
function io:setopts/2. So a file can be analyzed in latin1
encoding for, for example, a BOM, positioned beyond the BOM
and then be set for the right encoding before further
reading. For functions identifying BOMs, see module
unicode(3erl).
This option is not allowed on raw files.
ram:
File must be iodata(). Returns an fd(), which lets module
file operate on the data in-memory as if it is a file.
sync:
On platforms supporting it, enables the POSIX O_SYNC
synchronous I/O flag or its platform-dependent equivalent
(for example, FILE_FLAG_WRITE_THROUGH on Windows) so that
writes to the file block until the data is physically
written to disk. However, be aware that the exact semantics
of this flag differ from platform to platform. For example,
none of Linux or Windows guarantees that all file metadata
are also written before the call returns. For precise
semantics, check the details of your platform documentation.
On platforms with no support for POSIX O_SYNC or equivalent,
use of the sync flag causes open to return {error, enotsup}.
Returns:
{ok, IoDevice}:
The file is opened in the requested mode. IoDevice is a
reference to the file.
{error, Reason}:
The file cannot be opened.
IoDevice is really the pid of the process that handles the file.
This process is linked to the process that originally opened the
file. If any process to which the IoDevice is linked terminates,
the file is closed and the process itself is terminated. An
IoDevice returned from this call can be used as an argument to
the I/O functions (see io(3erl)).
Note:
In previous versions of file, modes were specified as one of the
atoms read, write, or read_write instead of a list. This is
still allowed for reasons of backwards compatibility, but is not
to be used for new code. Also note that 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 can be a directory,
a FIFO, or a device.
enotdir:
A component of the filename is not a directory. On some
platforms, enoent is returned instead.
enospc:
There is no space left on the device (if write access was
specified).
path_consult(Path, Filename) ->
{ok, Terms, FullName} | {error, Reason}
Types:
Path = [Dir]
Dir = Filename = name_all()
Terms = [term()]
FullName = filename_all()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
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 is successfully read. FullName is the full name of
the file.
{error, enoent}:
The file cannot be found in any of the directories in Path.
{error, atom()}:
An error occurred when opening the file or reading it. For a
list of typical error codes, see open/2.
{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.
The encoding of Filename can be set by a comment as described in
epp(3erl).
path_eval(Path, Filename) -> {ok, FullName} | {error, Reason}
Types:
Path = [Dir :: name_all()]
Filename = name_all()
FullName = filename_all()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
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 and evaluates Erlang expressions,
separated by '.' (or ',', a sequence of expressions is also an
expression), from the file. The result of evaluation is not
returned; any expression sequence in the file must be there for
its side effect.
Returns one of the following:
{ok, FullName}:
The file is read and evaluated. FullName is the full name of
the file.
{error, enoent}:
The file cannot be found in any of the directories in Path.
{error, atom()}:
An error occurred when opening the file or reading it. For a
list of typical error codes, see open/2.
{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.
The encoding of Filename can be set by a comment as described in
epp(3erl).
path_open(Path, Filename, Modes) ->
{ok, IoDevice, FullName} | {error, Reason}
Types:
Path = [Dir :: name_all()]
Filename = name_all()
Modes = [mode()]
IoDevice = io_device()
FullName = filename_all()
Reason = posix() | badarg | system_limit
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 opens the file in the mode determined by
Modes.
Returns one of the following:
{ok, IoDevice, FullName}:
The file is 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 cannot be found in any of the directories in Path.
{error, atom()}:
The file cannot be opened.
path_script(Path, Filename) ->
{ok, Value, FullName} | {error, Reason}
Types:
Path = [Dir :: name_all()]
Filename = name_all()
Value = term()
FullName = filename_all()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
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 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 is read and evaluated. FullName is the full name of
the file and Value the value of the last expression.
{error, enoent}:
The file cannot be found in any of the directories in Path.
{error, atom()}:
An error occurred when opening the file or reading it. For a
list of typical error codes, see open/2.
{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.
