file(3)

MODULE
file - File Interface Module

DESCRIPTION

The module file provides an interface to the file system.

On operating systems with thread support, it is possible to let file operations be 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).

DATA TYPES
iodata() = iolist() | binary()
  iolist() = [char() | binary() | iolist()]
io_device()
  as returned by file:open/2, a process handling IO protocols
name() = string() | atom() | DeepList
  DeepList = [char() | atom() | DeepList]
posix()
  an atom which is named from the POSIX error codes used in
  Unix, and in the runtime libraries of most C compilers
ext_posix() = posix() | badarg
time() = {{Year, Month, Day}, {Hour, Minute, Second}}
  Year = Month = Day = Hour = Minute = Second = int()
  Must denote a valid date and time

EXPORTS

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

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

    Changes group of a file. See write_file_info/2.

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

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

    Changes owner of a file. See write_file_info/2.

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

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

    Changes owner and group of a file. See write_file_info/2.

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

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

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

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

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

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

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

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

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

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

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

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

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

        {ok, Terms}
            The file was successfully read.

        {error, atom()}
            An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.

        {error, {Line, Mod, Term}}
            An error occurred when interpreting the Erlang terms in the file. Use 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(Source, Destination) ->
copy(Source, Destination, ByteCount) -> {ok, BytesCopied} | {error, Reason}

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

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

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

    If both Source and Destination refer to filenames, the files are opened with [read, binary] and [write, binary] prepended to their mode lists, respectively, to 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 actually was copied, which may be less than ByteCount if end of file was encountered on the source. If the operation fails, {error, Reason} is returned.

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

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

    Types
        Dir = name()
        Reason = ext_posix()

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

    Typical error reasons are:

        eacces
            Missing search or write permissions for the parent directories of Dir.

        eexist
            The directory is not empty.

        enoent
            The directory does not exist.

        enotdir
            A component of Dir is not a directory. On some platforms, enoent is returned instead.

        einval
            Attempt to delete the current directory. On some platforms, eacces is returned instead.

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

    Types
        Filename = name()
        Reason = ext_posix()

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

    Typical error reasons are:

        enoent
            The file does not exist.

        eacces
            Missing permission for the file or one of its parents.

        eperm
            The file is a directory and the user is not super-user.

        enotdir
            A component of the file name is not a directory. On some platforms, enoent is returned instead.

        einval
            Filename had an improper type, such as tuple.

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

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

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

    Reads and evaluates Erlang expressions, separated by '.' (or ',', a sequence of expressions is also an expression), from Filename. The actual result of the evaluation is not returned; any expression sequence in the file must be there for its side effect. Returns one of the following:

        ok  The file was read and evaluated.

        {error, atom()}
            An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.

        {error, {Line, Mod, Term}}
            An error occurred when interpreting the Erlang expressions in the file. Use format_error/1 to convert the three-element tuple to an English description of the error.

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

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

    The same as eval/1 but the variable bindings 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(Reason) -> Chars

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

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

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

    Types
        Dir = string()
        Reason = posix()

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

    Note:
        In rare circumstances, this function can fail on Unix. It may happen if read permission does not exist for the parent directories of the current directory.

    Typical error reasons are:

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

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

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

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

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

    Typical error reasons are:

        enotsup
            The operating system have no concept of drives.

        eacces
            The drive does not exist.

        einval
            The format of Drive is invalid.

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

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

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

    Typical error reasons are:

        eacces
            Missing search or write permissions for Dir or one of its parent directories.

        enoent
            The directory does not exist.

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

    Types
        Dir = name()
        Reason = ext_posix()

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

    Typical error reasons are:

        eacces
            Missing search or write permissions for the parent directories of Dir.

        eexist
            There is already a file or directory named Dir.

        enoent
            A component of Dir does not exist.

        enospc
            There is a no space left on the device.

        enotdir
            A component of Dir is not a directory. On some platforms, enoent is returned instead.

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

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

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

    Typical error reasons:

        eacces
            Missing read or write permissions for the parent directories of Existing or New.

        eexist
            New already exists.

        enotsup
            Hard links are not supported on this platform.

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

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

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

    Typical error reasons:

        eacces
            Missing read or write permissions for the parent directories of Name1 or Name2.

        eexist
            Name2 already exists.

        enotsup
            Symbolic links are not supported on this platform.

