Struct File

struct File { ... }

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

fn open<P: AsRef<Path>>(path: P) -> io::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()][self::read] or [std::fs::read_to_string()][self::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(())
}

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

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

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

If you only need to read the entire file contents, consider [std::fs::read()][self::read] or [std::fs::read_to_string()][self::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(())
}

fn create<P: AsRef<Path>>(path: P) -> io::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()][self::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(())
}

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

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][io::BufWriter] type for more details.

See also [std::fs::write()][self::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(())
}

fn create_new<P: AsRef<Path>>(path: P) -> io::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(())
}

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(())
}

fn sync_all(self: &Self) -> io::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(())
}

fn sync_data(self: &Self) -> io::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(())
}

fn lock(self: &Self) -> io::Result<()>

Acquire an exclusive lock on the file. Blocks until the lock can be acquired.

This acquires an exclusive lock; no other file handle to this file may acquire another lock.

This lock may be advisory or mandatory. This lock is meant to interact with lock, try_lock, lock_shared, try_lock_shared, and unlock. Its interactions with other methods, such as read and write are platform specific, and it may or may not cause non-lockholders to block.

If this file handle/descriptor, or a clone of it, already holds a lock the exact behavior is unspecified and platform dependent, including the possibility that it will deadlock. However, if this method returns, then an exclusive lock is held.

If the file is not open for writing, it is unspecified whether this function returns an error.

The lock will be released when this file (along with any other file descriptors/handles duplicated or inherited from it) is closed, or if the unlock method is called.

Platform-specific behavior

This function currently corresponds to the flock function on Unix with the LOCK_EX flag, and the LockFileEx function on Windows with the LOCKFILE_EXCLUSIVE_LOCK flag. Note that, this may change in the future.

On Windows, locking a file will fail if the file is opened only for append. To lock a file, open it with one of .read(true), .read(true).append(true), or .write(true).

Examples

use std::fs::File;

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

fn lock_shared(self: &Self) -> io::Result<()>

Acquire a shared (non-exclusive) lock on the file. Blocks until the lock can be acquired.

This acquires a shared lock; more than one file handle may hold a shared lock, but none may hold an exclusive lock at the same time.

This lock may be advisory or mandatory. This lock is meant to interact with lock, try_lock, lock_shared, try_lock_shared, and unlock. Its interactions with other methods, such as read and write are platform specific, and it may or may not cause non-lockholders to block.

If this file handle/descriptor, or a clone of it, already holds a lock, the exact behavior is unspecified and platform dependent, including the possibility that it will deadlock. However, if this method returns, then a shared lock is held.

The lock will be released when this file (along with any other file descriptors/handles duplicated or inherited from it) is closed, or if the unlock method is called.

Platform-specific behavior

This function currently corresponds to the flock function on Unix with the LOCK_SH flag, and the LockFileEx function on Windows. Note that, this may change in the future.

On Windows, locking a file will fail if the file is opened only for append. To lock a file, open it with one of .read(true), .read(true).append(true), or .write(true).

Examples

use std::fs::File;

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

fn try_lock(self: &Self) -> Result<(), TryLockError>

Try to acquire an exclusive lock on the file.

Returns Err(TryLockError::WouldBlock) if a different lock is already held on this file (via another handle/descriptor).

This acquires an exclusive lock; no other file handle to this file may acquire another lock.

This lock may be advisory or mandatory. This lock is meant to interact with lock, try_lock, lock_shared, try_lock_shared, and unlock. Its interactions with other methods, such as read and write are platform specific, and it may or may not cause non-lockholders to block.

If this file handle/descriptor, or a clone of it, already holds a lock, the exact behavior is unspecified and platform dependent, including the possibility that it will deadlock. However, if this method returns Ok(()), then it has acquired an exclusive lock.

If the file is not open for writing, it is unspecified whether this function returns an error.

The lock will be released when this file (along with any other file descriptors/handles duplicated or inherited from it) is closed, or if the unlock method is called.

