Struct File

pub struct File { /* private fields */ }

Available on crate feature fs only.

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 Seek to alter the logical cursor that the file contains internally.

Files are automatically closed when they go out of scope.

Examples

Create a new file and asynchronously write bytes to it:

use tokio::fs::File;
use tokio::prelude::*;

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::prelude::*;

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

pub async fn open(path: impl AsRef<Path>) -> Result<File>

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::prelude::*;

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

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

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

pub async fn create(path: impl AsRef<Path>) -> Result<File>

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::prelude::*;

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

pub fn from_std(std: File) -> File

Convert 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 seek(&mut self, pos: SeekFrom) -> Result<u64>

Seek to an offset, in bytes, in a stream.

Examples

use tokio::fs::File;
use tokio::prelude::*;

use std::io::SeekFrom;

let mut file = File::open("foo.txt").await?;
file.seek(SeekFrom::Start(6)).await?;

let mut contents = vec![0u8; 10];
file.read_exact(&mut contents).await?;

pub async fn sync_all(&mut self) -> Result<()>

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::prelude::*;

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

pub async fn sync_data(&mut self) -> Result<()>

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::prelude::*;

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

pub async fn set_len(&mut self, size: u64) -> Result<()>

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::prelude::*;

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

pub async fn metadata(&self) -> Result<Metadata>

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>

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

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>

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<()>

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

fn as_raw_fd(&self) -> RawFd

Extracts the raw file descriptor.

impl AsyncRead for File

fn poll_read( self: Pin<&mut Self>, cx: &mut Context<'_>, dst: &mut [u8], ) -> Poll<Result<usize>>

Attempt to read from the AsyncRead into buf.

unsafe fn prepare_uninitialized_buffer( &self, buf: &mut [MaybeUninit<u8>], ) -> bool

Prepares an uninitialized buffer to be safe to pass to read. Returns true if the supplied buffer was zeroed out.

fn poll_read_buf<B: BufMut>( self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &mut B, ) -> Poll<Result<usize>>

where Self: Sized,

Pull some bytes from this source into the specified BufMut, returning how many bytes were read.

impl AsyncSeek for File

fn start_seek( self: Pin<&mut Self>, cx: &mut Context<'_>, pos: SeekFrom, ) -> Poll<Result<()>>

Attempt to seek to an offset, in bytes, in a stream.

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

Wait for a seek operation to complete.

impl AsyncWrite for File

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

Attempt to write bytes from buf into the object.

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

Attempt to flush the object, ensuring that any buffered data reach their destination.

fn poll_shutdown( self: Pin<&mut Self>, _cx: &mut Context<'_>, ) -> Poll<Result<(), Error>>

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

fn poll_write_buf<B: Buf>( self: Pin<&mut Self>, cx: &mut Context<'_>, buf: &mut B, ) -> Poll<Result<usize, Error>>

where Self: Sized,

Write a Buf into this value, returning how many bytes were written.

impl Debug for File

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

Formats the value using the given formatter.

impl From<File> for File

fn from(std: File) -> Self

Converts to this type from the input type.