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

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

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

        read
            The file, which must exist, is opened for reading.

        write
            The file is opened for writing. It is created if it does not exist. If the file exists, and if write is not combined with read, the file will be truncated.

        append
            The file will be opened for writing, and it will be created if it does not exist. Every write operation to a file opened with append will take place at the end of the file.

        raw The raw option allows faster access to a file, because no Erlang process is needed to handle the file. However, a file opened in this way has the following limitations:

            *   The functions in the io module cannot be used, because they can only talk to an Erlang process. Instead, use the read/2, read_line/1 and write/2 functions.

            *   Especially if read_line/1 is to be used on a raw file, it is recommended to combine this option with the {read_ahead, Size} option as line oriented I/O is inefficient without buffering.

            *   Only the Erlang process which opened the file can use it.

            *   A remote Erlang file server cannot be used; the computer on which the Erlang node is running must have access to the file system (directly or through NFS).

        binary
            When this option has been given, read operations on the file will return binaries rather than lists.

        {delayed_write, Size, Delay}
            If this option is used, the data in subsequent write/2 calls is buffered until there are at least Size bytes buffered, or until the oldest buffered data is Delay milliseconds old. Then all buffered data is written in one operating system call. The buffered data is also flushed before some other file operation than write/2 is executed.
            The purpose of this option is to increase performance by reducing the number of 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. IoDevice is a reference to the file.

        {error, Reason}
            The file could not be opened.

    IoDevice is really the pid of the process which handles the file. This process is linked to the process which originally opened the file. If any process to which the IoDevice is linked terminates, the file will be closed and the process itself will be terminated. An IoDevice returned from this call can be used as an argument to the IO functions (see io(3)).

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

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

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

        {ok, Terms, FullName}
            The file was successfully read. FullName is the full name of the file.

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

        {error, atom()}
            An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.

        {error, {Line, Mod, Term}}
            An error occurred when interpreting the Erlang terms in the file. Use format_error/1 to convert the three-element tuple to an English description of the error.

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

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

    Searches the path Path (a list of directory names) until the file Filename is found. If Filename is an absolute file name, Path is ignored. Then reads and evaluates Erlang expressions, separated by '.' (or ',', a sequence of expressions is also an expression), from the file. The actual result of evaluation is not returned; any expression sequence in the file must be there for its side effect. Returns one of the following:

        {ok, FullName}
            The file was read and evaluated. FullName is the full name of the file.

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

        {error, atom()}
            An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.

        {error, {Line, Mod, Term}}
            An error occurred when interpreting the Erlang expressions in the file. Use format_error/1 to convert the three-element tuple to an English description of the error.

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

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

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

        {ok, IoDevice, FullName}
            The file has been opened in the requested mode. IoDevice is a reference to the file and FullName is the full name of the file.

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

        {error, atom()}
            The file could not be opened.

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

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

    Searches the path Path (a list of directory names) until the file Filename is found. If Filename is an absolute file name, Path is ignored. Then reads and evaluates Erlang expressions, separated by '.' (or ',', a sequence of expressions is also an expression), from the file. Returns one of the following:

        {ok, Value, FullName}
            The file was read and evaluated. FullName is the full name of the file and Value the value of the last expression.

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

        {error, atom()}
            An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.

        {error, {Line, Mod, Term}}
            An error occurred when interpreting the Erlang expressions in the file. Use format_error/1 to convert the three-element tuple to an English description of the error.

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

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

    The same as path_script/2 but the variable bindings Bindings are used in the evaluation. See erl_eval(3) about variable bindings.

pid2name(Pid) -> string() | undefined

    Types
        Pid = pid()

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

        {ok, Filename}
            If this node's file server is not a slave, the file was opened by this node's file server, (this implies that Pid must be a local pid) and the file is not closed. Filename is the filename in flat string format.

        undefined
            In all other cases.

    Warning:
        This function is intended for debugging only.

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

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

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

        Offset
            The same as {bof, Offset}.

        {bof, Offset}
            Absolute offset.

        {cur, Offset}
            Offset from the current position.

        {eof, Offset}
            Offset from the end of file.

        bof | cur | eof
            The same as above with Offset 0.

    Note that offsets are counted in bytes, not in characters. If the file is opened using some other encoding than latin1, one byte does not correspond to one character. Positioning in such a file can only be done to known character boundaries, i.e. to a position earlier retrieved by getting a current position, to the beginning/end of the file or to some other position known to be on a correct character boundary by some other means (typically beyond a byte order mark in the file, which has a known byte-size).

