Struct File

pub struct File { /* private fields */ }

An open file on the filesystem.

Depending on what options the file was opened with, this type can be used for reading and/or writing.

Files are automatically closed when they get dropped and any errors detected on closing are ignored. Use the sync_all() method before dropping a file if such errors need to be handled.

NOTE: If writing to a file, make sure to call [flush()][futures_lite::io::AsyncWriteExt::flush()], sync_data(), or sync_all() before dropping the file or else some written data might get lost!

Examples

Create a new file and write some bytes to it:

use async_fs::File;
use futures_lite::io::AsyncWriteExt;

let mut file = File::create("a.txt").await?;

file.write_all(b"Hello, world!").await?;
file.flush().await?;

Read the contents of a file into a vector of bytes:

use async_fs::File;
use futures_lite::io::AsyncReadExt;

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

let mut contents = Vec::new();
file.read_to_end(&mut contents).await?;

Implementations

impl File

pub async fn open<P>(path: P) -> Result<File, Error>

where P: AsRef<Path>,

Opens a file in read-only mode.

See the OpenOptions::open() function for more options.

Errors

An error will be returned in the following situations:

• path does not point to an existing file.
• The current process lacks permissions to read the file.
• Some other I/O error occurred.

For more details, see the list of errors documented by OpenOptions::open().

Examples

use async_fs::File;

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

pub async fn create<P>(path: P) -> Result<File, Error>

where P: AsRef<Path>,

Opens a file in write-only mode.

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

See the OpenOptions::open function for more options.

Errors

An error will be returned in the following situations:

• The file’s parent directory does not exist.
• The current process lacks permissions to write to the file.
• Some other I/O error occurred.

For more details, see the list of errors documented by OpenOptions::open().

Examples

use async_fs::File;

let file = File::create("a.txt").await?;

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

Synchronizes OS-internal buffered contents and metadata to disk.

This function will ensure that all in-memory data reaches the filesystem.

This can be used to handle errors that would otherwise only be caught by closing the file. When a file is dropped, errors in synchronizing this in-memory data are ignored.

Examples

use async_fs::File;
use futures_lite::io::AsyncWriteExt;

let mut file = File::create("a.txt").await?;

file.write_all(b"Hello, world!").await?;
file.sync_all().await?;

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

Synchronizes OS-internal buffered contents to disk.

This is similar to sync_all(), except that file metadata may not be synchronized.

This is intended for use cases that must synchronize the contents of the file, but don’t need the file metadata synchronized to disk.

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

Examples

use async_fs::File;
use futures_lite::io::AsyncWriteExt;

let mut file = File::create("a.txt").await?;

file.write_all(b"Hello, world!").await?;
file.sync_data().await?;

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

Truncates or extends the file.

If size is less than the current file size, then the file will be truncated. If it is greater than the current file size, then the file will be extended to size and have all intermediate data filled with zeros.

The file’s cursor stays at the same position, even if the cursor ends up being past the end of the file after this operation.

Examples

use async_fs::File;

let mut file = File::create("a.txt").await?;
file.set_len(10).await?;

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

Reads the file’s metadata.

Examples

use async_fs::File;

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

pub async fn set_permissions(&self, perm: Permissions) -> Result<(), Error>

Changes the permissions on the file.

Errors

An error will be returned in the following situations:

• The current process lacks permissions to change attributes on the file.
• Some other I/O error occurred.

Examples

use async_fs::File;

let file = File::create("a.txt").await?;

let mut perms = file.metadata().await?.permissions();
perms.set_readonly(true);
file.set_permissions(perms).await?;

Trait Implementations

impl AsFd for File

fn as_fd(&self) -> BorrowedFd<'_>

Borrows the file descriptor.

impl AsRawFd for File

fn as_raw_fd(&self) -> i32

Extracts the raw file descriptor.

impl AsyncRead for File

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

Attempt to read from the AsyncRead into buf.

fn poll_read_vectored( self: Pin<&mut Self>, cx: &mut Context<'_>, bufs: &mut [IoSliceMut<'_>], ) -> Poll<Result<usize, Error>>

Attempt to read from the AsyncRead into bufs using vectored IO operations.

impl AsyncSeek for File

fn poll_seek( self: Pin<&mut File>, cx: &mut Context<'_>, pos: SeekFrom, ) -> Poll<Result<u64, Error>>

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

impl AsyncWrite for File

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

Attempt to write bytes from buf into the object.

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

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

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

Attempt to close the object.

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

Attempt to write bytes from bufs into the object using vectored IO operations.

impl Debug for File

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

Formats the value using the given formatter.

impl From<File> for File

fn from(inner: File) -> File

Converts to this type from the input type.

impl From<OwnedFd> for File

fn from(fd: OwnedFd) -> File

Converts to this type from the input type.