| file(3erl) | Erlang Module Definition | file(3erl) |
file - File interface module.
This module provides an interface to the file system.
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, MacOS X and Android 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 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.
deep_list() = [char() | atom() | deep_list()]
fd()
A file descriptor representing a file opened in raw mode.
filename() = string()
See also the documentation of the name_all() type.
filename_all() = string() | binary()
See also the documentation of the name_all() type.
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. See also the documentation of the name_all() type.
name_all() =
string() | atom() | deep_list() | (RawFilename :: binary())
If VM is in Unicode filename mode, characters 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). Null characters (integer value zero) are not allowed in filenames (not even at the end).
posix() =
eacces | eagain | ebadf | ebadmsg | ebusy | edeadlk |
edeadlock | edquot | eexist | efault | efbig | eftype |
eintr | einval | eio | eisdir | eloop | emfile | emlink |
emultihop | enametoolong | enfile | enobufs | enodev |
enolck | enolink | enoent | enomem | enospc | enosr | enostr |
enosys | enotblk | enotdir | enotsup | enxio | eopnotsupp |
eoverflow | eperm | epipe | erange | erofs | espipe | esrch |
estale | etxtbsy | 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
advise(IoDevice, Offset, Length, Advise) -> ok | {error, Reason}
Types:
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:
allocate/3 can be used to preallocate space for a file.
This function only succeeds in platforms that provide this feature.
change_group(Filename, Gid) -> ok | {error, Reason}
Types:
Changes group of a file. See write_file_info/2.
change_mode(Filename, Mode) -> ok | {error, Reason}
Types:
Changes permissions of a file. See write_file_info/2.
change_owner(Filename, Uid) -> ok | {error, Reason}
Types:
Changes owner of a file. See write_file_info/2.
change_owner(Filename, Uid, Gid) -> ok | {error, Reason}
Types:
Changes owner and group of a file. See write_file_info/2.
change_time(Filename, Mtime) -> ok | {error, Reason}
Types:
Changes the modification and access times of a file. See write_file_info/2.
change_time(Filename, Atime, Mtime) -> ok | {error, Reason}
Types:
Changes the modification and last access times of a file. See write_file_info/2.
close(IoDevice) -> ok | {error, Reason}
Types:
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:
Reads Erlang terms, separated by '.', from Filename. Returns one of the following:
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:
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:
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:
Tries to delete directory Dir. The directory must be empty before it can be deleted. Returns ok if successful.
Typical error reasons:
del_dir_r(File) -> ok | {error, Reason}
Types:
Deletes file or directory File. If File is a directory, its contents is first recursively deleted. Returns:
delete(Filename) -> ok | {error, Reason}
Types:
Tries to delete file Filename. Returns ok if successful.
Typical error reasons:
eval(Filename) -> ok | {error, Reason}
Types:
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:
The encoding of Filename can be set by a comment, as described in epp(3erl).
eval(Filename, Bindings) -> ok | {error, Reason}
Types:
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:
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:
Returns {ok, Dir}, where Dir is the current working directory of the file server.
A typical error reason:
get_cwd(Drive) -> {ok, Dir} | {error, Reason}
Types:
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:
list_dir(Dir) -> {ok, Filenames} | {error, Reason}
Types:
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:
list_dir_all(Dir) -> {ok, Filenames} | {error, Reason}
Types:
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:
make_dir(Dir) -> ok | {error, Reason}
Types:
Tries to create directory Dir. Missing parent directories are not created. Returns ok if successful.
Typical error reasons:
make_link(Existing, New) -> ok | {error, Reason}
Types:
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:
make_symlink(Existing, New) -> ok | {error, Reason}
Types:
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:
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:
Opens file File in the mode determined by Modes, which can contain one or more of the following options:
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.
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.
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:
Bytes written to the file by any means are translated to UTF-8 encoding before being stored on the disk file.
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.
Returns:
IoDevice is really the pid of the process that handles the file. This process monitors the process that originally opened the file (the owner process). If the owner process terminates, the file is closed and the process itself terminates too. An IoDevice returned from this call can be used as an argument to the I/O functions (see io(3erl)).
Typical error reasons:
path_consult(Path, Filename) ->
{ok, Terms, FullName} | {error, Reason}
Types:
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:
The encoding of Filename can be set by a comment as described in epp(3erl).
path_eval(Path, Filename) -> {ok, FullName} | {error, Reason}
Types:
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:
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:
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:
path_script(Path, Filename) ->
{ok, Value, FullName} | {error, Reason}
Types:
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:
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:
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:
If Pid is an I/O device, that is, a pid returned from open/2, this function returns the filename, or rather:
position(IoDevice, Location) ->
{ok, NewPosition} | {error, Reason}
Types:
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:
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:
pread(IoDevice, LocNums) -> {ok, DataL} | eof | {error, Reason}
Types:
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:
Combines position/2 and read/2 in one operation, which is more efficient than calling them one at a time.
