Struct std::fs::OpenOptions

pub struct OpenOptions(/* private fields */);

Options and flags which can be used to configure how a file is opened.

This builder exposes the ability to configure how a File is opened and what operations are permitted on the open file. The File::open and File::create methods are aliases for commonly used options using this builder.

Generally speaking, when using OpenOptions, you’ll first call OpenOptions::new, then chain calls to methods to set each option, then call OpenOptions::open, passing the path of the file you’re trying to open. This will give you a io::Result with a File inside that you can further operate on.

Examples

Opening a file to read:

use std::fs::OpenOptions;

let file = OpenOptions::new().read(true).open("foo.txt");

Opening a file for both reading and writing, as well as creating it if it doesn’t exist:

use std::fs::OpenOptions;

let file = OpenOptions::new()
            .read(true)
            .write(true)
            .create(true)
            .open("foo.txt");

Implementations

impl OpenOptions

pub fn new() -> Self

Creates a blank new set of options ready for configuration.

All options are initially set to false.

Examples

use std::fs::OpenOptions;

let mut options = OpenOptions::new();
let file = options.read(true).open("foo.txt");

pub fn read(&mut self, read: bool) -> &mut Self

Sets the option for read access.

This option, when true, will indicate that the file should be read-able if opened.

Examples

use std::fs::OpenOptions;

let file = OpenOptions::new().read(true).open("foo.txt");

pub fn write(&mut self, write: bool) -> &mut Self

Sets the option for write access.

This option, when true, will indicate that the file should be write-able if opened.

If the file already exists, any write calls on it will overwrite its contents, without truncating it.

Examples

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true).open("foo.txt");

pub fn append(&mut self, append: bool) -> &mut Self

Sets the option for the append mode.

This option, when true, means that writes will append to a file instead of overwriting previous contents. Note that setting .write(true).append(true) has the same effect as setting only .append(true).

For most filesystems, the operating system guarantees that all writes are atomic: no writes get mangled because another process writes at the same time.

One maybe obvious note when using append-mode: make sure that all data that belongs together is written to the file in one operation. This can be done by concatenating strings before passing them to write(), or using a buffered writer (with a buffer of adequate size), and calling flush() when the message is complete.

If a file is opened with both read and append access, beware that after opening, and after every write, the position for reading may be set at the end of the file. So, before writing, save the current position (using seek(SeekFrom::Current(0))), and restore it before the next read.

Note

This function doesn’t create the file if it doesn’t exist. Use the OpenOptions::create method to do so.

Examples

use std::fs::OpenOptions;

let file = OpenOptions::new().append(true).open("foo.txt");

pub fn truncate(&mut self, truncate: bool) -> &mut Self

Sets the option for truncating a previous file.

If a file is successfully opened with this option set it will truncate the file to 0 length if it already exists.

The file must be opened with write access for truncate to work.

Examples

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true).truncate(true).open("foo.txt");

pub fn create(&mut self, create: bool) -> &mut Self

Sets the option to create a new file, or open it if it already exists.

In order for the file to be created, OpenOptions::write or OpenOptions::append access must be used.

See also std::fs::write() for a simple function to create a file with a given data.

Examples

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true).create(true).open("foo.txt");

pub fn create_new(&mut self, create_new: bool) -> &mut Self

Sets the option to create a new file, failing if it already exists.

No file is allowed to exist at the target location, also no (dangling) symlink. In this way, if the call succeeds, the file returned is guaranteed to be new.

This option is useful because it is atomic. Otherwise between checking whether a file exists and creating a new one, the file may have been created by another process (a TOCTOU race condition / attack).

If .create_new(true) is set, .create() and .truncate() are ignored.

The file must be opened with write or append access in order to create a new file.

Examples

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true)
                             .create_new(true)
                             .open("foo.txt");

pub fn open<P: AsRef<Path>>(&self, path: P) -> Result<File>

Opens a file at path with the options specified by self.

