std::fs::File - Rust
Description: API documentation for the Rust `File` struct in crate `std`.
Keywords: rust, rustlang, rust-lang, File

# 1.0.0[−][src]Struct std::fs::File

    pub struct File { /* fields omitted */ }

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.

# Examples

Create a new file and write bytes to it:

    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`:

    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 `Read`er. 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.

## Methods

### `impl File`[src]

#### `pub fn open<P: AsRef<Path>>(path: P) -> Result<File>`[src]

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

#### `pub fn create<P: AsRef<Path>>(path: P) -> Result<File>`[src]

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 std::fs::File;

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

#### `pub fn sync_all(&self) -> Result<()>`[src]

Attempts to sync all OS-internal metadata to disk.

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

# 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

#### `pub fn sync_data(&self) -> Result<()>`[src]

This function 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 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

#### `pub fn set_len(&self, size: u64) -> Result<()>`[src]

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.

# 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`.

#### `pub fn metadata(&self) -> Result<Metadata>`[src]

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

#### `pub fn try_clone(&self) -> Result<File>`
1.9.0
[src]

Create 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

Create 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

#### `pub fn set_permissions(&self, perm: Permissions) -> Result<()>`
1.16.0
[src]

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`.

## Trait Implementations

### `impl Read for File`[src]

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

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

#### `unsafe fn initializer(&self) -> Initializer`[src]

🔬 This is a nightly-only experimental API. (`read_initializer` #42788)

Determines if this `Read`er can work with buffers of uninitialized memory. Read more

#### `fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>`[src]

Read all bytes until EOF in this source, placing them into `buf`. Read more

#### `fn read_to_string(&mut self, buf: &mut String) -> Result<usize>`[src]

Read all bytes until EOF in this source, appending them to `buf`. Read more

#### `fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>`
1.6.0
[src]

Read the exact number of bytes required to fill `buf`. Read more

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn by_ref(&mut self) -> &mut Self where
Self: Sized,` [src]

Creates a "by reference" adaptor for this instance of `Read`. Read more

####
ⓘImportant traits for Bytes<R>

### Important traits for Bytes<R>

`impl<R: Read> Iterator for Bytes<R> type Item = Result<u8>;`
`fn bytes(self) -> Bytes<Self> where
Self: Sized,` [src]

Transforms this `Read` instance to an [`Iterator`] over its bytes. Read more

####
ⓘImportant traits for Chain<T, U>

### Important traits for Chain<T, U>

`impl<T: Read, U: Read> Read for Chain<T, U>`
`fn chain<R: Read>(self, next: R) -> Chain<Self, R> where
Self: Sized,` [src]

Creates an adaptor which will chain this stream with another. Read more

####
ⓘImportant traits for Take<T>

### Important traits for Take<T>

`impl<T: Read> Read for Take<T>`
`fn take(self, limit: u64) -> Take<Self> where
Self: Sized,` [src]

Creates an adaptor which will read at most `limit` bytes from it. Read more

### `impl<'a> Read for &'a File`[src]

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

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

#### `unsafe fn initializer(&self) -> Initializer`[src]

🔬 This is a nightly-only experimental API. (`read_initializer` #42788)

Determines if this `Read`er can work with buffers of uninitialized memory. Read more

#### `fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>`[src]

Read all bytes until EOF in this source, placing them into `buf`. Read more

#### `fn read_to_string(&mut self, buf: &mut String) -> Result<usize>`[src]

Read all bytes until EOF in this source, appending them to `buf`. Read more

#### `fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>`
1.6.0
[src]

Read the exact number of bytes required to fill `buf`. Read more

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn by_ref(&mut self) -> &mut Self where
Self: Sized,` [src]

Creates a "by reference" adaptor for this instance of `Read`. Read more

####
ⓘImportant traits for Bytes<R>

### Important traits for Bytes<R>

`impl<R: Read> Iterator for Bytes<R> type Item = Result<u8>;`
`fn bytes(self) -> Bytes<Self> where
Self: Sized,` [src]

Transforms this `Read` instance to an [`Iterator`] over its bytes. Read more

####
ⓘImportant traits for Chain<T, U>

### Important traits for Chain<T, U>

`impl<T: Read, U: Read> Read for Chain<T, U>`
`fn chain<R: Read>(self, next: R) -> Chain<Self, R> where
Self: Sized,` [src]

Creates an adaptor which will chain this stream with another. Read more

####
ⓘImportant traits for Take<T>

### Important traits for Take<T>

`impl<T: Read> Read for Take<T>`
`fn take(self, limit: u64) -> Take<Self> where
Self: Sized,` [src]

Creates an adaptor which will read at most `limit` bytes from it. Read more

### `impl Write for File`[src]

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

Write a buffer into this writer, returning how many bytes were written. Read more

#### `fn flush(&mut self) -> Result<()>`[src]

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more

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

Attempts to write an entire buffer into this writer. Read more

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`[src]

