File in std::fs - Rust (original) (raw)

Struct std::fs::File

pub struct File { /* private fields */ }

Expand description

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. 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 methodsync_all if these errors must be manually handled.

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(())
}

Run

Read 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(())
}

Run

It can be more efficient to read the contents of a file with a bufferedReader. This can be accomplished with BufReader:

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(())
}

Run

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.

Attempts to open a file in read-only mode.

See the OpenOptions::open method for more details.

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

use std::fs::File;

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

Run

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.

use std::fs::File;

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

Run

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 ofOpenOptions::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.

use std::fs::File;

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

Run

Attempts to sync all OS-internal 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. Dropping a file will ignore errors in synchronizing this in-memory data.

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(())
}

Run

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 ofsync_all.

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(())
}

Run

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.

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.

use std::fs::File;

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

Run

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

Queries metadata about the underlying file.

use std::fs::File;

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

Run

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.

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(())
}

Run

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(())
}

Run

Changes the permissions on the underlying file.

This function currently corresponds to the fchmod function on Unix and the SetFileInformationByHandle function on Windows. Note that, thismay change in the future.

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.

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(())
}

Run

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

🔬 This is a nightly-only experimental API. (io_safety #87074)

This is supported on Unix only.

Borrows the file descriptor. Read more

🔬 This is a nightly-only experimental API. (io_safety #87074)

This is supported on Unix only.

Extracts the raw file descriptor. Read more

Extracts the raw handle, without taking any ownership.

Formats the value using the given formatter. Read more

Reads a number of bytes starting from a given offset. Read more

Writes a number of bytes starting from a given offset. Read more

Reads the exact number of byte required to fill buf from the given offset. Read more

Attempts to write an entire buffer starting from a given offset. Read more