The encoding of Filename can be set by a comment as described in
epp(3erl).
path_script(Path, Filename, Bindings) ->
{ok, Value, FullName} | {error, Reason}
Types:
Path = [Dir :: name_all()]
Filename = name_all()
Bindings = erl_eval:binding_struct()
Value = term()
FullName = filename_all()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
The same as path_script/2 but the variable bindings Bindings are
used in the evaluation. See erl_eval(3erl) about variable
bindings.
pid2name(Pid) -> {ok, Filename} | undefined
Types:
Filename = filename_all()
Pid = pid()
If Pid is an I/O device, that is, a pid returned from open/2,
this function returns the filename, or rather:
{ok, Filename}:
If the file server of this node is not a slave, the file was
opened by the file server of this node (this implies that
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 = location()
NewPosition = integer()
Reason = posix() | badarg | 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.
Notice 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. That is, 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).
A typical error reason is:
einval:
Either Location is illegal, or it is evaluated to a negative
offset in the file. Notice that if the resulting position is
a negative value, the result is an error, and after the call
the file position is undefined.
pread(IoDevice, LocNums) -> {ok, DataL} | eof | {error, Reason}
Types:
IoDevice = io_device()
LocNums =
[{Location :: location(), Number :: integer() >= 0}]
DataL = [Data]
Data = string() | binary() | eof
Reason = posix() | badarg | 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
corresponding pread, is either a list or a binary depending on
the mode of the file, or eof if the requested position is beyond
end of file.
As the position is specified as a byte-offset, take special
caution when working with files where encoding is set to
something else than latin1, as not every byte position is a
valid character boundary on such a file.
pread(IoDevice, Location, Number) ->
{ok, Data} | eof | {error, Reason}
Types:
IoDevice = io_device()
Location = location()
Number = integer() >= 0
Data = string() | binary()
Reason = posix() | badarg | terminated
Combines position/2 and read/2 in one operation, which is more
efficient than calling them one at a time. If IoDevice is opened
in raw mode, some restrictions apply:
* Location is only allowed to be an integer.
* The current position of the file is undefined after the
operation.
As the position is specified as a byte-offset, take special
caution when working with files where encoding is set to
something else than latin1, as not every byte position is a
valid character boundary on such a file.
pwrite(IoDevice, LocBytes) -> ok | {error, {N, Reason}}
Types:
IoDevice = io_device()
LocBytes = [{Location :: location(), Bytes :: iodata()}]
N = integer() >= 0
Reason = posix() | badarg | 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 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. For details, see position/2.
pwrite(IoDevice, Location, Bytes) -> ok | {error, Reason}
Types:
IoDevice = io_device()
Location = location()
Bytes = iodata()
Reason = posix() | badarg | 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.
* 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. For details, see position/2.
read(IoDevice, Number) -> {ok, Data} | eof | {error, Reason}
Types:
IoDevice = io_device() | atom()
Number = integer() >= 0
Data = string() | binary()
Reason =
posix() |
badarg |
terminated |
{no_translation, unicode, latin1}
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 can be represented by more than one byte on the
file. The parameter Number always denotes the number of
characters read from the file, while the position in the file
can be moved much more than this number when reading a Unicode
file.
Also, if encoding is set to something else than latin1, the
read/3 call fails if the data contains characters larger than
255, which is why module io(3erl) 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 is 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 opened with another encoding than latin1 and the
data in the file cannot be translated to the byte-oriented
data that this function returns.
read_file(Filename) -> {ok, Binary} | {error, Reason}
Types:
Filename = name_all()
Binary = binary()
Reason = posix() | badarg | 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 filename 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}
read_file_info(Filename, Opts) -> {ok, FileInfo} | {error, Reason}
Types:
Filename = name_all()
Opts = [file_info_option()]
FileInfo = file_info()
Reason = posix() | badarg
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 function is
called:
-include_lib("kernel/include/file.hrl").
The time type returned in atime, mtime, and ctime is dependent
on the time type set in Opts :: {time, Type} as follows:
local:
Returns local time.
universal:
Returns universal time.
posix:
Returns seconds since or before Unix time epoch, which is
1970-01-01 00:00 UTC.