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

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn by_ref(&mut self) -> &mut Self where
Self: Sized,` [src]

Creates a "by reference" adaptor for this instance of `Write`. Read more

### `impl<'a> Write for &'a File`[src]

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

Write a buffer into this writer, returning how many bytes were written. Read more

#### `fn flush(&mut self) -> Result<()>`[src]

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more

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

Attempts to write an entire buffer into this writer. Read more

#### `fn write_fmt(&mut self, fmt: Arguments) -> Result<()>`[src]

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

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn by_ref(&mut self) -> &mut Self where
Self: Sized,` [src]

Creates a "by reference" adaptor for this instance of `Write`. Read more

### `impl Seek for File`[src]

#### `fn seek(&mut self, pos: SeekFrom) -> Result<u64>`[src]

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

### `impl<'a> Seek for &'a File`[src]

#### `fn seek(&mut self, pos: SeekFrom) -> Result<u64>`[src]

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

### `impl AsRawFd for File`[src]

#### `fn as_raw_fd(&self) -> RawFd`[src]

This is supported on Unix only.

Extracts the raw file descriptor. Read more

### `impl FromRawFd for File`
1.1.0
[src]

####
ⓘImportant traits for File

### Important traits for File

`impl Read for Fileimpl<'a> Read for &'a Fileimpl Write for Fileimpl<'a> Write for &'a File`
`unsafe fn from_raw_fd(fd: RawFd) -> File`[src]

This is supported on Unix only.

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

### `impl IntoRawFd for File`
1.4.0
[src]

#### `fn into_raw_fd(self) -> RawFd`[src]

This is supported on Unix only.

Consumes this object, returning the raw underlying file descriptor. Read more

### `impl FileExt for File`
1.15.0
[src]

#### `fn read_at(&self, buf: &mut [u8], offset: u64) -> Result<usize>`[src]

This is supported on Unix only.

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

#### `fn write_at(&self, buf: &[u8], offset: u64) -> Result<usize>`[src]

This is supported on Unix only.

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

#### `fn read_exact_at(&self, buf: &mut [u8], offset: u64) -> Result<()>`
1.33.0
[src]

This is supported on Unix only.

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

#### `fn write_all_at(&self, buf: &[u8], offset: u64) -> Result<()>`
1.33.0
[src]

This is supported on Unix only.

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

### `impl FileExt for File`
1.15.0
[src]

#### `fn seek_read(&self, buf: &mut [u8], offset: u64) -> Result<usize>`[src]

This is supported on Windows only.

Seeks to a given position and reads a number of bytes. Read more

#### `fn seek_write(&self, buf: &[u8], offset: u64) -> Result<usize>`[src]

This is supported on Windows only.

Seeks to a given position and writes a number of bytes. Read more

### `impl AsRawHandle for File`[src]

#### `fn as_raw_handle(&self) -> RawHandle`[src]

This is supported on Windows only.

Extracts the raw handle, without taking any ownership.

### `impl FromRawHandle for File`
1.1.0
[src]

####
ⓘImportant traits for File

### Important traits for File

`impl Read for Fileimpl<'a> Read for &'a Fileimpl Write for Fileimpl<'a> Write for &'a File`
`unsafe fn from_raw_handle(handle: RawHandle) -> File`[src]

This is supported on Windows only.

Constructs a new I/O object from the specified raw handle. Read more

### `impl IntoRawHandle for File`
1.4.0
[src]

#### `fn into_raw_handle(self) -> RawHandle`[src]

This is supported on Windows only.

Consumes this object, returning the raw underlying handle. Read more

### `impl From<File> for Stdio`
1.20.0
[src]

#### `fn from(file: File) -> Stdio`[src]

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

### `impl Debug for File`[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

## Auto Trait Implementations

### `impl Send for File`

### `impl Sync for File`

## Blanket Implementations

### `impl<T> From for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> TryFrom for T where
T: From<U>,` [src]

#### `type Error = !`

🔬 This is a nightly-only experimental API. (`try_from` #33417)

The type returned in the event of a conversion error.

#### `fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

🔬 This is a nightly-only experimental API. (`try_from` #33417)

Performs the conversion.

### `impl<T, U> TryInto for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

🔬 This is a nightly-only experimental API. (`try_from` #33417)

The type returned in the event of a conversion error.

#### `fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

🔬 This is a nightly-only experimental API. (`try_from` #33417)

Performs the conversion.

### `impl<T, U> Into for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

### `impl<T> Borrow for T where
T: ?Sized,` [src]

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut for T where
T: ?Sized,` [src]

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'a, R: Read + ?Sized> Read for &'a mut Rimpl<'a, W: Write + ?Sized> Write for &'a mut W`
`fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn get_type_id(&self) -> TypeId`[src]

🔬 This is a nightly-only experimental API. (`get_type_id` #27745)

this method will likely be replaced by an associated static

Gets the `TypeId` of `self`. Read more

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