Struct async_std::fs::OpenOptions

pub struct OpenOptions(_);

Options and flags which for configuring 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 with this builder.

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

This type is an async version of std::fs::OpenOptions.

Examples

Opening a file for reading:

use async_std::fs::OpenOptions;

let file = OpenOptions::new()
    .read(true)
    .open("a.txt")
    .await?;

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

use async_std::fs::OpenOptions;

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

Methods

impl OpenOptions

pub fn new() -> OpenOptions

Creates a blank new set of options.

All options are initially set to false.

Examples

use async_std::fs::OpenOptions;

let file = OpenOptions::new()
    .read(true)
    .open("a.txt")
    .await?;

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

Sets the option for read access.

This option, when true, will indicate that the file should be readable if opened.

Examples

use async_std::fs::OpenOptions;

let file = OpenOptions::new()
    .read(true)
    .open("a.txt")
    .await?;

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

Sets the option for write access.

This option, when true, will indicate that the file should be writable if opened.

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

Examples

use async_std::fs::OpenOptions;

let file = OpenOptions::new()
    .write(true)
    .open("a.txt")
    .await?;

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

Sets the option for 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 writing them, or using a buffered writer (with a buffer of adequate size), and flushing 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 by seeking with a zero offset, and restore it before the next read.

Note

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

Examples

use async_std::fs::OpenOptions;

let file = OpenOptions::new()
    .append(true)
    .open("a.txt")
    .await?;

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

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 truncation to work.

Examples

use async_std::fs::OpenOptions;

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

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

Sets the option for creating a new file.

This option indicates whether a new file will be created if the file does not yet exist.

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

Examples

use async_std::fs::OpenOptions;

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

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

Sets the option to always create a new file.

This option indicates whether a new file will be created. No file is allowed to exist at the target location, also no (dangling) symlink.

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 async_std::fs::OpenOptions;

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

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

Opens a file at specified path with the configured options.

Errors

This function will return an error under a number of different circumstances. Some of these error conditions are listed here, together with their ErrorKind. The mapping to ErrorKinds is not part of the compatibility contract of the function, especially the Other kind might change to more specific kinds in the future.

• 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.).
• Other: One of the directory components of the specified file path was not, in fact, a directory.
• Other: 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 async_std::fs::OpenOptions;

let file = OpenOptions::new().open("a.txt").await?;

Trait Implementations

impl OpenOptionsExt for OpenOptions

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;

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

This is supported on Unix only.

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

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;

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

This is supported on Unix only.

Pass custom flags to the flags argument of open.

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.