# file

### MODULE

file

### MODULE SUMMARY

File Interface Module

### DESCRIPTION

The module `file` provides an interface to the file system.

Most functions have a name argument such as a file name or directory name, which is either an atom, a string, or a deep list of characters and atoms. A path is a list of directory names. If the functions are successful, they return `ok`, or `{ok, Value}`.

If an error occurs, the return value has the format `{error, Reason}`. `Reason` is an atom which is named from the Posix error codes used in Unix, and in the runtime libraries of most C compilers. In the following descriptions of functions, the most typical error codes are listed. By matching the error code, applications can use this information for error recovery. To produce a readable error string, use `format_error/1`.

On operating systems with thread support (Solaris and Windows), 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 option `+A` in the manual page for `erl`.

### EXPORTS

`change_group(Filename, Gid)`

Change group of a file. See `write_file_info/2`.

`change_owner(Filename, Uid)`

Change owner of a file. See `write_file_info/2`.

`change_owner(Filename, Uid, Gid)`

Change owner and group of a file. See `write_file_info/2`.

`change_time(Filename, Mtime)`

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

`change_time(Filename, Mtime, Atime)`

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

`close(IoDevice)`

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)`

Opens file `Filename` and reads all the Erlang terms in it. Returns one of the following:

`{ok, TermList}`
  The file was successfully read.
`{error, Atom}`
  An error occurred when opening the file or reading it. The `Atom` is a Posix error code. See the description of `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 the `format_error/1` function to convert the three-element tuple to an English description of the error.

`copy(Source, Destination)`

Copies the contents of `Source` to `Destination`. `Source` and `Destination` are either filenames or open file references from e.g `open/2`.

The same as `copy/3` but with infinite byte count.

`copy(Source, Destination, ByteCount)`

Copies `ByteCount` bytes from `Source` to `Destination`. `Source` and `Destination` are either filenames or open file references from e.g `open/2`.

If `Source` is a tuple `{Filename, ModeList}` where `ModeList` is a mode list as for `open/2`, the source is opened with `read` mode prepended to the mode list before the copy, and closed when done.

If `Source` is a filename, it is interpreted as `{Source, []}`. The file is thereby read from the beginning.

If `Destination` is a tuple `{Filename, ModeList}` where `ModeList` is a mode list as for `open/2`, the destination is opened with `write` mode prepended to the mode list before the copy, and closed when done.

If `Destination` is a filename, it is interpreted as `{Destination, []}`. This implies that the previous file contents are overwritten.

If both `Source` and `Destination` are filenames or `{Filename, ModeList}` tuples, the files are opened with `[raw, read, binary]` and `[raw, write, binary]` prepended to their mode lists, respectively, to optimize the copy.

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(DirName)`

Tries to delete the directory `DirName`. 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 `DirName`.
`eexist`
  The directory is not empty.
`enoent`
  The directory does not exist.
`enotdir`
  A component of `DirName` 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)`

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.

`eval(Filename)`

Opens the file `Filename` and evaluates all the expression sequences in it. It returns one of the following:

`ok`
  The file was read and evaluated. The actual result of the evaluation is not returned; any expression sequence in the file must be there for its side effect.
`{error, Atom}`
  An error occurred when opening the file or reading it. The `Atom` is a Posix error code. See the description of `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 the `format_error/1` function to convert the three-element tuple to an English description of the error.

`file_info(Filename)`

| This function is obsolete. Use `read_file_info` instead.

Retrieves information about a file. Returns `{ok, FileInfo}` if successful, otherwise `{error, Reason}`. `FileInfo` is a tuple with the following fields:

              {Size,Type,Access,AccessTime,ModifyTime,UnUsed1,UnUsed2}

`Size`
  The size of the file in bytes.
`Type`
  The type of file which is `device`, `directory`, `regular`, or `other`.
`Access`
  The current system access to the file, which is one of the atoms `read`, `write`, `read_write`, or `none`.
`AccessTime`
  The last time the file was read, shown in the format `{Year, Month, Day, Hour, Minute, Second}`.
`ModifyTime`
  The last time the file was written, shown in the format `{Year, Month, Day, Hour, Minute, Second}`.
`UnUsed1, UnUsed2`
  These fields are not used, but reserved for future expansion. They probably contain `unused`.