Platform-specific behavior

This function currently corresponds to the flock function on Unix with the LOCK_EX and LOCK_NB flags, and the LockFileEx function on Windows with the LOCKFILE_EXCLUSIVE_LOCK and LOCKFILE_FAIL_IMMEDIATELY flags. Note that, this may change in the future.

On Windows, locking a file will fail if the file is opened only for append. To lock a file, open it with one of .read(true), .read(true).append(true), or .write(true).

Examples

use std::fs::{File, TryLockError};

fn main() -> std::io::Result<()> {
    let f = File::create("foo.txt")?;
    // Explicit handling of the WouldBlock error
    match f.try_lock() {
        Ok(_) => (),
        Err(TryLockError::WouldBlock) => (), // Lock not acquired
        Err(TryLockError::Error(err)) => return Err(err),
    }
    // Alternately, propagate the error as an io::Error
    f.try_lock()?;
    Ok(())
}

fn try_lock_shared(self: &Self) -> Result<(), TryLockError>

Try to acquire a shared (non-exclusive) lock on the file.

Returns Err(TryLockError::WouldBlock) if a different lock is already held on this file (via another handle/descriptor).

This acquires a shared lock; more than one file handle may hold a shared lock, but none may hold an exclusive lock at the same time.

This lock may be advisory or mandatory. This lock is meant to interact with lock, try_lock, lock_shared, try_lock_shared, and unlock. Its interactions with other methods, such as read and write are platform specific, and it may or may not cause non-lockholders to block.

If this file handle, or a clone of it, already holds a lock, the exact behavior is unspecified and platform dependent, including the possibility that it will deadlock. However, if this method returns Ok(()), then it has acquired a shared lock.

The lock will be released when this file (along with any other file descriptors/handles duplicated or inherited from it) is closed, or if the unlock method is called.

Platform-specific behavior

This function currently corresponds to the flock function on Unix with the LOCK_SH and LOCK_NB flags, and the LockFileEx function on Windows with the LOCKFILE_FAIL_IMMEDIATELY flag. Note that, this may change in the future.

On Windows, locking a file will fail if the file is opened only for append. To lock a file, open it with one of .read(true), .read(true).append(true), or .write(true).

Examples

use std::fs::{File, TryLockError};

fn main() -> std::io::Result<()> {
    let f = File::open("foo.txt")?;
    // Explicit handling of the WouldBlock error
    match f.try_lock_shared() {
        Ok(_) => (),
        Err(TryLockError::WouldBlock) => (), // Lock not acquired
        Err(TryLockError::Error(err)) => return Err(err),
    }
    // Alternately, propagate the error as an io::Error
    f.try_lock_shared()?;

    Ok(())
}

fn unlock(self: &Self) -> io::Result<()>

Release all locks on the file.

All locks are released when the file (along with any other file descriptors/handles duplicated or inherited from it) is closed. This method allows releasing locks without closing the file.

If no lock is currently held via this file descriptor/handle, this method may return an error, or may return successfully without taking any action.

Platform-specific behavior

This function currently corresponds to the flock function on Unix with the LOCK_UN flag, and the UnlockFile function on Windows. Note that, this may change in the future.

On Windows, locking a file will fail if the file is opened only for append. To lock a file, open it with one of .read(true), .read(true).append(true), or .write(true).

Examples

use std::fs::File;

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

fn set_len(self: &Self, size: u64) -> io::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.

fn metadata(self: &Self) -> io::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(())
}

fn try_clone(self: &Self) -> io::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(())
}

fn set_permissions(self: &Self, perm: Permissions) -> io::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.

fn set_times(self: &Self, times: FileTimes) -> io::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.

On most platforms, including UNIX and Windows platforms, this function can also change the timestamps of a directory. To get a File representing a directory in order to call set_times, open the directory with File::open without attempting to obtain write permission.