Location is only allowed to be an integer for raw and ram modes.
The current position of the file after the operation is undefined for raw mode and unchanged for ram mode.
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:
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:
Combines position/2 and write/2 in one operation, which is more efficient than calling them one at a time.
Location is only allowed to be an integer for raw and ram modes.
The current position of the file after the operation is undefined for raw mode and unchanged for ram mode.
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:
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:
Typical error reasons:
read_file(Filename) -> {ok, Binary} | {error, Reason}
Types:
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:
read_file_info(File) -> {ok, FileInfo} | {error, Reason}
read_file_info(File, Opts) -> {ok, FileInfo} | {error, Reason}
Types:
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:
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 that this will break this module's atomicity guarantees as it can race with a concurrent call to write_file_info/1,2.
This option has no effect when the function is given an I/O device instead of a file name. Use open/2 with the raw mode to obtain a file descriptor first.
The record file_info contains the following fields:
On Unix platforms, other bits than those listed above may be set.
Typical error reasons:
read_line(IoDevice) -> {ok, Data} | eof | {error, Reason}
Types:
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:
If the file is opened in binary mode, the read bytes are returned in a binary, otherwise in a list.
Typical error reasons:
read_link(Name) -> {ok, Filename} | {error, Reason}
Types:
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:
read_link_all(Name) -> {ok, Filename} | {error, Reason}
Types:
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:
read_link_info(Name) -> {ok, FileInfo} | {error, Reason}
read_link_info(Name, Opts) -> {ok, FileInfo} | {error, Reason}
Types:
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. Note that this will break this module's atomicity guarantees as it can race with a concurrent call to write_file_info/1,2
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:
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.
Typical error reasons:
script(Filename) -> {ok, Value} | {error, Reason}
Types:
Reads and evaluates Erlang expressions, separated by '.' (or ',', a sequence of expressions is also an expression), from the file.
Returns one of the following:
The encoding of Filename can be set by a comment as described in epp(3erl).
script(Filename, Bindings) -> {ok, Value} | {error, Reason}
Types:
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:
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:
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 non-blocking sendfile, an Erlang fallback using read/2 and gen_tcp:send/2 is used.
The option list can contain the following options:
set_cwd(Dir) -> ok | {error, Reason}
Types:
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:
sync(IoDevice) -> ok | {error, Reason}
Types:
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:
truncate(IoDevice) -> ok | {error, Reason}
Types:
Truncates the file referenced by IoDevice at the current position. Returns ok if successful, otherwise {error, Reason}.
write(IoDevice, Bytes) -> ok | {error, Reason}
Types:
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:
write_file(Filename, Bytes) -> ok | {error, Reason}
Types:
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:
write_file(Filename, Bytes, Modes) -> ok | {error, Reason}
Types:
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:
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:
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:
On Unix platforms, other bits than those listed above may be set.
Typical error reasons:
For increased performance, raw files are recommended.
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.
open/2 can be given the options delayed_write and read_ahead to turn on caching, which will reduce the number of operating system calls and greatly improve performance for small reads and writes. However, the overhead won't disappear completely and it's best to keep the number of file operations to a minimum. As a contrived example, the following function writes 4MB in 2.5 seconds when tested:
create_file_slow(Name) ->
{ok, Fd} = file:open(Name, [raw, write, delayed_write, binary]),
create_file_slow_1(Fd, 4 bsl 20),
file:close(Fd). create_file_slow_1(_Fd, 0) ->
ok; create_file_slow_1(Fd, M) ->
ok = file:write(Fd, <<0>>),
create_file_slow_1(Fd, M - 1).
The following functionally equivalent code writes 128 bytes per call to write/2 and so does the same work in 0.08 seconds, which is roughly 30 times faster:
create_file(Name) ->
{ok, Fd} = file:open(Name, [raw, write, delayed_write, binary]),
create_file_1(Fd, 4 bsl 20),
file:close(Fd),
ok. create_file_1(_Fd, 0) ->
ok; create_file_1(Fd, M) when M >= 128 ->
ok = file:write(Fd, <<0:(128)/unit:8>>),
create_file_1(Fd, M - 128); create_file_1(Fd, M) ->
ok = file:write(Fd, <<0:(M)/unit:8>>),
create_file_1(Fd, M - 1).
When writing data it's generally more efficient to write a list of binaries rather than a list of integers. 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.
| kernel 7.2 | Ericsson AB |