    Typical error reasons are:

        einval
            Either Location was illegal, or it evaluated to a negative offset in the file. Note that if the resulting position is a negative value, the result is an error, and after the call the file position is undefined.

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

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

    Performs a sequence of pread/3 in one operation, which is more efficient than calling them one at a time. Returns {ok, [Data, ...]} or {error, Reason}, where each Data, the result of the 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(IoDevice, Location, Number) -> {ok, Data} | eof | {error, Reason}

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

    Also if encoding is set to something else than latin1, the read/3 call will fail if the data contains characters larger than 255, why the io(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 Number>0 and end of file was reached before anything at all could be read.

        {error, Reason}
            An error occurred.

    Typical error reasons:

        ebadf
            The file is not opened for reading.

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

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

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

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

    Typical error reasons:

        enoent
            The file does not exist.

        eacces
            Missing permission for reading the file, or for searching one of the parent directories.

        eisdir
            The named file is a directory.

        enotdir
            A component of the file name is not a directory. On some platforms, enoent is returned instead.

        enomem
            There is not enough memory for the contents of the file.

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

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

    Retrieves information about a file. Returns {ok, FileInfo} if successful, otherwise {error, Reason}. FileInfo is a 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 = int()
            Size of file in bytes.

        type = device | directory | regular | other
            The type of the file.

        access = read | write | read_write | none
            The current system access to the file.

        atime = time()
            The last (local) time the file was read.

        mtime = time()
            The last (local) time the file was written.

        ctime = time()
            The interpretation of this time field depends on the operating system. On Unix, it is the last time the file or the inode was changed. In Windows, it is the create time.

        mode = int()
            The file permissions as the sum of the following bit values:

            8#00400
                read permission: owner

            8#00200
                write permission: owner

            8#00100
                execute permission: owner

            8#00040
                read permission: group

            8#00020
                write permission: group

            8#00010
                execute permission: group

            8#00004
                read permission: other

            8#00002
                write permission: other

            8#00001
                execute permission: other

            16#800
                set user id on execution

            16#400
                set group id on execution
        On Unix platforms, other bits than those listed above may be set.

        links = int()
            Number of links to the file (this will always be 1 for file systems which have no concept of links).

        major_device = int()
            Identifies the file system where the file is located. In Windows, the number indicates a drive as follows: 0 means A:, 1 means B:, and so on.

        minor_device = int()
            Only valid for character devices on Unix. In all other cases, this field is zero.

        inode = int()
            Gives the inode number. On non-Unix file systems, this field will be zero.

        uid = int()
            Indicates the owner of the file. Will be zero for non-Unix file systems.

        gid = int()
            Gives the group that the owner of the file belongs to. Will be zero for non-Unix file systems.

    Typical error reasons:

        eacces
            Missing search permission for one of the parent directories of the file.

        enoent
            The file does not exist.

        enotdir
            A component of the file name is not a directory. On some platforms, enoent is returned instead.

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

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

    Reads a line of bytes/characters from the file referenced by IoDevice. Lines are defined to be delimited by the linefeed (LF, \n) character, but any carriage return (CR, \r) followed by a newline is also treated as a single LF character (the carriage return is silently ignored). The line is returned including the LF, but excluding any CR immediately followed by a LF. This behaviour is consistent with the behaviour of io:get_line/2. If end of file is reached without any LF 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(Name) -> {ok, Filename} | {error, Reason}

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

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

    Typical error reasons:

        einval
            Linkname does not refer to a symbolic link.

        enoent
            The file does not exist.

        enotsup
            Symbolic links are not supported on this platform.

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

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

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

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

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

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

    Tries to rename the file Source to Destination. It can be used to move files (and directories) between directories, but it is not sufficient to specify the 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 Source or Destination. On some platforms, this error is given if either Source or Destination is open.

        eexist
            Destination is not an empty directory. On some platforms, also given when Source and Destination are not of the same type.

        einval
            Source is a root directory, or Destination is a sub-directory of Source.

        eisdir
            Destination is a directory, but Source is not.

        enoent
            Source does not exist.

        enotdir
            Source is a directory, but Destination is not.

        exdev
            Source and Destination are on different file systems.

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

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

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

        {ok, Value}
            The file was read and evaluated. Value is the value of the last expression.

        {error, atom()}
            An error occurred when opening the file or reading it. See open/2 for a list of typical error codes.

