file
(kernel)File Interface Module
The module file
provides an interface to the file system.
On operating systems with thread support, it is possible to let
file operations be performed in threads of their own, allowing
other Erlang processes to continue executing in parallel with
the file operations. See the command line flag
+A
in erl(1).
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
filename
) 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...
As returned by file:open/2, a process handling IO protocols.
If VM is in Unicode filename mode, string()
and char()
are allowed to be > 255.
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).
An atom which is named from the POSIX error codes used in Unix, and in the runtime libraries of most C compilers.
Must denote a valid date and time.
Functions
advise/4
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/2
Changes group of a file. See write_file_info/2.
change_mode/2
Changes permissions of a file. See write_file_info/2.
change_owner/2
Changes owner of a file. See write_file_info/2.
change_owner/3
Changes owner and group of a file. See write_file_info/2.
change_time/2
Changes the modification and access times of a file. See write_file_info/2.
change_time/3
Changes the modification and last access times of a file. See write_file_info/2.
close/1
Closes the file referenced by
. 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/1
Reads Erlang terms, separated by '.', from
. Returns one of the following:
{ok, Terms }
The file was successfully read.
{error, atom()}
An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.
{error, {Line , Mod ,
Term }}
An error occurred when interpreting the Erlang terms in
the file. Use format_error/1
to convert
the three-element tuple to an English description of
the error.
Example:
f.txt: {person, "kalle", 25}. {person, "pelle", 30}.
1> file:consult("f.txt").
{ok,[{person,"kalle",25},{person,"pelle",30}]}
copy/2
copy/3
Copies
bytes from
to
.
and
refer
to either filenames or IO devices from e.g. open/2
.
defaults to infinity
, denoting an
infinite number of bytes.
The argument
is a list of possible modes,
see open/2, and defaults to
[].
If both
and
refer to
filenames, the files are opened with [read, binary]
and [write, binary]
prepended to their mode lists,
respectively, to optimize the copy.
If
refers to a filename, it is opened with
read
mode prepended to the mode list before the copy,
and closed when done.
If
refers to a filename, it is opened
with write
mode prepended to the mode list before
the copy, and closed when done.
Returns {ok,
where
is
the number of bytes that actually was copied, which may be
less than
if end of file was
encountered on the source. If the operation fails,
{error,
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/1
Tries to delete the directory
.
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
.
eexist
The directory is not empty.
enoent
The directory does not exist.
enotdir
A component of
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/1
Tries to delete the file
.
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
had an improper type, such as tuple.
Warning!
In a future release, a bad type for the
argument will probably generate
an exception.
eval/1
Reads and evaluates Erlang expressions, separated by '.' (or
',', a sequence of expressions is also an expression), from
. The actual result of the evaluation
is not returned; any expression sequence in the file must be there
for its side effect. Returns one of the following:
ok
The file was read and evaluated.
{error, atom()}
An error occurred when opening the file or reading it.
See open/2
for a list of typical error codes.
{error, {Line , Mod ,
Term }}
An error occurred when interpreting the Erlang
expressions in the file. Use format_error/1
to
convert the three-element tuple to an English description
of the error.
eval/2
The same as eval/1
but the variable bindings
are used in the evaluation. See
erl_eval(3) about
variable bindings.
file_info(Filename) -> {ok, FileInfo} | {error, Reason}
This function is obsolete. Use read_file_info/1
instead.
format_error/1
Given the error reason returned by any function in this module, returns a descriptive string of the error in English.
get_cwd/0
Returns {ok,
, where
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/1
should be of the form
"Letter
:
",
for example "c:". Returns {ok,
or
{error,
, where
is the current
working directory of the drive specified.
This function returns {error, enotsup}
on platforms
which have no concept of current drive (Unix, for example).
Typical error reasons are:
enotsup
The operating system has no concept of drives.
eacces
The drive does not exist.
einval
The format of
is invalid.
list_dir/1
Lists all the files in a directory. Returns
{ok,
if successful.
