File in std::fs - Rust
pub struct File { /* private fields */ }
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.
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 Read er:
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).
1.0.0 · Source
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(())
}
Source
🔬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(())
}
1.0.0 · Source
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(())
}
Source
🔬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(())
}
1.77.0 · Source
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(())
}
1.58.0 · Source
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(())
}
1.0.0 · Source
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(())
}
1.0.0 · Source
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(())
}
Source
🔬This is a nightly-only experimental API. ( file_lock #130994)
Acquire an exclusive advisory lock on the file. Blocks until the lock can be acquired.
This acquires an exclusive advisory lock; no other file handle to this file may acquire another lock.
If this file handle/descriptor, or a clone of it, already holds an advisory 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 not open for writing, it is unspecified whether this function returns an error.
Note, this is an advisory lock meant to interact with lock_shared, try_lock,
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.
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.
§ Examples
#![feature(file_lock)]
use std::fs::File;
fn main() -> std::io::Result<()> {
let f = File::create("foo.txt")?;
f.lock()?;
Ok(())
}
Source
🔬This is a nightly-only experimental API. ( file_lock #130994)
Acquire a shared (non-exclusive) advisory lock on the file. Blocks until the lock can be acquired.
This acquires a shared advisory lock; more than one file handle may hold a shared lock, but none may hold an exclusive lock at the same time.
If this file handle/descriptor, or a clone of it, already holds an advisory 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.
Note, this is an advisory lock meant to interact with lock, try_lock,
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.
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.
§ Examples
#![feature(file_lock)]
use std::fs::File;
fn main() -> std::io::Result<()> {
let f = File::open("foo.txt")?;
f.lock_shared()?;
Ok(())
}
Source
🔬This is a nightly-only experimental API. ( file_lock #130994)
Try to acquire an exclusive advisory lock on the file.
Returns Ok(false) if a different lock is already held on this file (via another
handle/descriptor).
This acquires an exclusive advisory lock; no other file handle to this file may acquire another lock.
If this file handle/descriptor, or a clone of it, already holds an advisory lock, the exact
behavior is unspecified and platform dependent, including the possibility that it will
deadlock. However, if this method returns Ok(true), then it has acquired an exclusive
lock.
If the file not open for writing, it is unspecified whether this function returns an error.
Note, this is an advisory lock meant to interact with 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.
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.
§ Examples
#![feature(file_lock)]
use std::fs::File;
fn main() -> std::io::Result<()> {
let f = File::create("foo.txt")?;
f.try_lock()?;
Ok(())
}
Source
🔬This is a nightly-only experimental API. ( file_lock #130994)
Try to acquire a shared (non-exclusive) advisory lock on the file.
Returns Ok(false) if an exclusive lock is already held on this file (via another
handle/descriptor).
This acquires a shared advisory lock; more than one file handle may hold a shared lock, but none may hold an exclusive lock at the same time.
If this file handle, or a clone of it, already holds an advisory lock, the exact behavior is
unspecified and platform dependent, including the possibility that it will deadlock.
However, if this method returns Ok(true), then it has acquired a shared lock.
Note, this is an advisory lock meant to interact with lock, try_lock,
try_lock, 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.
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.
§ Examples
#![feature(file_lock)]
use std::fs::File;
fn main() -> std::io::Result<()> {
let f = File::open("foo.txt")?;
f.try_lock_shared()?;
Ok(())
}
Source
🔬This is a nightly-only experimental API. ( file_lock #130994)
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.
§ Examples
#![feature(file_lock)]
use std::fs::File;
fn main() -> std::io::Result<()> {
let f = File::open("foo.txt")?;
f.lock()?;
f.unlock()?;
Ok(())
}
1.0.0 · Source
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.
1.0.0 · Source
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(())
}
1.9.0 · Source
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(())
}
1.16.0 · Source
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.
1.75.0 · Source
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(())
}
1.75.0 · Source
Changes the modification time of the underlying file.
This is an alias for set_times(FileTimes::new().set_modified(time)).
1.63.0 · Source §1.63.0 · Source § Available on Windows only. 1.0.0 · Source §1.0.0 · Source § Available on Windows only. 1.0.0 · Source §1.15.0 · Source § Available on Unix only.
Reads a number of bytes starting from a given offset. Read more
Source §
🔬This is a nightly-only experimental API. ( unix_file_vectored_at #89517)
Like read_at, except that it reads into a slice of buffers. Read more
Writes a number of bytes starting from a given offset. Read more
Source §
🔬This is a nightly-only experimental API. ( unix_file_vectored_at #89517)
Like write_at, except that it writes from a slice of buffers. Read more
Reads the exact number of bytes required to fill buf from the given offset. Read more
Attempts to write an entire buffer starting from a given offset. Read more
Source § Available on WASI only.
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Reads a number of bytes starting from a given offset. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Writes a number of bytes starting from a given offset. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Returns the current position within the file. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Adjusts the flags associated with this file. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Adjusts the rights associated with this file. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Provides file advisory information on a file descriptor. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Forces the allocation of space in a file. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Reads the contents of a symbolic link. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Returns the attributes of a file or directory. Read more
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Source §
🔬This is a nightly-only experimental API. ( wasi_ext #71213)
Source §
🔬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
Source §
🔬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
1.15.0 · Source § Available on Windows only. 1.63.0 · Source §
Takes ownership of a File’s underlying file descriptor.
1.63.0 · Source § Available on Windows only.
Takes ownership of a File’s underlying file handle.
§ 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")?;
let reverse = Command::new("rev")
.stdin(file) // Implicit File conversion into a Stdio
.output()?;
assert_eq!(reverse.stdout, b"!dlrow ,olleH");
Returns a File that takes ownership of the given
file descriptor.
1.63.0 · Source § Available on Windows only.
Returns a File that takes ownership of the given handle.
Constructs a new instance of Self from the given raw file
descriptor. Read more
1.1.0 · Source § Available on Windows only. 1.4.0 · Source §
Consumes this object, returning the raw underlying file descriptor. Read more
1.4.0 · Source § Available on Windows only. 1.70.0 · Source §
Returns true if the descriptor/handle refers to a terminal/tty. Read more
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](https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior "mod std::io").
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.
Source §
🔬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.
Source §
🔬This is a nightly-only experimental API. ( read_buf #78485)
Pull some bytes from this source into the specified buffer. Read more
Reads all bytes until EOF in this source, placing them into buf. Read more
Reads all bytes until EOF in this source, appending them to buf. Read more
Reads the exact number of bytes required to fill buf. Read more
Source §
🔬This is a nightly-only experimental API. ( read_buf #78485)
Reads 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
Source §
🔬This is a nightly-only experimental API. ( read_buf #78485)
Pull some bytes from this source into the specified buffer. Read more
Source §
🔬This is a nightly-only experimental API. ( can_vector #69941)
Determines if this Read er has an efficient read_vectored
implementation. Read more
Reads all bytes until EOF in this source, placing them into buf. Read more
Reads all bytes until EOF in this source, appending them to buf. Read more
Reads the exact number of bytes required to fill buf. Read more
Source §
🔬This is a nightly-only experimental API. ( read_buf #78485)
Reads 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
1.0.0 · Source §1.0.0 · Source §1.0.0 · Source §
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](https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior "mod std::io").
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.
Source §
🔬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.
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](https://doc.rust-lang.org/std/io/index.html#platform-specific-behavior "mod std::io").
Attempts to write an entire buffer into this writer. Read more
Source §
🔬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
Writes 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
Source §
🔬This is a nightly-only experimental API. ( can_vector #69941)
Source §
Flushes 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
Source §
🔬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