In a future release, a bad type for the Filename argument will probably generate an exception.
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).
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
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:
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:
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:
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:
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.
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.
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:
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:
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:
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:
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:
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:
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:
Returns:
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)).
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:
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:
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:
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:
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:
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:
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:
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:
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. This function is the only way to read from a file opened in raw mode (although it works 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:
Typical error reasons:
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:
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.
Typical error reasons:
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:
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.
Renaming of open files is not allowed on most platforms (see eacces below).
Typical error reasons:
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:
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:
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:
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:
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:
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.
Typical error reasons:
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]).
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.
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.