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

# Struct tokio::fs::File [−] [src]

    pub struct File { /* fields omitted */ }

A reference to an open file on the filesystem.

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

An instance of a `File` can be read and/or written depending on what options it was opened with. Files also implement `AsyncSeek` to alter the logical cursor that the file contains internally.

A file will not be closed immediately when it goes out of scope if there are any IO operations that have not yet completed. To ensure that a file is closed immediately when it is dropped, you should call `flush` before dropping it. Note that this does not ensure that the file has been fully written to disk; the operating system might keep the changes around in an in-memory buffer. See the `sync_all` method for telling the OS to write the data to disk.

Reading and writing to a `File` is usually done using the convenience methods found on the `AsyncReadExt` and `AsyncWriteExt` traits.

# Examples

Create a new file and asynchronously write bytes to it:

    use tokio::fs::File;
    use tokio::io::AsyncWriteExt; // for write_all()

    let mut file = File::create("foo.txt").await?;
    file.write_all(b"hello, world!").await?;

Read the contents of a file into a buffer

    use tokio::fs::File;
    use tokio::io::AsyncReadExt; // for read_to_end()

    let mut file = File::open("foo.txt").await?;

    let mut contents = vec![];
    file.read_to_end(&mut contents).await?;

    println!("len = {}", contents.len());

## Implementations

### `impl File` [src]

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

Attempts to open a file in read-only mode.

See `OpenOptions` for more details.

# Errors

This function will return an error if called from outside of the Tokio runtime or if path does not already exist. Other errors may also be returned according to OpenOptions::open.

# Examples

    use tokio::fs::File;
    use tokio::io::AsyncReadExt;

    let mut file = File::open("foo.txt").await?;

    let mut contents = vec![];
    file.read_to_end(&mut contents).await?;

    println!("len = {}", contents.len());

The `read_to_end` method is defined on the `AsyncReadExt` trait.

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

Opens a file in write-only mode.

This function will create a file if it does not exist, and will truncate it if it does.

See `OpenOptions` for more details.

# Errors

Results in an error if called from outside of the Tokio runtime or if the underlying `create` call results in an error.

# Examples

    use tokio::fs::File;
    use tokio::io::AsyncWriteExt;

    let mut file = File::create("foo.txt").await?;
    file.write_all(b"hello, world!").await?;

The `write_all` method is defined on the `AsyncWriteExt` trait.

#### `pub fn from_std(std: File) -> File` [src]

Converts a `std::fs::File` to a `tokio::fs::File`.

# Examples

    // This line could block. It is not recommended to do this on the Tokio
    // runtime.
    let std_file = std::fs::File::open("foo.txt").unwrap();
    let file = tokio::fs::File::from_std(std_file);

#### `pub async fn sync_all(&self) -> Result<()>` [src]

Attempts to sync all OS-internal metadata to disk.

This function will attempt to ensure that all in-core data reaches the filesystem before returning.

# Examples

    use tokio::fs::File;
    use tokio::io::AsyncWriteExt;

    let mut file = File::create("foo.txt").await?;
    file.write_all(b"hello, world!").await?;
    file.sync_all().await?;

The `write_all` method is defined on the `AsyncWriteExt` trait.

#### `pub async fn sync_data(&self) -> Result<()>` [src]

This function is similar to `sync_all`, except that it may not synchronize file metadata to the filesystem.

This is intended for use cases that must synchronize content, but don't need the metadata on disk. The goal of this method is to reduce disk operations.

Note that some platforms may simply implement this in terms of `sync_all`.

# Examples

    use tokio::fs::File;
    use tokio::io::AsyncWriteExt;

    let mut file = File::create("foo.txt").await?;
    file.write_all(b"hello, world!").await?;
    file.sync_data().await?;

The `write_all` method is defined on the `AsyncWriteExt` trait.

#### `pub async fn set_len(&self, size: u64) -> Result<()>` [src]

Truncates or extends the underlying file, updating the size of this file to become size.

If the size is less than the current file's size, then the file will be shrunk. If it is greater than the current file's size, then the file will be extended to size and have all of the intermediate data filled in with 0s.

# Errors

This function will return an error if the file is not opened for writing.