        {error, {Line, Mod, Term}}
            An error occurred when interpreting the Erlang expressions in the file. Use format_error/1 to convert the three-element tuple to an English description of the error.

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

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

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

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

    Types
        Dir = name()
        Reason = ext_posix()

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

    Typical error reasons are:

        enoent
            The directory does not exist.

        enotdir
            A component of Dir is not a directory. On some platforms, enoent is returned.

        eacces
            Missing permission for the directory or one of its parents.

        badarg
            Filename had an improper type, such as tuple.

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

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

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

    Makes sure that any buffers kept by the operating system (not by the Erlang runtime system) are written to disk. On some platforms, this function might have no effect.

    Typical error reasons are:

        enospc
            Not enough space left to write the file.

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

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

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

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

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

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

    If the file is opened with encoding set to something else than latin1, each byte written might result in several bytes actually being written to the file, as the byte range 0..255 might represent anything between one and four bytes depending on value and UTF encoding type.

    Typical error reasons are:

        ebadf
            The file is not opened for writing.

        enospc
            There is a no space left on the device.

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

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

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

    Typical error reasons are:

        enoent
            A component of the file name does not exist.

        enotdir
            A component of the file name is not a directory. On some platforms, enoent is returned instead.

        enospc
            There is a no space left on the device.

        eacces
            Missing permission for writing the file or searching one of the parent directories.

        eisdir
            The named file is a directory.

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

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

    Same as write_file/2, but takes a third argument Modes, a list of possible modes, see open/2. The mode flags binary and write are implicit, so they should not be used.

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

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

    Change file information. Returns ok if successful, otherwise {error, Reason}. FileInfo is a record file_info, defined in the Kernel include file file.hrl. Include the following directive in the module from which the function is called:
    -include_lib("kernel/include/file.hrl").

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

        atime = time()
            The last (local) time the file was read.

        mtime = time()
            The last (local) time the file was written.

        ctime = time()
            On Unix, any value give for this field will be ignored (the "ctime" for the file will be set to the current time). On Windows, this field is the new creation time to set for the file.

        mode = int()
            The file permissions as the sum of the following bit values:

            8#00400
                read permission: owner

            8#00200
                write permission: owner

            8#00100
                execute permission: owner

            8#00040
                read permission: group

            8#00020
                write permission: group

            8#00010
                execute permission: group

            8#00004
                read permission: other

            8#00002
                write permission: other

            8#00001
                execute permission: other

            16#800
                set user id on execution

            16#400
                set group id on execution
        On Unix platforms, other bits than those listed above may be set.

        uid = int()
            Indicates the owner of the file. Ignored for non-Unix file systems.

        gid = int()
            Gives the group that the owner of the file belongs to. Ignored non-Unix file systems.

    Typical error reasons:

        eacces
            Missing search permission for one of the parent directories of the file.

        enoent
            The file does not exist.

        enotdir
            A component of the file name is not a directory. On some platforms, enoent is returned instead.

POSIX ERROR CODES

    *   eacces - permission denied

    *   eagain - resource temporarily unavailable

    *   ebadf - bad file number

    *   ebusy - file busy

    *   edquot - disk quota exceeded

    *   eexist - file already exists

    *   efault - bad address in system call argument

    *   efbig - file too large

    *   eintr - interrupted system call

    *   einval - invalid argument

    *   eio - IO error

    *   eisdir - illegal operation on a directory

    *   eloop - too many levels of symbolic links

    *   emfile - too many open files

    *   emlink - too many links

    *   enametoolong - file name too long

    *   enfile - file table overflow

    *   enodev - no such device

    *   enoent - no such file or directory

    *   enomem - not enough memory

    *   enospc - no space left on device

    *   enotblk - block device required

    *   enotdir - not a directory

    *   enotsup - operation not supported

    *   enxio - no such device or address

    *   eperm - not owner

    *   epipe - broken pipe

    *   erofs - read-only file system

    *   espipe - invalid seek

    *   esrch - no such process

    *   estale - stale remote file handle

    *   exdev - cross-domain link

PERFORMANCE

Some operating system file operations, for example 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 =&lt; N ->
    create_file(FD, M, M + 1024, []),
    create_file(FD, M + 1024, N);
create_file(FD, M, N) ->
    create_file(FD, M, N, []).

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

Note:
    Trust only your own benchmarks. If the list length in create_file/2 above is increased, it will run slightly faster, but consume more memory and cause more memory fragmentation. How much this affects your application is something that this simple benchmark can not 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

filename(3)