Default is {time, local}.
If the option raw is set, the file server is not called and only
information about local files is returned.
Note:
As file times are stored in POSIX time on most OS, it is faster
to query file information with option posix.
The record file_info contains the following fields:
size = integer() >= 0:
Size of file in bytes.
type = device | directory | other | regular | symlink:
The type of the file.
access = read | write | read_write | none:
The current system access to the file.
atime = date_time() | integer() >= 0:
The last time the file was read.
mtime = date_time() | integer() >= 0:
The last time the file was written.
ctime = date_time() | integer() >=0:
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 = integer() >= 0:
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, the following bits can also be set:
links = integer() >= 0:
Number of links to the file (this is always 1 for file
systems that have no concept of links).
major_device = integer() >= 0:
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 = integer() >= 0:
Only valid for character devices on Unix. In all other
cases, this field is zero.
inode = integer() >= 0:
Gives the inode number. On non-Unix file systems, this field
is zero.
uid = integer() >= 0:
Indicates the owner of the file. On non-Unix file systems,
this field is zero.
gid = integer() >= 0:
Gives the group that the owner of the file belongs to. On
non-Unix file systems, this field is zero.
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 filename is not a directory. On some
platforms, enoent is returned instead.
read_line(IoDevice) -> {ok, Data} | eof | {error, Reason}
Types:
IoDevice = io_device() | atom()
Data = string() | binary()
Reason =
posix() |
badarg |
terminated |
{no_translation, unicode, latin1}
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 an LF. This
behaviour is consistent with the behaviour of io: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. However,
it is inefficient to use it on raw files if the file is not
opened with option {read_ahead, Size} specified. Thus, 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 fails if the data contains characters larger
than 255, why module io(3erl) 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 is 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 opened with another encoding than latin1 and the
data on the file cannot be translated to the byte-oriented
data that this function returns.
read_link(Name) -> {ok, Filename} | {error, Reason}
Types:
Name = name_all()
Filename = filename()
Reason = posix() | badarg
Returns {ok, Filename} if Name refers to a symbolic link that is
not a raw filename, or {error, Reason} otherwise. On platforms
that do not support symbolic links, the return value is
{error,enotsup}.
Typical error reasons:
einval:
Name does not refer to a symbolic link or the name of the
file that it refers to does not conform to the expected
encoding.
enoent:
The file does not exist.
enotsup:
Symbolic links are not supported on this platform.
read_link_all(Name) -> {ok, Filename} | {error, Reason}
Types:
Name = name_all()
Filename = filename_all()
Reason = posix() | badarg
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 is {error,enotsup}.
Notice that Filename can be either a list or a binary.
Typical error reasons:
einval:
Name 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}
read_link_info(Name, Opts) -> {ok, FileInfo} | {error, Reason}
Types:
Name = name_all()
Opts = [file_info_option()]
FileInfo = file_info()
Reason = posix() | badarg
Works like read_file_info/1,2 except that if Name is a symbolic
link, information about the link is returned in the file_info
record and the type field of the record is set to symlink.
If the option raw is set, the file server is not called and only
information about local files is returned.
If Name is not a symbolic link, this function returns 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_all()
Reason = posix() | badarg
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
filename 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 subdirectory
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_all()
Value = term()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
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 is read and evaluated. Value is the value of the
last expression.
{error, atom()}:
An error occurred when opening the file or reading it. For a
list of typical error codes, see open/2.
{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.
