Struct File

pub struct File { /* private fields */ }

A reference to an open file on the filesystem.

An instance of a File can be read and/or written depending on what options it was opened with. The File type provides positional read and write operations. The file does not maintain an internal cursor. The caller is required to specify an offset when issuing an operation.

While files are automatically closed when they go out of scope, the operation happens asynchronously in the background. It is recommended to call the close() function in order to guarantee that the file successfully closed before exiting the scope. Closing a file does not guarantee writes have persisted to disk. Use sync_all to ensure all writes have reached the filesystem.

Examples

Creates a new file and write data to it:

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Open a file
    let file = File::create("hello.txt").await?;

    // Write some data
    let (res, buf) = file.write_at(&b"hello world"[..], 0).await;
    let n = res?;

    println!("wrote {} bytes", n);

    // Sync data to the file system.
    file.sync_all().await?;

    // Close the file
    file.close().await?;

    Ok(())
}

Implementations

impl File

pub fn from_std(std: StdFile) -> Result<File>

Converts a std::fs::File to a monoio::fs::File.

Examples

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

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

Queries metadata about the underlying file.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> std::io::Result<()> {
    let mut f = File::open("foo.txt").await?;
    let metadata = f.metadata().await?;
    Ok(())
}

impl File

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

Attempts to open a file in read-only mode.

See the OpenOptions::open method for more details.

Errors

This function will return an error if path does not already exist. Other errors may also be returned according to OpenOptions::open.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let f = File::open("foo.txt").await?;

    // Close the file
    f.close().await?;
    Ok(())
}

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 the OpenOptions::open function for more details.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let f = File::create("foo.txt").await?;

    // Close the file
    f.close().await?;
    Ok(())
}

pub async fn read_at<T: IoBufMut>( &self, buf: T, pos: u64, ) -> BufResult<usize, T>

Read some bytes at the specified offset from the file into the specified buffer, returning how many bytes were read.

Return

The method returns the operation result and the same buffer value passed as an argument.

If the method returns [Ok(n)], then the read was successful. A nonzero n value indicates that the buffer has been filled with n bytes of data from the file. If n is 0, then one of the following happened:

1. The specified offset is the end of the file.
2. The buffer specified was 0 bytes in length.

It is not an error if the returned value n is smaller than the buffer size, even when the file contains enough data to fill the buffer.

Platform-specific behavior

• On unix-like platform
  • this function will not change the file pointer, and the pos always start from the begin of file.
• On windows
  • this function will change the file pointer, but the pos always start from the begin of file.

Addtionally,

• On Unix and Windows (without the iouring feature enabled or not support the iouring):

  • If the sync feature is enabled and the thread pool is attached, this operation will be executed on the blocking thread pool, preventing it from blocking the current thread.
  • If the sync feature is enabled but the thread pool is not attached, or if the sync feature is disabled, the operation will be executed on the local thread, blocking the current thread.

• On Linux (with iouring enabled and supported):

  This operation will use io-uring to execute the task asynchronously.

Errors

If this function encounters any form of I/O or other error, an error variant will be returned. The buffer is returned on error.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let f = File::open("foo.txt").await?;
    let buffer = vec![0; 10];

    // Read up to 10 bytes
    let (res, buffer) = f.read_at(buffer, 0).await;
    let n = res?;

    println!("The bytes: {:?}", &buffer[..n]);

    // Close the file
    f.close().await?;
    Ok(())
}

pub async fn read_exact_at<T: IoBufMut>( &self, buf: T, pos: u64, ) -> BufResult<(), T>

Read the exact number of bytes required to fill buf at the specified offset from the file.

This function reads as many as bytes as necessary to completely fill the specified buffer buf.

Return

The method returns the operation result and the same buffer value passed as an argument.

If the method returns [Ok(())], then the read was successful.

Errors

If this function encounters an error of the kind ErrorKind::Interrupted then the error is ignored and the operation will continue.

If this function encounters an “end of file” before completely filling the buffer, it returns an error of the kind ErrorKind::UnexpectedEof. The buffer is returned on error.

If this function encounters any form of I/O or other error, an error variant will be returned. The buffer is returned on error.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let f = File::open("foo.txt").await?;
    let buffer = vec![0; 10];

    // Read up to 10 bytes
    let (res, buffer) = f.read_exact_at(buffer, 0).await;
    res?;

    println!("The bytes: {:?}", buffer);

    // Close the file
    f.close().await?;
    Ok(())
}

pub async fn write_at<T: IoBuf>(&self, buf: T, pos: u64) -> BufResult<usize, T>

Write a buffer into this file at the specified offset, returning how many bytes were written.

This function will attempt to write the entire contents of buf, but the entire write may not succeed, or the write may also generate an error. The bytes will be written starting at the specified offset.

