Enum OpenOptionsWriteMode

#[non_exhaustive]
pub enum OpenOptionsWriteMode {
    None,
    Write,
    Append,
}

Controls the way writes to an opened file are performed. Write modes do not affect how the file is opened - creating the file or truncating it require separate options.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

None

No writing permitted. Allows opening files where the process lacks write permissions, and attempts to write will fail.

Write

Writes permitted. The file location pointer tracked by the OS determines where writes in the file will take place.

Append

Writes permitted. The OS will place each write at the current end of the file. These may still change the file location pointer, so if reads are being used as well, be sure to seek to the desired location before reading. One way to do this is to use seek to save the file location pointer (seek(SeekFrom::Current(0))) and then apply the result before the next read.

Most OSes and filesystems make these writes atomically, such that different threads or even processes can collaborate safely on a single file, as long as each write call provides a full unit of data (e.g. a line, or a binary struct etc). This can be done by building up the data to write, or using a buffered writer that is large enough and calling flush after each unit is complete.

In particular NFS on Linux is documented as not providing atomic appends.

use std::fs::OpenOptions;

let file = OpenOptions::new().write(OpenOptionsWriteMode::Append).open_at(&mut parent, "foo.txt");

Trait Implementations

impl Clone for OpenOptionsWriteMode

fn clone(&self) -> OpenOptionsWriteMode

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for OpenOptionsWriteMode

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for OpenOptionsWriteMode

fn default() -> OpenOptionsWriteMode

Returns the “default value” for a type.

impl PartialEq for OpenOptionsWriteMode

fn eq(&self, other: &OpenOptionsWriteMode) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl PartialOrd for OpenOptionsWriteMode

fn partial_cmp(&self, other: &OpenOptionsWriteMode) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Copy for OpenOptionsWriteMode

impl Eq for OpenOptionsWriteMode

impl StructuralPartialEq for OpenOptionsWriteMode