Struct OpenOptions 

pub struct OpenOptions(/* private fields */);

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 new, then chain calls to methods to set each option, then call 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.

Implementations

impl OpenOptions

pub fn new() -> Self

Creates a blank new set of options ready for configuration. All options are initially set to false.

pub fn read(&mut self, read: bool) -> &mut OpenOptions

Sets the option for read access.

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

pub fn write(&mut self, write: bool) -> &mut OpenOptions

Sets the option for write access.

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

pub fn append(&mut self, append: bool) -> &mut OpenOptions

Sets the option for 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).

For most filesystems, the operating system guarantees that all writes are atomic: no writes get mangled because another process writes at the same time.

One maybe obvious note when using append-mode: make sure that all data that belongs together is written to the file in one operation. This can be done by concatenating strings before passing them to Self::write, or using a buffered writer (with a buffer of adequate size), and calling flush() when the message is complete.

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(SeekFrom::Current(0))), and restore it before the next read.

Note

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

pub fn truncate(&mut self, truncate: bool) -> &mut OpenOptions

Sets the option for truncating a previous file.

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

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

pub fn create(&mut self, create: bool) -> &mut OpenOptions

Sets the option for creating a new file.

This option indicates whether a new file will be created if the file does not yet already exist.

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

pub fn create_new(&mut self, create_new: bool) -> &mut OpenOptions

Sets the option to always create a new file.

This option indicates whether a new file will be created. No file is allowed to exist at the target location, also no (dangling) symlink.

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 Self::write or Self::append access in order to create a new file.

pub async fn open(&self, path: impl AsRef<Path>) -> Result<File>

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

Errors

This function will return an error under a number of different circumstances. Some of these error conditions are listed here, together with their [ErrorKind]. The mapping to [ErrorKind]s is not part of the compatibility contract of the function, especially the Other kind might change to more specific kinds in the future.

• [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.).
• [Other]: One of the directory components of the specified file path was not, in fact, a directory.
• [Other]: 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.

pub fn mode(&mut self, mode: u32) -> &mut OpenOptions

Available on Unix only.

Sets the mode bits that a new file will be created with.

If a new file is created as part of an Self::open call then this specified mode will be used as the permission bits for the new file. If no mode is set, the default of 0o666 will be used. The operating system masks out bits with the system’s umask, to produce the final permissions.

pub fn custom_flags(&mut self, flags: i32) -> &mut OpenOptions

Available on Unix only.

Passes custom flags to the flags argument of open.

The bits that define the access mode are masked out with O_ACCMODE, to ensure they do not interfere with the access mode set by Rusts options.

Custom flags can only set flags, not remove flags set by Rusts options. This options overwrites any previously set custom flags.

Trait Implementations

impl Clone for OpenOptions

fn clone(&self) -> OpenOptions

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for OpenOptions

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

Formats the value using the given formatter.

impl Default for OpenOptions

fn default() -> Self

Returns the “default value” for a type.

impl From<OpenOptions> for OpenOptions

fn from(options: OpenOptions) -> Self

Converts to this type from the input type.

impl Unwrap for OpenOptions

type StdImpl = OpenOptions

type TokioImpl = OpenOptions

fn unwrap_std(self) -> Self::StdImpl

Unwraps the std underlying implementation of the MaybeFut type.

fn unwrap_tokio(self) -> Self::TokioImpl

Unwraps the tokio underlying implementation of the MaybeFut type.

fn unwrap_std_ref(&self) -> &Self::StdImpl

Unwraps the std underlying implementation of the MaybeFut type as a reference.

fn unwrap_tokio_ref(&self) -> &Self::TokioImpl

Unwraps the tokio underlying implementation of the MaybeFut type as a reference.

fn unwrap_std_mut(&mut self) -> &mut Self::StdImpl

Unwraps the std underlying implementation of the MaybeFut type as a mutable reference.

fn unwrap_tokio_mut(&mut self) -> &mut Self::TokioImpl

Unwraps the tokio underlying implementation of the MaybeFut type as a mutable reference.

fn get_std(self) -> Option<Self::StdImpl>

Safely unwraps the std underlying implementation of the MaybeFut type.

fn get_tokio(self) -> Option<Self::TokioImpl>

Safely unwraps the tokio underlying implementation of the MaybeFut type.

fn get_std_ref(&self) -> Option<&Self::StdImpl>

Safely unwraps the std underlying implementation of the MaybeFut type as a reference.

fn get_tokio_ref(&self) -> Option<&Self::TokioImpl>

Safely unwraps the tokio underlying implementation of the MaybeFut type as a reference.

fn get_std_mut(&mut self) -> Option<&mut Self::StdImpl>

Safely unwraps the std underlying implementation of the MaybeFut type as a mutable reference.

fn get_tokio_mut(&mut self) -> Option<&mut Self::TokioImpl>

Safely unwraps the tokio underlying implementation of the MaybeFut type as a mutable reference.