Return

The method returns the operation result and the same buffer value passed in as an argument. A return value of 0 typically means that the underlying file is no longer able to accept bytes and will likely not be able to in the future as well, or that the buffer provided is empty.

Platform-specific behavior

• On Unix and Windows (without the iouring feature enabled or not support the iouring):

  • If the sync feature is enabled and the thread pool is attached, this operation will be executed on the blocking thread pool, preventing it from blocking the current thread.
  • If the sync feature is enabled but the thread pool is not attached, or if the sync feature is disabled, the operation will be executed on the local thread, blocking the current thread.

• On Linux (with iouring enabled and supported):

  This operation will use io-uring to execute the task asynchronously.

Errors

Each call to write may generate an I/O error indicating that the operation could not be completed. If an error is returned then no bytes in the buffer were written to this writer.

It is not considered an error if the entire buffer could not be written to this writer.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let file = File::create("foo.txt").await?;

    // Writes some prefix of the byte string, not necessarily all of it.
    let (res, _) = file.write_at(&b"some bytes"[..], 0).await;
    let n = res?;

    println!("wrote {} bytes", n);

    // Close the file
    file.close().await?;
    Ok(())
}

pub async fn write_all_at<T: IoBuf>(&self, buf: T, pos: u64) -> BufResult<(), T>

Attempts to write an entire buffer into this file at the specified offset.

This method will continuously call write_at until there is no more data to be written or an error of non-ErrorKind::Interrupted kind is returned. This method will not return until the entire buffer has been successfully written or such an error occurs.

If the buffer contains no data, this will never call write_at.

Return

The method returns the operation result and the same buffer value passed in as an argument.

Errors

This function will return the first error of non-ErrorKind::Interrupted kind that write_at returns.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let file = File::create("foo.txt").await?;

    // Writes some prefix of the byte string, not necessarily all of it.
    let (res, _) = file.write_all_at(&b"some bytes"[..], 0).await;
    res?;

    println!("wrote all bytes");

    // Close the file
    file.close().await?;
    Ok(())
}

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

Attempts to sync all OS-internal metadata to disk.

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

This can be used to handle errors that would otherwise only be caught when the File is closed. Dropping a file will ignore errors in synchronizing this in-memory data.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let f = File::create("foo.txt").await?;
    let (res, buf) = f.write_at(&b"Hello, world!"[..], 0).await;
    let n = res?;

    f.sync_all().await?;

    // Close the file
    f.close().await?;
    Ok(())
}

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

Attempts to sync file data to disk.

This method 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 monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let f = File::create("foo.txt").await?;
    let (res, buf) = f.write_at(&b"Hello, world!"[..], 0).await;
    let n = res?;

    f.sync_data().await?;

    // Close the file
    f.close().await?;
    Ok(())
}

pub async fn close(self) -> Result<()>

Closes the file.

The method completes once the close operation has completed, guaranteeing that resources associated with the file have been released.

If close is not called before dropping the file, the file is closed in the background, but there is no guarantee as to when the close operation will complete.

Examples

use monoio::fs::File;

#[monoio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Open the file
    let f = File::open("foo.txt").await?;
    // Close the file
    f.close().await?;

    Ok(())
}

Trait Implementations

impl AsRawFd for File

fn as_raw_fd(&self) -> RawFd

Extracts the raw file descriptor.

impl AsyncReadRent for File

async fn read<T: IoBufMut>(&mut self, buf: T) -> BufResult<usize, T>

Reads bytes from the file at the current file pointer into the specified buffer, returning the number of bytes read.

Return

The method returns a tuple with the result of the operation and the same buffer passed as an argument.

If the method returns [(Ok(n), buf)], a non-zero n means the buffer has been filled with n bytes of data from the file. If n is 0, it indicates one of the following:

1. The current file pointer is at the end of the file.
2. The provided buffer was 0 bytes in length.

It is not an error if n is smaller than the buffer size, even if there is enough data in the file to fill the buffer.

Platform-specific behavior

• On Unix and Windows (without the iouring feature enabled or not support the iouring):

  • If the sync feature is enabled and the thread pool is attached, this operation will be executed on the blocking thread pool, preventing it from blocking the current thread.
  • If the sync feature is enabled but the thread pool is not attached, or if the sync feature is disabled, the operation will be executed on the local thread, blocking the current thread.

• On Linux (with iouring enabled and supported):

  This operation will use io-uring to execute the task asynchronously.

Errors

If an I/O or other error occurs, an error variant will be returned, and the buffer will also be returned.

Example

use monoio::io::AsyncReadRent;

#[monoio::main]
async fn main() -> std::io::Result<()> {
    let buf = Vec::with_capacity(1024);
    let mut file = monoio::fs::File::open("example.txt").await?;
    let (res, buf) = file.read(buf).await;
    println!("bytes read: {}", res?);
    Ok(())
}

async fn readv<T: IoVecBufMut>(&mut self, buf: T) -> BufResult<usize, T>

Read some bytes at the specified offset from the file into the specified array of buffers, returning how many bytes were read.