The encoding of Filename can be set by a comment as described in
epp(3erl).
script(Filename, Bindings) -> {ok, Value} | {error, Reason}
Types:
Filename = name_all()
Bindings = erl_eval:binding_struct()
Value = term()
Reason =
posix() |
badarg |
terminated |
system_limit |
{Line :: integer(), Mod :: module(), Term :: term()}
The same as script/1 but the variable bindings Bindings are used
in the evaluation. See erl_eval(3erl) about variable bindings.
sendfile(Filename, Socket) ->
{ok, integer() >= 0} |
{error, inet:posix() | closed | badarg | not_owner}
Types:
Filename = name_all()
Socket = inet:socket()
Sends the file Filename to Socket. Returns {ok, BytesSent} if
successful, otherwise {error, Reason}.
sendfile(RawFile, Socket, Offset, Bytes, Opts) ->
{ok, integer() >= 0} |
{error, inet:posix() | closed | badarg | not_owner}
Types:
RawFile = fd()
Socket = inet:socket()
Offset = Bytes = integer() >= 0
Opts = [sendfile_option()]
sendfile_option() =
{chunk_size, integer() >= 0} | {use_threads, boolean()}
Sends Bytes from the file referenced by RawFile beginning at
Offset to Socket. Returns {ok, BytesSent} if successful,
otherwise {error, Reason}. If Bytes is set to 0 all data after
the specified Offset is sent.
The file used must be opened using the raw flag, and the process
calling sendfile must be the controlling process of the socket.
See gen_tcp:controlling_process/2.
If the OS used does not support sendfile, an Erlang fallback
using read/2 and gen_tcp:send/2 is used.
The option list can contain the following options:
chunk_size:
The chunk size used by the Erlang fallback to send data. If
using the fallback, set this to a value that comfortably
fits in the systems memory. Default is 20 MB.
use_threads:
Instructs the emulator to use the async thread pool for the
sendfile system call. This can be useful if the OS you are
running on does not properly support non-blocking sendfile
calls. Notice that using async threads potentially makes
your system vulnerable to slow client attacks. If set to
true and no async threads are available, the sendfile call
returns {error,einval}. Introduced in Erlang/OTP 17.0.
Default is false.
set_cwd(Dir) -> ok | {error, Reason}
Types:
Dir = name() | EncodedBinary
EncodedBinary = binary()
Reason = posix() | badarg | no_translation
Sets the current working directory of the file server to Dir.
Returns ok if successful.
The functions in the module file usually treat binaries as raw
filenames, that is, they are passed "as is" even when the
encoding of the binary does not agree with
native_name_encoding(). However, this function expects binaries
to be encoded according to the value returned by
native_name_encoding().
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:
Dir has an improper type, such as tuple.
no_translation:
Dir is a binary() with characters coded in ISO-latin-1 and
the VM is operating with unicode filename encoding.
Warning:
In a future release, a bad type for argument Dir will probably
generate an exception.
sync(IoDevice) -> ok | {error, Reason}
Types:
IoDevice = io_device()
Reason = posix() | badarg | terminated
Ensures that any buffers kept by the operating system (not by
the Erlang runtime system) are written to disk. On some
platforms, this function might have no effect.
A typical error reason is:
enospc:
Not enough space left to write the file.
truncate(IoDevice) -> ok | {error, Reason}
Types:
IoDevice = io_device()
Reason = posix() | badarg | 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() | atom()
Bytes = iodata()
Reason = posix() | badarg | 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 can result in many bytes being written
to the file, as the byte range 0..255 can represent anything
between one and four bytes depending on value and UTF encoding
type.
Typical error reasons:
ebadf:
The file is not opened for writing.
enospc:
No space is left on the device.
write_file(Filename, Bytes) -> ok | {error, Reason}
Types:
Filename = name_all()
Bytes = iodata()
Reason = posix() | badarg | terminated | system_limit
Writes the contents of the iodata term Bytes to file Filename.
The file is created if it does not exist. If it exists, the
previous contents are overwritten. Returns ok if successful,
otherwise {error, Reason}.
Typical error reasons:
enoent:
A component of the filename does not exist.
enotdir:
A component of the filename is not a directory. On some
platforms, enoent is returned instead.
enospc:
No space is 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_all()
Bytes = iodata()
Modes = [mode()]
Reason = posix() | badarg | 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 are not to be used.
write_file_info(Filename, FileInfo) -> ok | {error, Reason}
write_file_info(Filename, FileInfo, Opts) -> ok | {error, Reason}
Types:
Filename = name_all()
Opts = [file_info_option()]
FileInfo = file_info()
Reason = posix() | badarg
Changes 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 time type set in atime, mtime, and ctime depends on the time
type set in Opts :: {time, Type} as follows:
local:
Interprets the time set as local.
universal:
Interprets it as universal time.
posix:
Must be seconds since or before Unix time epoch, which is
1970-01-01 00:00 UTC.
