Struct File

pub struct File { /* private fields */ }

An object providing access 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. Files also implement Seek to alter the logical cursor that the file contains internally.

Files are automatically closed when they go out of scope. Errors detected on closing are ignored by the implementation of Drop. Use the method sync_all if these errors must be manually handled.

File does not buffer reads and writes. For efficiency, consider wrapping the file in a BufReader or BufWriter when performing many small read or write calls, unless unbuffered reads and writes are required.

Examples

Creates a new file and write bytes to it (you can also use write):

use std::fs::File;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut file = File::create("foo.txt")?;
    file.write_all(b"Hello, world!")?;
    Ok(())
}

Reads the contents of a file into a String (you can also use read):

use std::fs::File;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut file = File::open("foo.txt")?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    assert_eq!(contents, "Hello, world!");
    Ok(())
}

Using a buffered Reader:

use std::fs::File;
use std::io::BufReader;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let file = File::open("foo.txt")?;
    let mut buf_reader = BufReader::new(file);
    let mut contents = String::new();
    buf_reader.read_to_string(&mut contents)?;
    assert_eq!(contents, "Hello, world!");
    Ok(())
}

Note that, although read and write methods require a &mut File, because of the interfaces for Read and Write, the holder of a &File can still modify the file, either through methods that take &File or by retrieving the underlying OS object and modifying the file that way. Additionally, many operating systems allow concurrent modification of files by different processes. Avoid assuming that holding a &File means that the file will not change.

Platform-specific behavior

On Windows, the implementation of Read and Write traits for File perform synchronous I/O operations. Therefore the underlying file must not have been opened for asynchronous I/O (e.g. by using FILE_FLAG_OVERLAPPED).

Implementations

impl File

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

Attempts to open a file in read-only mode.

See the OpenOptions::open method for more details.

If you only need to read the entire file contents, consider std::fs::read() or std::fs::read_to_string() instead.

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 std::fs::File;
use std::io::Read;

fn main() -> std::io::Result<()> {
    let mut f = File::open("foo.txt")?;
    let mut data = vec![];
    f.read_to_end(&mut data)?;
    Ok(())
}

pub fn open_buffered<P: AsRef<Path>>(path: P) -> Result<BufReader<File>>

🔬This is a nightly-only experimental API. (file_buffered #130804)

Attempts to open a file in read-only mode with buffering.

See the OpenOptions::open method, the BufReader type, and the BufRead trait for more details.

If you only need to read the entire file contents, consider std::fs::read() or std::fs::read_to_string() instead.

Errors

This function will return an error if path does not already exist, or if memory allocation fails for the new buffer. Other errors may also be returned according to OpenOptions::open.

Examples

#![feature(file_buffered)]
use std::fs::File;
use std::io::BufRead;

fn main() -> std::io::Result<()> {
    let mut f = File::open_buffered("foo.txt")?;
    assert!(f.capacity() > 0);
    for (line, i) in f.lines().zip(1..) {
        println!("{i:6}: {}", line?);
    }
    Ok(())
}

pub fn create<P: AsRef<Path>>(path: P) -> 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.

Depending on the platform, this function may fail if the full directory path does not exist. See the OpenOptions::open function for more details.

See also std::fs::write() for a simple function to create a file with some given data.

Examples

use std::fs::File;
use std::io::Write;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    f.write_all(&1234_u32.to_be_bytes())?;
    Ok(())
}

pub fn create_buffered<P: AsRef<Path>>(path: P) -> Result<BufWriter<File>>

🔬This is a nightly-only experimental API. (file_buffered #130804)

Opens a file in write-only mode with buffering.

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

Depending on the platform, this function may fail if the full directory path does not exist.

See the OpenOptions::open method and the BufWriter type for more details.

See also std::fs::write() for a simple function to create a file with some given data.

Examples

#![feature(file_buffered)]
use std::fs::File;
use std::io::Write;

fn main() -> std::io::Result<()> {
    let mut f = File::create_buffered("foo.txt")?;
    assert!(f.capacity() > 0);
    for i in 0..100 {
        writeln!(&mut f, "{i}")?;
    }
    f.flush()?;
    Ok(())
}

pub fn create_new<P: AsRef<Path>>(path: P) -> Result<File>

Creates a new file in read-write mode; error if the file exists.

This function will create a file if it does not exist, or return an error if it does. This way, if the call succeeds, the file returned is guaranteed to be new. If a file exists at the target location, creating a new file will fail with AlreadyExists or another error based on the situation. See OpenOptions::open for a non-exhaustive list of likely errors.

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).

This can also be written using File::options().read(true).write(true).create_new(true).open(...).

Examples

use std::fs::File;
use std::io::Write;

fn main() -> std::io::Result<()> {
    let mut f = File::create_new("foo.txt")?;
    f.write_all("Hello, world!".as_bytes())?;
    Ok(())
}

pub fn options() -> OpenOptions

Returns a new OpenOptions object.

This function returns a new OpenOptions object that you can use to open or create a file with specific options if open() or create() are not appropriate.