Return

The method returns the operation result and the same array of buffers passed as an argument.

If the method returns [Ok(n)], then the read was successful. A nonzero n value indicates that the buffers have been filled with n bytes of data from the file. If n is 0, then one of the following happened:

1. The specified offset is the end of the file.
2. The buffers specified were 0 bytes in length.

It is not an error if the returned value n is smaller than the buffer size, even when the file contains enough data to fill the buffer.

Platform-specific behavior

• On windows

  • due to windows does not have syscall like readv, so the implement of this function on windows is by internally calling the ReadFile syscall to fill each buffer.

• On Unix and Windows (without the iouring feature enabled or not support the iouring):

  • If the sync feature is enabled and the thread pool is attached, this operation will be executed on the blocking thread pool, preventing it from blocking the current thread.
  • If the sync feature is enabled but the thread pool is not attached, or if the sync feature is disabled, the operation will be executed on the local thread, blocking the current thread.

• On Linux (with iouring enabled and supported):

  This operation will use io-uring to execute the task asynchronously.

Errors

If this function encounters any form of I/O or other error, an error variant will be returned. The buffer is returned on error.

Example

use monoio::io::AsyncReadRent;

#[monoio::main]
async fn main() -> std::io::Result<()> {
    let mut file = monoio::fs::File::open("example.txt").await?;
    let buffers = monoio::buf::VecBuf::from(vec![
        Vec::<u8>::with_capacity(10),
        Vec::<u8>::with_capacity(10),
    ]);

    let (res, buffer) = file.readv(buffers).await;

    println!("bytes read: {}", res?);
    Ok(())
}

impl AsyncReadRentAt for File

fn read_at<T: IoBufMut>( &mut self, buf: T, pos: usize, ) -> impl Future<Output = BufResult<usize, T>>

Same as pread(2)

impl AsyncWriteRent for File

async fn write<T: IoBuf>(&mut self, buf: T) -> BufResult<usize, T>

Writes the contents of a buffer to a file, returning the number of bytes written.

This function attempts to write the entire buffer buf, but the write may not fully succeed, and it might also result in an error. A call to write represents at most one attempt to write to the underlying object.

Return

If the return value is (Ok(n), buf), it guarantees that n <= buf.len(). A return value of 0 typically indicates that the underlying object can no longer accept bytes and likely won’t be able to in the future, or that the provided buffer is empty.

Errors

Each write call may result in an I/O error, indicating that the operation couldn’t be completed. If an error occurs, no bytes from the buffer were written to the file.

It is not considered an error if the entire buffer could not be written to the file.

Example

use monoio::io::AsyncWriteRent;

#[monoio::main]
async fn main() -> std::io::Result<()> {
    let mut file = monoio::fs::File::create("example.txt").await?;
    let (res, buf) = file.write("Hello, world").await;
    res?;
    Ok(())
}

async fn writev<T: IoVecBuf>(&mut self, buf_vec: T) -> BufResult<usize, T>

This function attempts to write the entire contents of buf_vec, but the write may not fully succeed, and it might also result in an error. The bytes will be written starting at the current file pointer.

Return

The method returns the result of the operation along with the same array of buffers passed as an argument. A return value of 0 typically indicates that the underlying file can no longer accept bytes and likely won’t be able to in the future, or that the provided buffer is empty.

Platform-specific behavior

• On windows
  • due to windows does not have syscall like writev, so the implement of this function on windows is by internally calling the WriteFile syscall to write each buffer into file.

Errors

Each write call may result in an I/O error, indicating the operation couldn’t be completed. If an error occurs, no bytes from the buffer were written to the file.

It is not considered an error if the entire buffer could not be written to the file.

Example

use monoio::io::AsyncWriteRent;

#[monoio::main]
async fn main() -> std::io::Result<()> {
    let buf_vec = monoio::buf::VecBuf::from(vec![
        "Hello".to_owned().into_bytes(),
        "World".to_owned().into_bytes(),
    ]);
    let mut file = monoio::fs::File::create("example.txt").await?;
    let (res, buf_vec) = file.writev(buf_vec).await;
    res?;
    Ok(())
}

async fn flush(&mut self) -> Result<()>

Flushes the file, ensuring that all buffered contents are written to their destination.

Platform-specific behavior

Since the File structure doesn’t contain any internal buffers, this function is currently a no-op.

async fn shutdown(&mut self) -> Result<()>

This function will call [flush] inside.

impl AsyncWriteRentAt for File

fn write_at<T: IoBuf>( &mut self, buf: T, pos: usize, ) -> impl Future<Output = BufResult<usize, T>>

Write buf at given offset

impl Debug for File

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

Formats the value using the given formatter.