Struct std::fs::OpenOptions
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pub struct OpenOptions(_);1.0.0
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:
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().read(true).open("foo.txt"); }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:
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new() .read(true) .write(true) .create(true) .open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new() .read(true) .write(true) .create(true) .open("foo.txt");
Methods
impl OpenOptions
fn new() -> OpenOptions
Creates a blank new set of options ready for configuration.
All options are initially set to false.
Examples
fn main() { use std::fs::OpenOptions; let mut options = OpenOptions::new(); let file = options.read(true).open("foo.txt"); }use std::fs::OpenOptions; let mut options = OpenOptions::new(); let file = options.read(true).open("foo.txt");
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
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().read(true).open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new().read(true).open("foo.txt");
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
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().write(true).open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new().write(true).open("foo.txt");
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
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().append(true).open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new().append(true).open("foo.txt");
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
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().write(true).truncate(true).open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new().write(true).truncate(true).open("foo.txt");
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
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().write(true).create(true).open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new().write(true).create(true).open("foo.txt");
fn create_new(&mut self, create_new: bool) -> &mut OpenOptions1.9.0
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 as 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
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().write(true) .create_new(true) .open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new().write(true) .create_new(true) .open("foo.txt");
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
createorcreate_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
fn main() { use std::fs::OpenOptions; let file = OpenOptions::new().open("foo.txt"); }use std::fs::OpenOptions; let file = OpenOptions::new().open("foo.txt");