Default is {time, local}.
If the option raw is set, the file server is not called and only
information about local files is returned.
The following fields are used from the record, if they are
specified:
atime = date_time() | integer() >= 0:
The last time the file was read.
mtime = date_time() | integer() >= 0:
The last time the file was written.
ctime = date_time() | integer() >= 0:
On Unix, any value specified for this field is ignored (the
"ctime" for the file is set to the current time). On
Windows, this field is the new creation time to set for the
file.
mode = integer() >= 0:
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, the following bits can also be set.
uid = integer() >= 0:
Indicates the file owner. Ignored for non-Unix file systems.
gid = integer() >= 0:
Gives the group that the file owner belongs to. Ignored 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 filename 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 - I/O error
* eisdir - Illegal operation on a directory
* eloop - Too many levels of symbolic links
* emfile - Too many open files
* emlink - Too many links
* enametoolong - Filename 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, can block their calling thread for seconds. If this
affects the emulator main thread, the response time is no longer in the
order of milliseconds, depending on the definition of "soft" in soft
real-time system.
If the device driver thread pool is active, file operations are done
through those threads instead, so the emulator can go on executing
Erlang processes. Unfortunately, the time for serving a file operation
increases because of the extra scheduling required from the operating
system.
If the device driver thread pool is disabled or of size 0, large file
reads and writes are segmented into many smaller, which enable the
emulator to serve other processes during the file operation. This has
the same effect as when using the thread pool, but with larger
overhead. Other file operations, for example, sync/1 or close/1 on a
huge file, still are a problem.
For increased performance, raw files are recommended. Raw files use the
file system of the host machine of the node.
Note:
For normal files (non-raw), the file server is used to find the files,
and if the node is running its file server as slave to the file server
of another node, and the other node runs on some other host machine,
they can have different file systems. However, this is seldom a
problem.
A normal file is really a process so it can be used as an I/O 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 the file server of another node, also binaries
are copied.
Caching data to reduce the number of file operations, or rather the
number of calls to the file driver, generally increases performance.
The following function writes 4 MBytes in 23 seconds when tested:
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 write/2 and so
does the same work in 0.52 seconds, 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 =< 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 runs slightly faster, but consumes more memory
and causes more memory fragmentation. How much this affects your
application is something that this simple benchmark cannot predict.
If the size of each binary is increased to 64 bytes, it also runs
slightly faster, but the code is then twice as clumsy. In the current
implementation, binaries larger than 64 bytes are stored in memory
common to all processes and not copied when sent between processes,
while these smaller binaries are stored on the process heap and copied
when sent like any other term.
So, with a binary size of 68 bytes, create_file/2 runs 30 percent
slower than with 64 bytes, and causes much more memory fragmentation.
Notice that if the binaries were to be sent between processes (for
example, a non-raw file), the results would probably be completely
different.
A raw file is really a port. When writing data to a port, it is
efficient to write a list of binaries. It is not needed to flatten a
deep list before writing. On Unix hosts, scatter output, which writes a
set of buffers in one operation, is used when possible. In this way
write(FD, [Bin1, Bin2 | Bin3]) writes 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 open/2 make the file driver
cache data to reduce the number of operating system calls. The function
create_file/2 in the recent example takes 60 seconds without option
delayed_write, which is 2.6 times slower.
As a bad example, create_file_slow/2 without options raw, binary, and
delayed_write, meaning it calls 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.
Warning:
If an error occurs when accessing an open file with module io, the
process handling the file exits. The dead file process can hang if a
process tries to access it later. This will be fixed in a future
release.
SEE ALSO
filename(3erl)
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