It is equivalent to OpenOptions::new(), but allows you to write more readable code. Instead of OpenOptions::new().append(true).open("example.log"), you can write File::options().append(true).open("example.log"). This also avoids the need to import OpenOptions.

See the OpenOptions::new function for more details.

Examples

use std::fs::File;
use std::io::Write;

fn main() -> std::io::Result<()> {
    let mut f = File::options().append(true).open("example.log")?;
    writeln!(&mut f, "new line")?;
    Ok(())
}

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

Attempts to sync all OS-internal file content and metadata to disk.

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

This can be used to handle errors that would otherwise only be caught when the File is closed, as dropping a File will ignore all errors. Note, however, that sync_all is generally more expensive than closing a file by dropping it, because the latter is not required to block until the data has been written to the filesystem.

If synchronizing the metadata is not required, use sync_data instead.

Examples

use std::fs::File;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    f.write_all(b"Hello, world!")?;

    f.sync_all()?;
    Ok(())
}

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

This function is similar to sync_all, except that it might 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 std::fs::File;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    f.write_all(b"Hello, world!")?;

    f.sync_data()?;
    Ok(())
}

pub fn set_len(&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.

The file’s cursor isn’t changed. In particular, if the cursor was at the end and the file is shrunk using this operation, the cursor will now be past the end.

Errors

This function will return an error if the file is not opened for writing. Also, std::io::ErrorKind::InvalidInput will be returned if the desired length would cause an overflow due to the implementation specifics.

Examples

use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    f.set_len(10)?;
    Ok(())
}

Note that this method alters the content of the underlying file, even though it takes &self rather than &mut self.

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

Queries metadata about the underlying file.

Examples

use std::fs::File;

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

pub fn try_clone(&self) -> Result<File>

Creates 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

Creates two handles for a file named foo.txt:

use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut file = File::open("foo.txt")?;
    let file_copy = file.try_clone()?;
    Ok(())
}

Assuming there’s a file named foo.txt with contents abcdef\n, create two handles, seek one of them, and read the remaining bytes from the other handle:

use std::fs::File;
use std::io::SeekFrom;
use std::io::prelude::*;

fn main() -> std::io::Result<()> {
    let mut file = File::open("foo.txt")?;
    let mut file_copy = file.try_clone()?;

    file.seek(SeekFrom::Start(3))?;

    let mut contents = vec![];
    file_copy.read_to_end(&mut contents)?;
    assert_eq!(contents, b"def\n");
    Ok(())
}

pub 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

fn main() -> std::io::Result<()> {
    use std::fs::File;

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

Note that this method alters the permissions of the underlying file, even though it takes &self rather than &mut self.

pub fn set_times(&self, times: FileTimes) -> Result<()>

Changes the timestamps of the underlying file.

Platform-specific behavior

This function currently corresponds to the futimens function on Unix (falling back to futimes on macOS before 10.13) and the SetFileTime function on Windows. Note that this may change in the future.

Errors

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

This function may return an error if the operating system lacks support to change one or more of the timestamps set in the FileTimes structure.

Examples

fn main() -> std::io::Result<()> {
    use std::fs::{self, File, FileTimes};

    let src = fs::metadata("src")?;
    let dest = File::options().write(true).open("dest")?;
    let times = FileTimes::new()
        .set_accessed(src.accessed()?)
        .set_modified(src.modified()?);
    dest.set_times(times)?;
    Ok(())
}

pub fn set_modified(&self, time: SystemTime) -> Result<()>

Changes the modification time of the underlying file.

This is an alias for set_times(FileTimes::new().set_modified(time)).

Trait Implementations

impl AsFd for File

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

Borrows the file descriptor.

impl AsRawFd for File

fn as_raw_fd(&self) -> RawFd

Extracts the raw file descriptor.

impl Debug for File

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

Formats the value using the given formatter.

impl From<File> for OwnedFd

fn from(file: File) -> OwnedFd

Takes ownership of a File’s underlying file descriptor.

impl From<File> for Stdio

fn from(file: File) -> Stdio

Converts a File into a Stdio.

Examples

File will be converted to Stdio using Stdio::from under the hood.

use std::fs::File;
use std::process::Command;

// With the `foo.txt` file containing "Hello, world!"
let file = File::open("foo.txt").unwrap();

let reverse = Command::new("rev")
    .stdin(file)  // Implicit File conversion into a Stdio
    .output()
    .expect("failed reverse command");

assert_eq!(reverse.stdout, b"!dlrow ,olleH");

impl From<OwnedFd> for File

fn from(owned_fd: OwnedFd) -> Self

Returns a File that takes ownership of the given file descriptor.

impl FromRawFd for File

unsafe fn from_raw_fd(fd: RawFd) -> File

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

impl IntoRawFd for File

fn into_raw_fd(self) -> RawFd

Consumes this object, returning the raw underlying file descriptor.

impl IsTerminal for File

fn is_terminal(&self) -> bool

Returns true if the descriptor/handle refers to a terminal/tty.

impl Read for &File

fn read(&mut self, buf: &mut [u8]) -> Result<usize>

Reads some bytes from the file.