Typical error reasons: Same as for `read_file_info/1`.

`format_error(ErrorDescriptor)`

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

`get_cwd()`

Returns `{ok, CurDir}`, where `CurDir` (a string) 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:

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

`get_cwd(Drive)`

`Drive` should be of the form "`Letter``:`", for example "c:". Returns `{ok, CurDir}` or `{error, Reason}`, where `CurDir` (a string) 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.

`ipread_s32bu_p32bu(IoDevice, Location, MaxSize)`

Specialised indirect read function for Dets. Equivalent to `pread/3` of a header from `Location` followed by another `pread/3` of the buffer specified by the header.

| This function is not intended to be used by others than Dets. It is therefore not well documented.

`list_dir(DirName)`

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

Typical error reasons are:

`eacces`
  Missing search or write permissions for `DirName` or one of its parent directories.
`enoent`
  The directory does not exist.

`make_dir(DirName)`

Tries to create the directory `DirName`. 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 `DirName`.
`eexist`
  There is already a file or directory named `DirName`.
`enoent`
  A component of `DirName` does not exist.
`enospc`
  There is a no space left on the device.
`enotdir`
  A component of `DirName` is not a directory. On some platforms, `enoent` is returned instead.

`make_link(Existing, New)`

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}` will be 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)`

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}` will be returned.

Typical error reasons:

`eacces`
  Missing read or write permissions for the parent directories of `Existing` or `New`.
`eexist`
  `new` already exists.
`enotsup`
  Symbolic links are not supported on this platform.

`open(Filename, ModeList)`

Opens the file `Filename` in the mode determined by `ModeList`. `ModeList` 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. Otherwise, it is truncated (unless combined with `read`).
`append`
  The file will be opened for writing, and it will be created it does not exist. Every write operation to a file openeded 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` and `write/2` functions.
  - 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`
  This option can only be used if the `raw` option is specified as well. When specified, read operations on the file using the `read/2` function 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.
  E.g 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.
  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)

If both `read` and `write` are specified, the file is created if it does not exists. It is not truncated if it exists.

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.

A `file descriptor` is the Pid of the process which handles the file. The file process is linked to the process which originally opened the file. If any process to which the file process is linked terminates, the file will be closed by the file process and the process itself will be terminated. The file descriptor returned from this call can be used as an argument to the I/O functions (see `io`).

| In previous versions of `file`, modes were given as on 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 a directory.
`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)`

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. The file is opened and all the terms in it are read. The function returns one of the following:

`{ok, TermList, FullName}`
  The file was successfully read. `FullName` is the full name of the file which was opened and read.
`{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. The `Atom` is a Posix error code. See the description of `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 the `format_error/1` function to convert the three-element tuple to an English description of the error.

`path_eval(Path, Filename)`

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. The file is opened and all the expression sequences in it are evaluated. The function returns one of the following:

`{ok, FullName}`
  The file was read. `FullName` is the full name of the file which was opened and evaluated.
`{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. The `Atom` is a Posix error code. See the description of `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 the `format_error/1` function to convert the three-element tuple to an English description of the error.

`path_open(Path, Filename, Mode)`

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. The function returns one of the following:

`{ok, IoDevice, FullName}`
  The file was opened in the requested mode. `IoDevice` is a reference to the file and `FullName` is the full name of the file which was opened.
`{error, enoent}`
  `Filename` was not found in the path.
`{error, Reason}`
  There was an error opening `Filename`.

`position(IoDevice, Location)`

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:

`{bof, Offset}`
  Absolute offset
`{cur, Offset}`
  Offset from the current position
`{eof, Offset}`
  Offset from the end of file
`Integer`
  The same as `{bof, Integer}`
`bof || cur || eof`
  The same as above with `Offset` 0.

Typical error reasons are:

`einval`
  Either the `Location` was illegal, or it evaluated to a negative offset in the file. Note that if the resulting position is a negative value you will get an error but after the call it is undefined where the file position will be.

`pread(IoDevice, [{Location, Number}, ...])`

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 `Data` 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.

`pread(IoDevice, Location, Number)`

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.

`pwrite(IoDevice, [{Location, Bytes}, ...])`

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

`pwrite(IoDevice, Location, Bytes)`

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.

