# tokio::fs::OpenOptions - Rust
API documentation for the Rust `OpenOptions` struct in crate `tokio`.
Keywords: rust, rustlang, rust-lang, OpenOptions

# Struct tokio::fs::OpenOptions [−] [src]

    pub struct OpenOptions(_);

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 `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 `io::Result` with a `File` inside that you can further operate on.

This is a specialized version of `std::fs::OpenOptions` for usage from the Tokio runtime.

`From<std::fs::OpenOptions>` is implemented for more advanced configuration than the methods provided here.

# Examples

Opening a file to read:

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .read(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

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

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .read(true)
            .write(true)
            .create(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

## Implementations

### `impl OpenOptions` [src]

#### `pub fn new() -> OpenOptions` [src]

Creates a blank new set of options ready for configuration.

All options are initially set to `false`.

This is an async version of `std::fs::OpenOptions::new`

# Examples

    use tokio::fs::OpenOptions;

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

#### `pub fn read(&mut self, read: bool) -> &mut OpenOptions` [src]

Sets the option for read access.

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

This is an async version of `std::fs::OpenOptions::read`

# Examples

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .read(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

#### `pub fn write(&mut self, write: bool) -> &mut OpenOptions` [src]

Sets the option for write access.

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

This is an async version of `std::fs::OpenOptions::write`

# Examples

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .write(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

#### `pub fn append(&mut self, append: bool) -> &mut OpenOptions` [src]

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.

This is an async version of `std::fs::OpenOptions::append`

## Note

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

# Examples

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .append(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

#### `pub fn truncate(&mut self, truncate: bool) -> &mut OpenOptions` [src]

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.

This is an async version of `std::fs::OpenOptions::truncate`

# Examples

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .write(true)
            .truncate(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

#### `pub fn create(&mut self, create: bool) -> &mut OpenOptions` [src]

Sets the option for creating a new file.

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

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

This is an async version of `std::fs::OpenOptions::create`

# Examples

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .write(true)
            .create(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

#### `pub fn create_new(&mut self, create_new: bool) -> &mut OpenOptions` [src]

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.

This is an async version of `std::fs::OpenOptions::create_new`

# Examples

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new()
            .write(true)
            .create_new(true)
            .open("foo.txt")
            .await?;

        Ok(())
    }

#### `pub async fn open(&self, path: impl AsRef<Path>) -> Result<File>` [src]

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

This is an async version of `std::fs::OpenOptions::open`

# 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 `ErrorKind`s 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 tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let file = OpenOptions::new().open("foo.txt").await?;
        Ok(())
    }

### `impl OpenOptions` [src]

#### `pub fn mode(&mut self, mode: u32) -> &mut OpenOptions` [src]

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

If a new file is created as part of an `OpenOptions::open` call then this specified `mode` will be used as the permission bits for the new file. If no `mode` is set, the default of `0o666` will be used. The operating system masks out bits with the system's `umask`, to produce the final permissions.

# Examples

    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let mut options = OpenOptions::new();
        options.mode(0o644); // Give read/write for owner and read for others.
        let file = options.open("foo.txt").await?;

        Ok(())
    }

#### `pub fn custom_flags(&mut self, flags: i32) -> &mut OpenOptions` [src]

Pass custom flags to the `flags` argument of `open`.

The bits that define the access mode are masked out with `O_ACCMODE`, to ensure they do not interfere with the access mode set by Rusts options.

Custom flags can only set flags, not remove flags set by Rusts options. This options overwrites any previously set custom flags.

# Examples

    use libc;
    use tokio::fs::OpenOptions;
    use std::io;

    #[tokio::main]
    async fn main() -> io::Result<()> {
        let mut options = OpenOptions::new();
        options.write(true);
        if cfg!(unix) {
            options.custom_flags(libc::O_NOFOLLOW);
        }
        let file = options.open("foo.txt").await?;

        Ok(())
    }

## Trait Implementations

### `impl Clone for OpenOptions` [src]

#### `fn clone(&self) -> OpenOptions` [src]

Returns a copy of the value. Read more

#### `pub fn clone_from(&mut self, source: &Self)`1.0.0 [src]

Performs copy-assignment from `source`. Read more

### `impl Debug for OpenOptions` [src]

#### `fn fmt(&self, f: &mut Formatter<'_>) -> Result` [src]

Formats the value using the given formatter. Read more

### `impl Default for OpenOptions` [src]

#### `fn default() -> Self` [src]

Returns the "default value" for a type. Read more

### `impl From<OpenOptions> for OpenOptions` [src]

#### `fn from(options: OpenOptions) -> OpenOptions` [src]

Performs the conversion.

## Auto Trait Implementations

### `impl RefUnwindSafe for OpenOptions` [src]

### `impl Send for OpenOptions` [src]

### `impl Sync for OpenOptions` [src]

### `impl Unpin for OpenOptions` [src]

### `impl UnwindSafe for OpenOptions` [src]

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId` [src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

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

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

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

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T` [src]

#### `pub fn from(t: T) -> T` [src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U` [src]

Performs the conversion.

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

#### `pub fn to_owned(&self) -> T` [src]

Creates owned data from borrowed data, usually by cloning. Read more

#### `pub fn clone_into(&self, target: &mut T)` [src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into`)

recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>` [src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>` [src]

Performs the conversion.