See Read::read docs for more info.

Platform-specific behavior

This function currently corresponds to the read function on Unix and the NtReadFile function on Windows. Note that this may change in the future.

fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like read, except that it reads into a slice of buffers.

See Read::read_vectored docs for more info.

Platform-specific behavior

This function currently corresponds to the readv function on Unix and falls back to the read implementation on Windows. Note that this may change in the future.

fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector #69941)

Determines if File has an efficient read_vectored implementation.

See Read::is_read_vectored docs for more info.

Platform-specific behavior

This function currently returns true on Unix an false on Windows. Note that this may change in the future.

fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (read_buf #78485)

Pull some bytes from this source into the specified buffer.

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Reads all bytes until EOF in this source, placing them into buf.

fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Reads all bytes until EOF in this source, appending them to buf.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Reads the exact number of bytes required to fill buf.

fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (read_buf #78485)

Reads the exact number of bytes required to fill cursor.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adaptor for this instance of Read.

fn bytes(self) -> Bytes<Self>

where Self: Sized,

Transforms this Read instance to an Iterator over its bytes.

fn chain<R: Read>(self, next: R) -> Chain<Self, R>

where Self: Sized,

Creates an adapter which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

where Self: Sized,

Creates an adapter which will read at most limit bytes from it.

impl Read for File

fn read(&mut self, buf: &mut [u8]) -> Result<usize>

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

fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like read, except that it reads into a slice of buffers.

fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (read_buf #78485)

Pull some bytes from this source into the specified buffer.

fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector #69941)

Determines if this Reader has an efficient read_vectored implementation.

fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Reads all bytes until EOF in this source, placing them into buf.

fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Reads all bytes until EOF in this source, appending them to buf.

fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Reads the exact number of bytes required to fill buf.

fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (read_buf #78485)

Reads the exact number of bytes required to fill cursor.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adaptor for this instance of Read.

fn bytes(self) -> Bytes<Self>

where Self: Sized,

Transforms this Read instance to an Iterator over its bytes.

fn chain<R: Read>(self, next: R) -> Chain<Self, R>

where Self: Sized,

Creates an adapter which will chain this stream with another.

fn take(self, limit: u64) -> Take<Self>

where Self: Sized,

Creates an adapter which will read at most limit bytes from it.

impl Seek for &File

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

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

fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream.

fn stream_len(&mut self) -> Result<u64>

🔬This is a nightly-only experimental API. (seek_stream_len #59359)

Returns the length of this stream (in bytes).

fn stream_position(&mut self) -> Result<u64>

Returns the current seek position from the start of the stream.

fn seek_relative(&mut self, offset: i64) -> Result<()>

Seeks relative to the current position.

impl Seek for File

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

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

fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream.

fn stream_len(&mut self) -> Result<u64>

🔬This is a nightly-only experimental API. (seek_stream_len #59359)

Returns the length of this stream (in bytes).

fn stream_position(&mut self) -> Result<u64>

Returns the current seek position from the start of the stream.

fn seek_relative(&mut self, offset: i64) -> Result<()>

Seeks relative to the current position.

impl Write for &File

fn write(&mut self, buf: &[u8]) -> Result<usize>

Writes some bytes to the file.

See Write::write docs for more info.

Platform-specific behavior

This function currently corresponds to the write function on Unix and the NtWriteFile function on Windows. Note that this may change in the future.

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like write, except that it writes into a slice of buffers.

See Write::write_vectored docs for more info.

Platform-specific behavior

This function currently corresponds to the writev function on Unix and falls back to the write implementation on Windows. Note that this may change in the future.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector #69941)

Determines if File has an efficient write_vectored implementation.

See Write::is_write_vectored docs for more info.

Platform-specific behavior

This function currently returns true on Unix an false on Windows. Note that this may change in the future.

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

Flushes the file, ensuring that all intermediately buffered contents reach their destination.

See Write::flush docs for more info.

Platform-specific behavior

Since a File structure doesn’t contain any buffers, this function is currently a no-op on Unix and Windows. Note that this may change in the future.

fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>

🔬This is a nightly-only experimental API. (write_all_vectored #70436)

Attempts to write multiple buffers into this writer.

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

Writes a formatted string into this writer, returning any error encountered.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adapter for this instance of Write.

impl Write for File

fn write(&mut self, buf: &[u8]) -> Result<usize>

Writes a buffer into this writer, returning how many bytes were written.

fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>

Like write, except that it writes from a slice of buffers.

fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (can_vector #69941)

Determines if this Writer has an efficient write_vectored implementation.

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

Flushes this output stream, ensuring that all intermediately buffered contents reach their destination.

fn write_all(&mut self, buf: &[u8]) -> Result<()>

Attempts to write an entire buffer into this writer.

fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>

🔬This is a nightly-only experimental API. (write_all_vectored #70436)

Attempts to write multiple buffers into this writer.

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

Writes a formatted string into this writer, returning any error encountered.

fn by_ref(&mut self) -> &mut Self

where Self: Sized,

Creates a “by reference” adapter for this instance of Write.