Struct Once

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

A synchronization primitive which can be used to run a one-time initialization. Useful for one-time initialization for globals, FFI or related functionality.

Differences from the standard library Once

• Only requires 1 byte of space, instead of 1 word.
• Not required to be 'static.
• Relaxed memory barriers in the fast path, which can significantly improve performance on some architectures.
• Efficient handling of micro-contention using adaptive spinning.

Examples

use parking_lot::Once;

static START: Once = Once::new();

START.call_once(|| {
    // run initialization here
});

Implementations

impl Once

pub const fn new() -> Once

Creates a new Once value.

pub fn state(&self) -> OnceState

Returns the current state of this Once.

pub fn call_once<F>(&self, f: F)

where F: FnOnce(),

Performs an initialization routine once and only once. The given closure will be executed if this is the first time call_once has been called, and otherwise the routine will not be invoked.

This method will block the calling thread if another initialization routine is currently running.

When this function returns, it is guaranteed that some initialization has run and completed (it may not be the closure specified). It is also guaranteed that any memory writes performed by the executed closure can be reliably observed by other threads at this point (there is a happens-before relation between the closure and code executing after the return).

Examples

use parking_lot::Once;

static mut VAL: usize = 0;
static INIT: Once = Once::new();

// Accessing a `static mut` is unsafe much of the time, but if we do so
// in a synchronized fashion (e.g. write once or read all) then we're
// good to go!
//
// This function will only call `expensive_computation` once, and will
// otherwise always return the value returned from the first invocation.
fn get_cached_val() -> usize {
    unsafe {
        INIT.call_once(|| {
            VAL = expensive_computation();
        });
        VAL
    }
}

fn expensive_computation() -> usize {
    // ...
}

Panics

The closure f will only be executed once if this is called concurrently amongst many threads. If that closure panics, however, then it will poison this Once instance, causing all future invocations of call_once to also panic.

pub fn call_once_force<F>(&self, f: F)

where F: FnOnce(OnceState),

Performs the same function as call_once except ignores poisoning.

If this Once has been poisoned (some initialization panicked) then this function will continue to attempt to call initialization functions until one of them doesn’t panic.

The closure f is yielded a structure which can be used to query the state of this Once (whether initialization has previously panicked or not).

Trait Implementations

impl Debug for Once

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

Formats the value using the given formatter.

impl Default for Once

fn default() -> Once

Returns the “default value” for a type.