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

# Struct std::fs::OpenOptions 1.0.0 [−] [src]

    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 `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.

# Examples

Opening a file to read:

    use std::fs::OpenOptions;

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

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");Run

## Methods

### `impl OpenOptions`
[src]

#### `fn new() -> OpenOptions`

Creates a blank new set of options ready for configuration.

All options are initially set to `false`.

# Examples

    use std::fs::OpenOptions;

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

#### `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.

# Examples

    use std::fs::OpenOptions;

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

#### `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.

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

# Examples

    use std::fs::OpenOptions;

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

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

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)`.

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 `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.

# Examples

    use std::fs::OpenOptions;

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

#### `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.

# Examples

    use std::fs::OpenOptions;

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

#### `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, `write` or `append` access must be used.

# Examples

    use std::fs::OpenOptions;

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

#### `fn create_new(&mut self, create_new: bool) -> &mut OpenOptions`
1.9.0

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

# Examples

    use std::fs::OpenOptions;

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

#### `fn open<P: AsRef<Path>>(&self, path: P) -> 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, to include but not limited to:

- Opening a file that does not exist without setting `create` or `create_new`.
- Attempting to open a file with access that the user lacks permissions for
- Filesystem-level errors (full disk, etc)
- Invalid combinations of open options (truncate without write access, no access mode set, etc)

# Examples

    use std::fs::OpenOptions;

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

## Trait Implementations

### `impl Clone for OpenOptions`
[src]

#### `fn clone(&self) -> OpenOptions`

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

### `impl OpenOptionsExt for OpenOptions`
1.1.0
[src]

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

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

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

Pass custom flags to the `flags` agument of `open`. Read more

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