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::open("dest")?;
    let times = FileTimes::new()
        .set_accessed(src.accessed()?)
        .set_modified(src.modified()?);
    dest.set_times(times)?;
    Ok(())
}

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

Changes the modification time of the underlying file.

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

impl AsFd for fs::File

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

impl AsHandle for fs::File

fn as_handle(self: &Self) -> BorrowedHandle<'_>

impl AsRawFd for fs::File

fn as_raw_fd(self: &Self) -> RawFd

impl AsRawHandle for fs::File

fn as_raw_handle(self: &Self) -> RawHandle

impl Debug for File

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

impl FileExt for crate::fs::File

fn read_at(self: &Self, buf: &mut [u8], offset: u64) -> io::Result<usize>
fn read_buf_at(self: &Self, buf: BorrowedCursor<'_>, offset: u64) -> io::Result<()>
fn read_vectored_at(self: &Self, bufs: &mut [IoSliceMut<'_>], offset: u64) -> io::Result<usize>
fn write_at(self: &Self, buf: &[u8], offset: u64) -> io::Result<usize>
fn write_vectored_at(self: &Self, bufs: &[IoSlice<'_>], offset: u64) -> io::Result<usize>

impl FileExt for fs::File

fn seek_read(self: &Self, buf: &mut [u8], offset: u64) -> io::Result<usize>
fn seek_read_buf(self: &Self, buf: BorrowedCursor<'_>, offset: u64) -> io::Result<()>
fn seek_write(self: &Self, buf: &[u8], offset: u64) -> io::Result<usize>

impl FileExt for fs::File

fn read_at(self: &Self, buf: &mut [u8], offset: u64) -> io::Result<usize>
fn read_buf_at(self: &Self, buf: BorrowedCursor<'_>, offset: u64) -> io::Result<()>
fn read_vectored_at(self: &Self, bufs: &mut [io::IoSliceMut<'_>], offset: u64) -> io::Result<usize>
fn write_at(self: &Self, buf: &[u8], offset: u64) -> io::Result<usize>
fn write_vectored_at(self: &Self, bufs: &[io::IoSlice<'_>], offset: u64) -> io::Result<usize>

impl Freeze for File

impl From for fs::File

fn from(owned_fd: OwnedFd) -> Self

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

impl From for fs::File

fn from(owned: OwnedHandle) -> Self

Returns a File that takes ownership of the given handle.

impl FromRawFd for fs::File

unsafe fn from_raw_fd(fd: RawFd) -> fs::File

impl FromRawHandle for fs::File

unsafe fn from_raw_handle(handle: RawHandle) -> fs::File

impl IntoRawFd for fs::File

fn into_raw_fd(self: Self) -> RawFd

impl IntoRawHandle for fs::File

fn into_raw_handle(self: Self) -> RawHandle

impl IsTerminal for crate::fs::File

fn is_terminal(self: &Self) -> bool

impl Read for File

fn read(self: &mut Self, buf: &mut [u8]) -> io::Result<usize>
fn read_vectored(self: &mut Self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize>
fn read_buf(self: &mut Self, cursor: BorrowedCursor<'_>) -> io::Result<()>
fn is_read_vectored(self: &Self) -> bool
fn read_to_end(self: &mut Self, buf: &mut Vec<u8>) -> io::Result<usize>
fn read_to_string(self: &mut Self, buf: &mut String) -> io::Result<usize>

impl RefUnwindSafe for File

impl Seek for File

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

impl Send for File

impl Sync for File

impl Unpin for File

impl UnwindSafe for File

impl Write for File

fn write(self: &mut Self, buf: &[u8]) -> io::Result<usize>
fn write_vectored(self: &mut Self, bufs: &[IoSlice<'_>]) -> io::Result<usize>
fn is_write_vectored(self: &Self) -> bool
fn flush(self: &mut Self) -> io::Result<()>

impl<T> Any for File

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for File

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for File

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> From for File

fn from(t: T) -> T

Returns the argument unchanged.

impl<T, U> Into for File

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for File

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for File

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>