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Showing content from https://doc.rust-lang.org/nightly/std/fs/struct.OpenOptions.html below:

OpenOptions in std::fs - Rust

pub struct OpenOptions();

Options and flags which can be used to configure how a file is opened.

This builder exposes the ability to configure how a File is opened and what operations are permitted on the open file. The File::open and File::create methods are aliases for commonly used options using this builder.

Generally speaking, when using OpenOptions, you’ll first call OpenOptions::new, then chain calls to methods to set each option, then call OpenOptions::open, passing the path of the file you’re trying to open. This will give you a io::Result with a File inside that you can further operate on.

Opening a file to read:

use std::fs::OpenOptions;

let file = OpenOptions::new().read(true).open("foo.txt");

Opening a file for both reading and writing, as well as creating it if it doesn’t exist:

use std::fs::OpenOptions;

let file = OpenOptions::new()
            .read(true)
            .write(true)
            .create(true)
            .open("foo.txt");

Creates a blank new set of options ready for configuration.

All options are initially set to false.

use std::fs::OpenOptions;

let mut options = OpenOptions::new();
let file = options.read(true).open("foo.txt");

Sets the option for read access.

This option, when true, will indicate that the file should be read-able if opened.

use std::fs::OpenOptions;

let file = OpenOptions::new().read(true).open("foo.txt");

Sets the option for write access.

This option, when true, will indicate that the file should be write-able if opened.

If the file already exists, any write calls on it will overwrite its contents, without truncating it.

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true).open("foo.txt");

Sets the option for the append mode.

This option, when true, means that writes will append to a file instead of overwriting previous contents. Note that setting .write(true).append(true) has the same effect as setting only .append(true).

Append mode guarantees that writes will be positioned at the current end of file, even when there are other processes or threads appending to the same file. This is unlike seek(SeekFrom::End(0)) followed by write(), which has a race between seeking and writing during which another writer can write, with our write() overwriting their data.

Keep in mind that this does not necessarily guarantee that data appended by different processes or threads does not interleave. The amount of data accepted a single write() call depends on the operating system and file system. A successful write() is allowed to write only part of the given data, so even if you’re careful to provide the whole message in a single call to write(), there is no guarantee that it will be written out in full. If you rely on the filesystem accepting the message in a single write, make sure that all data that belongs together is written in one operation. This can be done by concatenating strings before passing them to write().

If a file is opened with both read and append access, beware that after opening, and after every write, the position for reading may be set at the end of the file. So, before writing, save the current position (using Seek::stream_position), and restore it before the next read.

This function doesn’t create the file if it doesn’t exist. Use the OpenOptions::create method to do so.

use std::fs::OpenOptions;

let file = OpenOptions::new().append(true).open("foo.txt");

Sets the option for truncating a previous file.

If a file is successfully opened with this option set to true, it will truncate the file to 0 length if it already exists.

The file must be opened with write access for truncate to work.

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true).truncate(true).open("foo.txt");

Sets the option to create a new file, or open it if it already exists.

In order for the file to be created, OpenOptions::write or OpenOptions::append access must be used.

See also std::fs::write() for a simple function to create a file with some given data.

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true).create(true).open("foo.txt");

Sets the option to create a new file, failing if it already exists.

No file is allowed to exist at the target location, also no (dangling) symlink. In 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).

If .create_new(true) is set, .create() and .truncate() are ignored.

The file must be opened with write or append access in order to create a new file.

use std::fs::OpenOptions;

let file = OpenOptions::new().write(true)
                             .create_new(true)
                             .open("foo.txt");

Opens a file at path with the options specified by self.

This function will return an error under a number of different circumstances. Some of these error conditions are listed here, together with their io::ErrorKind. The mapping to io::ErrorKinds is not part of the compatibility contract of the function.

• NotFound: The specified file does not exist and neither create or create_new is set.
• NotFound: One of the directory components of the file path does not exist.
• PermissionDenied: The user lacks permission to get the specified access rights for the file.
• PermissionDenied: The user lacks permission to open one of the directory components of the specified path.
• AlreadyExists: create_new was specified and the file already exists.
• InvalidInput: Invalid combinations of open options (truncate without write access, no access mode set, etc.).

The following errors don’t match any existing io::ErrorKind at the moment:

• One of the directory components of the specified file path was not, in fact, a directory.
• Filesystem-level errors: full disk, write permission requested on a read-only file system, exceeded disk quota, too many open files, too long filename, too many symbolic links in the specified path (Unix-like systems only), etc.

use std::fs::OpenOptions;

let file = OpenOptions::new().read(true).open("foo.txt");

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