Struct std::fs::File

1.0.0 · source · []
pub struct File {
    inner: File,
}
Expand description

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.

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(())
}
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 buffered Reader. This can be accomplished with BufReader<R>:

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.

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).

Fields

inner: File

Implementations

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 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.

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

See the OpenOptions::open function for more details.

Examples
use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut f = File::create("foo.txt")?;
    Ok(())
}
Run
🔬This is a nightly-only experimental API. (file_create_new)

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.

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
#![feature(file_create_new)]

use std::fs::File;

fn main() -> std::io::Result<()> {
    let mut f = File::create_new("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 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;

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.

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(())
}
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 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(())
}
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.

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(())
}
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.

Examples
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.

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(())
}
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.

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(())
}
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. (file_set_times #98245)

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
#![feature(file_set_times)]

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(())
}
Run
🔬This is a nightly-only experimental API. (file_set_times #98245)

Changes the modification time of the underlying file.

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

Trait Implementations

Available on Unix only.
Borrows the file descriptor. Read more
Borrows the handle. Read more
Available on Unix only.
Extracts the raw file descriptor. Read more
Extracts the raw handle. Read more
Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
Implementations that contain buffers (i.e. BufReader) must transfer data from their internal buffers into writer until either the buffers are emptied or limit bytes have been transferred, whichever occurs sooner. If nested buffers are present the outer buffers must be drained first. Read more
Updates Take wrappers to remove the number of bytes copied.
The minimum of the limit of all Take<_> wrappers, u64::MAX otherwise. This method does not account for data BufReader buffers and would underreport the limit of a Take<BufReader<Take<_>>> type. Thus its result is only valid after draining the buffers via drain_to. Read more
Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
Implementations that contain buffers (i.e. BufReader) must transfer data from their internal buffers into writer until either the buffers are emptied or limit bytes have been transferred, whichever occurs sooner. If nested buffers are present the outer buffers must be drained first. Read more
Updates Take wrappers to remove the number of bytes copied.
The minimum of the limit of all Take<_> wrappers, u64::MAX otherwise. This method does not account for data BufReader buffers and would underreport the limit of a Take<BufReader<Take<_>>> type. Thus its result is only valid after draining the buffers via drain_to. Read more
Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.
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
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Reads a number of bytes starting from a given offset. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Writes a number of bytes starting from a given offset. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Returns the current position within the file. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Adjust the flags associated with this file. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Adjust the rights associated with this file. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Provide file advisory information on a file descriptor. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Force the allocation of space in a file. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Create a directory. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Read the contents of a symbolic link. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Return the attributes of a file or directory. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Unlink a file. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Remove a directory. Read more
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Reads 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
🔬This is a nightly-only experimental API. (wasi_ext #71213)
Writes a number of bytes starting from a given offset. Read more
Attempts to write an entire buffer starting from a given offset. Read more
Seeks to a given position and reads a number of bytes. Read more
Seeks to a given position and writes a number of bytes. Read more
Converts to this type from the input type.
Converts to this type from the input type.

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");
Run
Converts to this type from the input type.
Converts to this type from the input type.
Available on Unix only.
Constructs a new instance of Self from the given raw file descriptor. Read more
Constructs a new I/O object from the specified raw handle. Read more
Available on Unix only.
Consumes this object, returning the raw underlying file descriptor. Read more
Consumes this object, returning the raw underlying handle. Read more
Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more
🔬This is a nightly-only experimental API. (read_buf #78485)
Pull some bytes from this source into the specified buffer. Read more
Like read, except that it reads into a slice of buffers. Read more
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Reader has an efficient read_vectored implementation. Read more
Read all bytes until EOF in this source, placing them into buf. Read more
Read all bytes until EOF in this source, appending them to buf. Read more
Read the exact number of bytes required to fill buf. Read more
🔬This is a nightly-only experimental API. (read_buf #78485)
Read the exact number of bytes required to fill cursor. Read more
Creates a “by reference” adaptor for this instance of Read. Read more
Transforms this Read instance to an Iterator over its bytes. Read more
Creates an adapter which will chain this stream with another. Read more
Creates an adapter which will read at most limit bytes from it. Read more
Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more
Like read, except that it reads into a slice of buffers. Read more
🔬This is a nightly-only experimental API. (read_buf #78485)
Pull some bytes from this source into the specified buffer. Read more
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Reader has an efficient read_vectored implementation. Read more
Read all bytes until EOF in this source, placing them into buf. Read more
Read all bytes until EOF in this source, appending them to buf. Read more
Read the exact number of bytes required to fill buf. Read more
🔬This is a nightly-only experimental API. (read_buf #78485)
Read the exact number of bytes required to fill cursor. Read more
Creates a “by reference” adaptor for this instance of Read. Read more
Transforms this Read instance to an Iterator over its bytes. Read more
Creates an adapter which will chain this stream with another. Read more
Creates an adapter which will read at most limit bytes from it. Read more
Seek to an offset, in bytes, in a stream. Read more
Rewind to the beginning of a stream. Read more
🔬This is a nightly-only experimental API. (seek_stream_len #59359)
Returns the length of this stream (in bytes). Read more
Returns the current seek position from the start of the stream. Read more
Seek to an offset, in bytes, in a stream. Read more
Rewind to the beginning of a stream. Read more
🔬This is a nightly-only experimental API. (seek_stream_len #59359)
Returns the length of this stream (in bytes). Read more
Returns the current seek position from the start of the stream. Read more
Write a buffer into this writer, returning how many bytes were written. Read more
Like write, except that it writes from a slice of buffers. Read more
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Writer has an efficient write_vectored implementation. Read more
Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
Attempts to write an entire buffer into this writer. Read more
🔬This is a nightly-only experimental API. (write_all_vectored #70436)
Attempts to write multiple buffers into this writer. Read more
Writes a formatted string into this writer, returning any error encountered. Read more
Creates a “by reference” adapter for this instance of Write. Read more
Write a buffer into this writer, returning how many bytes were written. Read more
Like write, except that it writes from a slice of buffers. Read more
🔬This is a nightly-only experimental API. (can_vector #69941)
Determines if this Writer has an efficient write_vectored implementation. Read more
Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
Attempts to write an entire buffer into this writer. Read more
🔬This is a nightly-only experimental API. (write_all_vectored #70436)
Attempts to write multiple buffers into this writer. Read more
Writes a formatted string into this writer, returning any error encountered. Read more
Creates a “by reference” adapter for this instance of Write. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

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

The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.