Otherwise, it returns {error,
.
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
or one of its parent directories.
enoent
The directory does not exist.
make_dir/1
Tries to create the directory
. 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
.
eexist
There is already a file or directory named
.
enoent
A component of
does not exist.
enospc
There is a no space left on the device.
enotdir
A component of
is not a directory.
On some platforms, enoent
is returned instead.
make_link/2
Makes a hard link from
to
, on
platforms that support links (Unix). This function returns
ok
if the link was successfully created, or
{error,
. 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
or
.
eexist
already exists.
enotsup
Hard links are not supported on this platform.
make_symlink/2
This function creates a symbolic link
to
the file or directory
, on platforms that
support
symbolic links (most Unix systems).
need not
exist. This function returns ok
if the link was
successfully created, or {error,
.
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
or
.
eexist
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. binaries).
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/2
Opens the file
in the mode determined
by
, 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 theread/2
,read_line/1
andwrite/2
functions. - Especially if
read_line/1
is to be used on araw
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 operating 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 (Unicode) encoding. Note that the data supplied to file:write or returned by file:read still is byte oriented, this option only denotes how data is actually stored in the disk file.
Depending on the encoding, different methods of reading and writing data is preferred. The default encoding of latin1
implies using this (the file) module for reading and writing data, as the interfaces provided here work with byte-oriented data, while using other (Unicode) encodings makes the io(3) module's get_chars
, get_line
and put_chars
functions more suitable, as they can work with the full Unicode range.
If data is sent to an io_device()
in a format that cannot be converted to the specified encoding, or if data is read by a function that returns data in a format that cannot cope with the character range of the data, an error occurs and the file will be closed.
The allowed values for Encoding
are:
latin1
The default encoding. Bytes supplied to i.e. file:write are written as is on the file, likewise bytes read from the file are returned to i.e. file:read as is. If the io(3) module is used for writing, the file can only cope with Unicode characters up to codepoint 255 (the ISO-latin-1 range).
unicode
or utf8
Characters are translated to and from the UTF-8 encoding before being written to or read from the file. A file opened in this way might be readable using the file:read function, as long as no data stored on the file lies beyond the ISO-latin-1 range (0..255), but failure will occur if the data contains Unicode codepoints beyond that range. The file is best read with the functions in the Unicode aware io(3) module.
Bytes written to the file by any means are translated to UTF-8 encoding before actually being stored on the disk file.
utf16
or {utf16,big}
Works like unicode
, but translation is done to and from big endian UTF-16 instead of UTF-8.
{utf16,little}
Works like unicode
, but translation is done to and from little endian UTF-16 instead of UTF-8.
utf32
or {utf32,big}
Works like unicode
, but translation is done to and from big endian UTF-32 instead of UTF-8.
{utf32,little}
Works like unicode
, but translation is done to and from little endian UTF-32 instead of UTF-8.
The Encoding can be changed for a file "on the fly" by using the io:setopts/2 function, why a file can be analyzed in latin1 encoding for i.e. a BOM, positioned beyond the BOM and then be set for the right encoding before further reading.See the unicode(3) module for functions identifying BOM's.
This option is not allowed on raw
files.
Returns:
{ok, IoDevice }
The file has been opened in the requested mode.
is a reference to the file.
{error, Reason }
The file could not be opened.
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
is linked terminates, the file will
be closed and the process itself will be terminated.
An
returned from this call can be used
as an argument to the IO functions (see
io(3)).
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 compatibility, but should not be
used for new code. Also note that read_write
is not
allowed in a mode list.
Typical error reasons:
enoent
The file does not exist.
eacces
Missing permission for reading the file or searching one of the parent directories.
eisdir
The named file is not a regular file. It may be a directory, a fifo, or a device.
enotdir
A component of the file name is not a directory. On some
platforms, enoent
is returned instead.
enospc
There is a no space left on the device (if write
access was specified).
path_consult/2
Searches the path
(a list of directory
names) until the file
is found.
