      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171

    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use fmt;
    use sync::{Mutex, Condvar};

    /// A barrier enables multiple threads to synchronize the beginning
    /// of some computation.
    ///
    /// ```
    /// use std::sync::{Arc, Barrier};
    /// use std::thread;
    ///
    /// let mut handles = Vec::with_capacity(10);
    /// let barrier = Arc::new(Barrier::new(10));
    /// for _ in 0..10 {
    ///     let c = barrier.clone();
    ///     // The same messages will be printed together.
    ///     // You will NOT see any interleaving.
    ///     handles.push(thread::spawn(move|| {
    ///         println!("before wait");
    ///         c.wait();
    ///         println!("after wait");
    ///     }));
    /// }
    /// // Wait for other threads to finish.
    /// for handle in handles {
    ///     handle.join().unwrap();
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Barrier {
        lock: Mutex<BarrierState>,
        cvar: Condvar,
        num_threads: usize,
    }

    // The inner state of a double barrier
    struct BarrierState {
        count: usize,
        generation_id: usize,
    }

    /// A result returned from wait.
    ///
    /// Currently this opaque structure only has one method, `.is_leader()`. Only
    /// one thread will receive a result that will return `true` from this function.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BarrierWaitResult(bool);

    #[stable(feature = "std_debug", since = "1.15.0")]
    impl fmt::Debug for Barrier {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Barrier { .. }")
        }
    }

    impl Barrier {
        /// Creates a new barrier that can block a given number of threads.
        ///
        /// A barrier will block `n`-1 threads which call `wait` and then wake up
        /// all threads at once when the `n`th thread calls `wait`.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(n: usize) -> Barrier {
            Barrier {
                lock: Mutex::new(BarrierState {
                    count: 0,
                    generation_id: 0,
                }),
                cvar: Condvar::new(),
                num_threads: n,
            }
        }

        /// Blocks the current thread until all threads have rendezvoused here.
        ///
        /// Barriers are re-usable after all threads have rendezvoused once, and can
        /// be used continuously.
        ///
        /// A single (arbitrary) thread will receive a `BarrierWaitResult` that
        /// returns `true` from `is_leader` when returning from this function, and
        /// all other threads will receive a result that will return `false` from
        /// `is_leader`
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn wait(&self) -> BarrierWaitResult {
            let mut lock = self.lock.lock().unwrap();
            let local_gen = lock.generation_id;
            lock.count += 1;
            if lock.count < self.num_threads {
                // We need a while loop to guard against spurious wakeups.
                // http://en.wikipedia.org/wiki/Spurious_wakeup
                while local_gen == lock.generation_id &&
                      lock.count < self.num_threads {
                    lock = self.cvar.wait(lock).unwrap();
                }
                BarrierWaitResult(false)
            } else {
                lock.count = 0;
                lock.generation_id += 1;
                self.cvar.notify_all();
                BarrierWaitResult(true)
            }
        }
    }

    #[stable(feature = "std_debug", since = "1.15.0")]
    impl fmt::Debug for BarrierWaitResult {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("BarrierWaitResult")
                .field("is_leader", &self.is_leader())
                .finish()
        }
    }

    impl BarrierWaitResult {
        /// Returns whether this thread from `wait` is the "leader thread".
        ///
        /// Only one thread will have `true` returned from their result, all other
        /// threads will have `false` returned.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_leader(&self) -> bool { self.0 }
    }

    #[cfg(test)]
    mod tests {
        use sync::{Arc, Barrier};
        use sync::mpsc::{channel, TryRecvError};
        use thread;

        #[test]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn test_barrier() {
            const N: usize = 10;

            let barrier = Arc::new(Barrier::new(N));
            let (tx, rx) = channel();

            for _ in 0..N - 1 {
                let c = barrier.clone();
                let tx = tx.clone();
                thread::spawn(move|| {
                    tx.send(c.wait().is_leader()).unwrap();
                });
            }

            // At this point, all spawned threads should be blocked,
            // so we shouldn't get anything from the port
            assert!(match rx.try_recv() {
                Err(TryRecvError::Empty) => true,
                _ => false,
            });

            let mut leader_found = barrier.wait().is_leader();

            // Now, the barrier is cleared and we should get data.
            for _ in 0..N - 1 {
                if rx.recv().unwrap() {
                    assert!(!leader_found);
                    leader_found = true;
                }
            }
            assert!(leader_found);
        }
    }

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result
+
  Collapse/expand all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)