Errors

This function will return an error under a number of different circumstances. Some of these error conditions are listed here, together with their io::ErrorKind. The mapping to io::ErrorKinds is not part of the compatibility contract of the function.

• NotFound: The specified file does not exist and neither create or create_new is set.
• NotFound: One of the directory components of the file path does not exist.
• PermissionDenied: The user lacks permission to get the specified access rights for the file.
• PermissionDenied: The user lacks permission to open one of the directory components of the specified path.
• AlreadyExists: create_new was specified and the file already exists.
• InvalidInput: Invalid combinations of open options (truncate without write access, no access mode set, etc.).

The following errors don’t match any existing io::ErrorKind at the moment:

• One of the directory components of the specified file path was not, in fact, a directory.
• Filesystem-level errors: full disk, write permission requested on a read-only file system, exceeded disk quota, too many open files, too long filename, too many symbolic links in the specified path (Unix-like systems only), etc.

Examples

use std::fs::OpenOptions;

let file = OpenOptions::new().read(true).open("foo.txt");

Trait Implementations

impl Clone for OpenOptions

fn clone(&self) -> OpenOptions

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for OpenOptions

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl OpenOptionsExt for OpenOptions

Available on Windows only.

fn access_mode(&mut self, access: u32) -> &mut OpenOptions

Overrides the dwDesiredAccess argument to the call to CreateFile with the specified value.

fn share_mode(&mut self, share: u32) -> &mut OpenOptions

Overrides the dwShareMode argument to the call to CreateFile with the specified value.

fn custom_flags(&mut self, flags: u32) -> &mut OpenOptions

Sets extra flags for the dwFileFlags argument to the call to CreateFile2 to the specified value (or combines it with attributes and security_qos_flags to set the dwFlagsAndAttributes for CreateFile).

fn attributes(&mut self, attributes: u32) -> &mut OpenOptions

Sets the dwFileAttributes argument to the call to CreateFile2 to the specified value (or combines it with custom_flags and security_qos_flags to set the dwFlagsAndAttributes for CreateFile).

fn security_qos_flags(&mut self, flags: u32) -> &mut OpenOptions

Sets the dwSecurityQosFlags argument to the call to CreateFile2 to the specified value (or combines it with custom_flags and attributes to set the dwFlagsAndAttributes for CreateFile).

impl OpenOptionsExt for OpenOptions

Available on WASI only.

fn lookup_flags(&mut self, flags: u32) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Pass custom dirflags argument to path_open.

fn directory(&mut self, dir: bool) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Indicates whether OpenOptions must open a directory or not.

fn dsync(&mut self, enabled: bool) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Indicates whether __WASI_FDFLAG_DSYNC is passed in the fs_flags field of path_open.

fn nonblock(&mut self, enabled: bool) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Indicates whether __WASI_FDFLAG_NONBLOCK is passed in the fs_flags field of path_open.

fn rsync(&mut self, enabled: bool) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Indicates whether __WASI_FDFLAG_RSYNC is passed in the fs_flags field of path_open.

fn sync(&mut self, enabled: bool) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Indicates whether __WASI_FDFLAG_SYNC is passed in the fs_flags field of path_open.

fn fs_rights_base(&mut self, rights: u64) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Indicates the value that should be passed in for the fs_rights_base parameter of path_open.

fn fs_rights_inheriting(&mut self, rights: u64) -> &mut OpenOptions

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Indicates the value that should be passed in for the fs_rights_inheriting parameter of path_open.

fn open_at<P: AsRef<Path>>(&self, file: &File, path: P) -> Result<File>

🔬This is a nightly-only experimental API. (wasi_ext #71213)

Open a file or directory.

impl OpenOptionsExt for OpenOptions

Available on Unix only.

fn mode(&mut self, mode: u32) -> &mut OpenOptions

Sets the mode bits that a new file will be created with.

fn custom_flags(&mut self, flags: i32) -> &mut OpenOptions

Pass custom flags to the flags argument of open.