If
is an absolute filename,
is ignored.
Then reads Erlang terms, separated by '.', from the file.
Returns one of the following:
{ok, Terms , FullName }
The file was successfully read.
is
the full name of the file.
{error, enoent}
The file could not be found in any of the directories in
.
{error, atom()}
An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.
{error, {Line , Mod ,
Term }}
An error occurred when interpreting the Erlang terms in
the file. Use format_error/1
to convert
the three-element tuple to an English description of
the error.
path_eval/2
Searches the path
(a list of directory
names) until the file
is found.
If
is an absolute file name,
is ignored. Then reads
and evaluates Erlang expressions, separated by '.' (or ',', a
sequence of expressions is also an expression), from the file.
The actual result of evaluation is not returned; any
expression sequence in the file must be there for its side
effect. Returns one of the following:
{ok, FullName }
The file was read and evaluated.
is
the full name of the file.
{error, enoent}
The file could not be found in any of the directories in
.
{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/3
Searches the path
(a list of directory
names) until the file
is found.
If
is an absolute file name,
is ignored.
Then opens the file in the mode determined by
.
Returns one of the following:
{ok, IoDevice , FullName }
The file has been opened in the requested mode.
is a reference to the file and
is the full name of the file.
{error, enoent}
The file could not be found in any of the directories in
.
{error, atom()}
The file could not be opened.
path_script/2
Searches the path
(a list of directory
names) until the file
is found.
If
is an absolute file name,
is ignored. Then reads
and evaluates Erlang expressions, separated by '.' (or ',', a
sequence of expressions is also an expression), from the file.
Returns one of the following:
{ok, Value , FullName }
The file was read and evaluated.
is
the full name of the file and
the value of
the last expression.
{error, enoent}
The file could not be found in any of the directories in
.
{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/3
The same as path_script/2
but the variable bindings
are used in the evaluation. See
erl_eval(3) about
variable bindings.
pid2name/1
If
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
must be a local pid) and the file is not
closed.
is the filename in flat string
format.
undefined
In all other cases.
Warning!
This function is intended for debugging only.
position/2
Sets the position of the file referenced by
to
. Returns
{ok,
(as
absolute offset) if successful, otherwise
{error,
.
is
one of the following:
Offset
The same as {bof, Offset}
.
{bof, Offset}
Absolute offset.
{cur, Offset}
Offset from the current position.
{eof, Offset}
Offset from the end of file.
bof | cur | eof
The same as above with Offset
0.
Note that offsets are counted in bytes, not in characters. If the file is opened using some other encoding
than latin1
, one byte does not correspond to one character. Positioning in such a file can only be done to known character boundaries, i.e. to a position earlier retrieved by getting a current position, to the beginning/end of the file or to some other position known to be on a correct character boundary by some other means (typically beyond a byte order mark in the file, which has a known byte-size).
Typical error reasons are:
einval
Either
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/2
Performs a sequence of pread/3
in one operation,
which is more efficient than calling them one at a time.
Returns {ok, [
or
{error,
,
where each
, 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
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/3
Combines position/2
and read/2
in one
operation, which is more efficient than calling them one at a
time. If
has been opened in raw mode,
some restrictions apply:
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/2
Performs a sequence of pwrite/3
in one operation,
which is more efficient than calling them one at a time.
Returns ok
or {error, {
, where
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/3
Combines position/2
and write/2
in one
operation, which is more efficient than calling them one at a
time. If
has been opened in raw mode,
some restrictions apply:
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/2
Reads
bytes/characters from the file
referenced by
. The functions
read/2
, pread/3
and read_line/1
are the only ways to read from a file
opened in raw mode (although they work for normally opened
files, too).
