Struct Once

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

A synchronization primitive which can be used to run a one-time global initialization. Useful for one-time initialization for FFI or related functionality. This type can only be constructed with Once::new().

Implementations

impl Once

pub const fn new() -> Self

Creates a new Once value.

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

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).

If the given closure recursively invokes call_once on the same Once instance the exact behavior is not specified, allowed outcomes are a panic or a deadlock.

Note specific to the Linux version: recursive calls currently cause deadlock. This information is only intended to help debugging and must not be relied on.

pub fn is_completed(&self) -> bool

Returns true if some call_once() call has completed successfully. Specifically, is_completed will return false in the following situations:

• call_once() was not called at all,
• call_once() was called, but has not yet completed,
• the Once instance is poisoned

This function returning false does not mean that Once has not been executed. For example, it may have been executed in the time between when is_completed starts executing and when it returns, in which case the false return value would be stale (but still permissible).