`read(IoDevice, Number)`

Reads `Number` bytes from the file described 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). Returns:

`{ok, ListOrBinary}`
  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 the number of bytes requested if the end of the file is reached.
`eof`
  `eof` is returned if the `Number` was greater than zero and end of file was reached before anything at all could be read.
`{error, Reason}`
  A Posix error code will be returned if an error occurred.
  Typical error reasons:
  `ebadf`
    The file is not opened for reading.

`read_file(Filename)`

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)`

Retrieves information about a file. Returns `{ok, FileInfo}` if successful, otherwise `{error, Reason}`. `FileInfo` is a record. Its definition can be found by including `file.hrl` from the kernel application:

              -include_lib("kernel/include/file.hrl").

The record contains the following fields.

`size`
  Size of file in bytes.
`type`
  The type of the file which can be `device`, `directory`, `regular`, or `other`.
`access`
  The current system access to the file, which is one of the atoms `read`, `write`, `read_write`, or `none`.
`atime`
  The last (local) time the file was read, in the format `{{Year, Month, Day}, {Hour, Minute, Second}}`.
`mtime`
  The last (local) time the file was written, in the format `{{Year, Month, Day}, {Hour, Minute, Second}}`.
`ctime`
  The interpretation of this time field depends on the operating system. On Unix, it is the last time the file or or the inode was changed. In Windows, it is the create time. The format is `{{Year, Month, Day}, {Hour, Minute, Second}}`.
`mode`
  An integer which gives the file permissions as a 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`
  Number of links to the file (this will always be 1 for file systems which have no concept of links).
`major_device`
  An integer which 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`
  Only valid for character devices on Unix. In all other cases, this field is zero.
`inode`
  An integer which gives the `inode` number. On non-Unix file systems, this field will be zero.
`uid`
  An integer which indicates the owner of the file. Will be zero for non-Unix file systems.
`gid`
  An integer which 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_link(Linkname)`

This function returns `{ok,Filename}` if `Linkname` 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(Filename)`

This function works like `read_file_info/1`, except that if `Filename` 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 `Filename` is not a symbolic link, this function returns exactly the same result as `read_file_info/1`. On platforms that do not support symbolic link, this function is always equvivalent to `read_file_info/1`.

`rename(Source, Destination)`

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:

`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.

`set_cwd(DirName)`

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

Typical error reasons are:

`enoent`
  The directory does not exist.
`enotdir`
  A component of `DirName` is not a directory. On some platforms, `enoent` is returned.
`eacces`
  Missing permission for the directory or one of its parents.

`sync(IoDevice)`

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)`

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

`write(IoDevice, Bytes)`

Writes `Bytes` (possibly a deep list of characters, or a binary) to the file described 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).

This function returns `ok` if successful, and `{error, Reason}` otherwise.

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, Binary)`

Writes the contents of the binary data object `Binary` to the file `Filename`. The file is created if it does not exist already. 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_info(Filename, FileInfo)`

Change file information. Returns `ok` if successful, otherwise `{error, Reason}`. `FileInfo` is a record. Its definition can be found by including `file.hrl` from the kernel application:

              -include_lib("kernel/include/file.hrl").

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

`atime`
  The last (local) time the file was read, in the format `{{Year, Month, Day}, {Hour, Minute, Second}}`.
`mtime`
  The last (local) time the file was written, in the format `{{Year, Month, Day}, {Hour, Minute, Second}}`.
`ctime`
  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. The format is `{{Year, Month, Day}, {Hour, Minute, Second}}`.
`mode`
  An integer which gives the file permissions as a 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`
  An integer which indicates the owner of the file. Ignored for non-Unix file systems.
`gid`
  An integer which 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
  I/O error
eisdir
  illegal operation on a directory
eloop
  too many levels of symbolic links
emfile
  too many open files
emlink
  too many links
enametoolong
  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 is 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 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 is 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 on my machine:

          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 functionallyequivalent 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`.

### 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)

### AUTHORS

Robert Virding - support@erlang.ericsson.se
Claes Wikström - support@erlang.ericsson.se
Björn Gustavsson - support@erlang.ericsson.se

kernel 2.7.2
Copyright © 1991-2002 Ericsson Utvecklings AB