1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! A "once initialization" primitive
13 //! This primitive is meant to be used to run one-time initialization. An
14 //! example use case would be for initializing an FFI library.
20 use sync::atomic::{AtomicIsize, Ordering};
21 use sync::StaticMutex;
23 /// A synchronization primitive which can be used to run a one-time global
24 /// initialization. Useful for one-time initialization for FFI or related
25 /// functionality. This type can only be constructed with the `ONCE_INIT`
31 /// use std::sync::{Once, ONCE_INIT};
33 /// static START: Once = ONCE_INIT;
35 /// START.call_once(|| {
36 /// // run initialization here
39 #[stable(feature = "rust1", since = "1.0.0")]
43 lock_cnt: AtomicIsize,
46 /// Initialization value for static `Once` values.
47 #[stable(feature = "rust1", since = "1.0.0")]
48 pub const ONCE_INIT: Once = Once::new();
51 /// Creates a new `Once` value.
52 #[stable(feature = "once_new", since = "1.2.0")]
53 pub const fn new() -> Once {
55 mutex: StaticMutex::new(),
56 cnt: AtomicIsize::new(0),
57 lock_cnt: AtomicIsize::new(0),
61 /// Performs an initialization routine once and only once. The given closure
62 /// will be executed if this is the first time `call_once` has been called,
63 /// and otherwise the routine will *not* be invoked.
65 /// This method will block the calling thread if another initialization
66 /// routine is currently running.
68 /// When this function returns, it is guaranteed that some initialization
69 /// has run and completed (it may not be the closure specified). It is also
70 /// guaranteed that any memory writes performed by the executed closure can
71 /// be reliably observed by other threads at this point (there is a
72 /// happens-before relation between the closure and code executing after the
74 #[stable(feature = "rust1", since = "1.0.0")]
75 pub fn call_once<F>(&'static self, f: F) where F: FnOnce() {
76 // Optimize common path: load is much cheaper than fetch_add.
77 if self.cnt.load(Ordering::SeqCst) < 0 {
81 // Implementation-wise, this would seem like a fairly trivial primitive.
82 // The stickler part is where our mutexes currently require an
83 // allocation, and usage of a `Once` shouldn't leak this allocation.
85 // This means that there must be a deterministic destroyer of the mutex
86 // contained within (because it's not needed after the initialization
89 // The general scheme here is to gate all future threads once
90 // initialization has completed with a "very negative" count, and to
91 // allow through threads to lock the mutex if they see a non negative
92 // count. For all threads grabbing the mutex, exactly one of them should
93 // be responsible for unlocking the mutex, and this should only be done
94 // once everyone else is done with the mutex.
96 // This atomicity is achieved by swapping a very negative value into the
97 // shared count when the initialization routine has completed. This will
98 // read the number of threads which will at some point attempt to
99 // acquire the mutex. This count is then squirreled away in a separate
100 // variable, and the last person on the way out of the mutex is then
101 // responsible for destroying the mutex.
103 // It is crucial that the negative value is swapped in *after* the
104 // initialization routine has completed because otherwise new threads
105 // calling `call_once` will return immediately before the initialization
108 let prev = self.cnt.fetch_add(1, Ordering::SeqCst);
110 // Make sure we never overflow, we'll never have isize::MIN
111 // simultaneous calls to `call_once` to make this value go back to 0
112 self.cnt.store(isize::MIN, Ordering::SeqCst);
116 // If the count is negative, then someone else finished the job,
117 // otherwise we run the job and record how many people will try to grab
119 let guard = self.mutex.lock();
120 if self.cnt.load(Ordering::SeqCst) > 0 {
122 let prev = self.cnt.swap(isize::MIN, Ordering::SeqCst);
123 self.lock_cnt.store(prev, Ordering::SeqCst);
127 // Last one out cleans up after everyone else, no leaks!
128 if self.lock_cnt.fetch_add(-1, Ordering::SeqCst) == 1 {
129 unsafe { self.mutex.destroy() }
140 use sync::mpsc::channel;
144 static O: Once = Once::new();
146 O.call_once(|| a += 1);
148 O.call_once(|| a += 1);
154 static O: Once = Once::new();
155 static mut run: bool = false;
157 let (tx, rx) = channel();
160 thread::spawn(move|| {
161 for _ in 0..4 { thread::yield_now() }
169 tx.send(()).unwrap();