# Examples

    use tokio::fs::File;
    use tokio::io::AsyncWriteExt;

    let mut file = File::create("foo.txt").await?;
    file.write_all(b"hello, world!").await?;
    file.set_len(10).await?;

The `write_all` method is defined on the `AsyncWriteExt` trait.

#### `pub async fn metadata(&self) -> Result<Metadata>` [src]

Queries metadata about the underlying file.

# Examples

    use tokio::fs::File;

    let file = File::open("foo.txt").await?;
    let metadata = file.metadata().await?;

    println!("{:?}", metadata);

#### `pub async fn try_clone(&self) -> Result<File>` [src]

Create a new `File` instance that shares the same underlying file handle as the existing `File` instance. Reads, writes, and seeks will affect both File instances simultaneously.

# Examples

    use tokio::fs::File;

    let file = File::open("foo.txt").await?;
    let file_clone = file.try_clone().await?;

#### `pub async fn into_std(self) -> File` [src]

Destructures `File` into a `std::fs::File`. This function is async to allow any in-flight operations to complete.

Use `File::try_into_std` to attempt conversion immediately.

# Examples

    use tokio::fs::File;

    let tokio_file = File::open("foo.txt").await?;
    let std_file = tokio_file.into_std().await;

#### `pub fn try_into_std(self) -> Result<File, Self>` [src]

Tries to immediately destructure `File` into a `std::fs::File`.

# Errors

This function will return an error containing the file if some operation is in-flight.

# Examples

    use tokio::fs::File;

    let tokio_file = File::open("foo.txt").await?;
    let std_file = tokio_file.try_into_std().unwrap();

#### `pub async fn set_permissions(&self, perm: Permissions) -> Result<()>` [src]

Changes the permissions on the underlying file.

# Platform-specific behavior

This function currently corresponds to the `fchmod` function on Unix and the `SetFileInformationByHandle` function on Windows. Note that, this may change in the future.

# Errors

This function will return an error if the user lacks permission change attributes on the underlying file. It may also return an error in other os-specific unspecified cases.

# Examples

    use tokio::fs::File;

    let file = File::open("foo.txt").await?;
    let mut perms = file.metadata().await?.permissions();
    perms.set_readonly(true);
    file.set_permissions(perms).await?;

## Trait Implementations

### `impl AsRawFd for File` [src]

#### `fn as_raw_fd(&self) -> RawFd` [src]

Extracts the raw file descriptor. Read more

### `impl AsyncRead for File` [src]

#### `fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
dst: &mut ReadBuf<'_>
) -> Poll<Result<()>>` [src]

Attempts to read from the `AsyncRead` into `buf`. Read more

### `impl AsyncSeek for File` [src]

#### `fn start_seek(self: Pin<&mut Self>, pos: SeekFrom) -> Result<()>` [src]

Attempts to seek to an offset, in bytes, in a stream. Read more

#### `fn poll_complete(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Result<u64>>` [src]

Waits for a seek operation to complete. Read more

### `impl AsyncWrite for File` [src]

#### `fn poll_write(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
src: &[u8]
) -> Poll<Result<usize>>` [src]

Attempt to write bytes from `buf` into the object. Read more

#### `fn poll_flush(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Result<(), Error>>` [src]

Attempts to flush the object, ensuring that any buffered data reach their destination. Read more

#### `fn poll_shutdown(
self: Pin<&mut Self>,
cx: &mut Context<'_>
) -> Poll<Result<(), Error>>` [src]

Initiates or attempts to shut down this writer, returning success when the I/O connection has completely shut down. Read more

#### `fn poll_write_vectored(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
bufs: &[IoSlice<'_>]
) -> Poll<Result<usize, Error>>` [src]

Like `poll_write`, except that it writes from a slice of buffers. Read more

#### `fn is_write_vectored(&self) -> bool` [src]

Determines if this writer has an efficient `poll_write_vectored` implementation. Read more

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

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

Formats the value using the given formatter. Read more

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

#### `fn from(std: File) -> Self` [src]

Performs the conversion.

### `impl FromRawFd for File` [src]

#### `unsafe fn from_raw_fd(fd: RawFd) -> Self` [src]

Constructs a new instance of `Self` from the given raw file descriptor. Read more

## Auto Trait Implementations

### `impl !RefUnwindSafe for File` [src]

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

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

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

### `impl !UnwindSafe for File` [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, 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.