For files where encoding
is set to something else than latin1
, one character might be represented by more than one byte on the file. The parameter Number
always denotes the number of characters read from the file, why the position in the file might be moved a lot more than this number when reading a Unicode file.
Also if encoding
is set to something else than latin1
, the read/3
call will fail if the data contains characters larger than 255, why the io(3) 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
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/1
Returns {ok,
, where
is a binary
data object that contains the contents of
, or
{error,
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/1
Retrieves information about a file. Returns
{ok,
if successful, otherwise
{error,
.
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 record file_info
contains the following fields.
size = integer()
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 = date_time()
The last (local) time the file was read.
mtime = date_time()
The last (local) time the file was written.
ctime = date_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 = integer()
The file permissions as the sum of the following bit values:
On Unix platforms, other bits than those listed above may be set.
links = integer()
Number of links to the file (this will always be 1 for file systems which have no concept of links).
major_device = integer()
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()
Only valid for character devices on Unix. In all other cases, this field is zero.
inode = integer()
Gives the inode
number. On non-Unix file systems,
this field will be zero.
uid = integer()
Indicates the owner of the file. Will be zero for non-Unix file systems.
gid = integer()
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/1
Reads a line of bytes/characters from the file referenced by
. 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 ending the last line, a line with no trailing LF is returned.
The function can be used on files opened in raw
mode. It is however inefficient to use it on raw
files if the file is not opened with the option {read_ahead, Size}
specified, why combining raw
and {read_ahead, Size}
is highly recommended when opening a text file for raw line oriented reading.
If encoding
is set to something else than latin1
, the read_line/1
call will fail if the data contains characters larger than 255, why the io(3) 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/1
This function returns {ok,
if
refers to a symbolic link or
{error,
otherwise.
On platforms that do not support symbolic links, the return
value will be {error,enotsup}
.
Typical error reasons:
einval
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/1
This function works like read_file_info/1
, except that
if
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
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/2
Tries to rename the file
to
.
It can be used to move files (and directories) between
directories, but it is not sufficient to specify
the destination only. The destination file name must also be
specified. For example, if bar
is a normal file 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
or
. On
some platforms, this error is given if either
or
is open.
eexist
is not an empty directory.
On some platforms, also given when
and
are not of the same type.
einval
is a root directory, or
is a sub-directory of
.
eisdir
is a directory, but
is not.
enoent
Source
does not exist.
enotdir
is a directory, but
is not.
exdev
and
are on different file systems.
script/1
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.
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/2
The same as script/1
but the variable bindings
are used in the evaluation. See
erl_eval(3) about
variable bindings.
set_cwd/1
Sets the current working directory of the file server to
. Returns ok
if successful.
Typical error reasons are:
enoent
The directory does not exist.
enotdir
A component of
is not a directory.
On some platforms, enoent
is returned.
eacces
Missing permission for the directory or one of its parents.
badarg
had an improper type,
such as tuple.
Warning!
In a future release, a bad type for the
argument will probably generate an exception.
sync/1
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/1
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/1
Truncates the file referenced by
at
the current position. Returns ok
if successful,
otherwise {error,
.
write/2
Writes
to the file referenced by
. 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,
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/2
Writes the contents of the iodata term
to the file
.
The file is created if it does not
exist. If it exists, the previous contents are
overwritten. Returns ok
, or {error,
.
Typical error reasons are:
enoent
A component of the file name does not exist.
enotdir
A component of the file name is not a directory. On some
platforms, enoent
is returned instead.
enospc
There is a no space left on the device.
eacces
Missing permission for writing the file or searching one of the parent directories.
eisdir
The named file is a directory.
write_file/3
Same as write_file/2
, but takes a third argument
, 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/2
Change file information. Returns ok
if successful,
otherwise {error,
.
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 = date_time()
The last (local) time the file was read.
mtime = date_time()
The last (local) time the file was written.
ctime = date_time()
On Unix, any value give for this field will be ignored (the "ctime" for the file will be set to the current time). On Windows, this field is the new creation time to set for the file.
mode = integer()
The file permissions as the sum of the following bit values:
On Unix platforms, other bits than those listed above may be set.
uid = integer()
Indicates the owner of the file. Ignored for non-Unix file systems.
gid = integer()
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 deniedeagain
- resource temporarily unavailableebadf
- bad file numberebusy
- file busyedquot
- disk quota exceededeexist
- file already existsefault
- bad address in system call argumentefbig
- file too largeeintr
- interrupted system calleinval
- invalid argumenteio
- IO erroreisdir
- illegal operation on a directoryeloop
- too many levels of symbolic linksemfile
- too many open filesemlink
- too many linksenametoolong
- file name too longenfile
- file table overflowenodev
- no such deviceenoent
- no such file or directoryenomem
- not enough memoryenospc
- no space left on deviceenotblk
- block device requiredenotdir
- not a directoryenotsup
- operation not supportedenxio
- no such device or addresseperm
- not ownerepipe
- broken pipeerofs
- read-only file systemespipe
- invalid seekesrch
- no such processestale
- stale remote file handleexdev
- 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 during the file
operation. This gives the same effect as when using the thread
pool, but with larger overhead. Other file operations, for
example sync/1
or close/1
on a huge file, still are
a problem.
For increased performance, raw files are recommended. Raw files uses the file system of the node's host machine. For normal files (non-raw), the file server is used to find the files, and if the node is running its file server as slave to another node's, and the other node runs on some other host machine, they may have different file systems. This is seldom a problem, but you have now been warned.
A normal file is really a process so it can be used as an IO
device (see io
). Therefore when data is written to a
normal file, the sending of the data to the file process, copies
all data that are not binaries. Opening the file in binary mode
and writing binaries is therefore recommended. If the file is
opened on another node, or if the file server runs as slave to
another node's, also binaries are copied.
Caching data to reduce the number of file operations, or rather the number of calls to the file driver, will generally increase performance. The following function writes 4 MBytes in 23 seconds when tested:
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 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 will run slightly
faster, but consume more memory and cause more memory
fragmentation. How much this affects your application is
something that this simple benchmark can not predict.
If the size of each binary is increased to 64 bytes, it will also run slightly faster, but the code will be twice as clumsy. In the current implementation are binaries larger than 64 bytes stored in memory common to all processes and not copied when sent between processes, while these smaller binaries are stored on the process heap and copied when sent like any other term.
So, with a binary size of 68 bytes create_file/2
runs
30 percent slower then with 64 bytes, and will cause much more
memory fragmentation. Note that if the binaries were to be sent
between processes (for example a non-raw file) the results
would probably be completely different.
A raw file is really a port. When writing data to a port, it is
efficient to write a list of binaries. There is no need to
flatten a deep list before writing. On Unix hosts, scatter output,
which writes a set of buffers in one operation, is used when
possible. In this way file:write(FD, [Bin1, Bin2 | Bin3])
will write the contents of the binaries without copying the data
at all except for perhaps deep down in the operating 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 handles the file will exit. The dead
file process might hang if a process tries to access it later.
This will be fixed in a future release.
SEE ALSO
- advise
- change_group
- change_mode
- change_owner
- change_owner-1
- change_time
- change_time-1
- close
- consult
- copy
- copy-1
- del_dir
- delete
- eval
- eval-1
- file_info/1
- format_error
- get_cwd
- get_cwd-1
- list_dir
- make_dir
- make_link
- make_symlink
- native_name_encoding/0
- open
- path_consult
- path_eval
- path_open
- path_script
- path_script-1
- pid2name
- position
- pread
- pread-1
- pwrite
- pwrite-1
- read
- read_file
- read_file_info
- read_line
- read_link
- read_link_info
- rename
- script
- script-1
- set_cwd
- sync
- datasync
- truncate
- write
- write_file
- write_file-1
- write_file_info