std/thread/
id.rs

1use crate::num::NonZero;
2use crate::sync::atomic::{Atomic, Ordering};
3
4/// A unique identifier for a running thread.
5///
6/// A `ThreadId` is an opaque object that uniquely identifies each thread
7/// created during the lifetime of a process. `ThreadId`s are guaranteed not to
8/// be reused, even when a thread terminates. `ThreadId`s are under the control
9/// of Rust's standard library and there may not be any relationship between
10/// `ThreadId` and the underlying platform's notion of a thread identifier --
11/// the two concepts cannot, therefore, be used interchangeably. A `ThreadId`
12/// can be retrieved from the [`id`] method on a [`Thread`].
13///
14/// # Examples
15///
16/// ```
17/// use std::thread;
18///
19/// let other_thread = thread::spawn(|| {
20///     thread::current().id()
21/// });
22///
23/// let other_thread_id = other_thread.join().unwrap();
24/// assert!(thread::current().id() != other_thread_id);
25/// ```
26///
27/// [`Thread`]: super::Thread
28/// [`id`]: super::Thread::id
29#[stable(feature = "thread_id", since = "1.19.0")]
30#[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
31pub struct ThreadId(NonZero<u64>);
32
33impl ThreadId {
34    // Generate a new unique thread ID.
35    pub(crate) fn new() -> ThreadId {
36        #[cold]
37        fn exhausted() -> ! {
38            panic!("failed to generate unique thread ID: bitspace exhausted")
39        }
40
41        cfg_select! {
42            target_has_atomic = "64" => {
43                use crate::sync::atomic::AtomicU64;
44
45                static COUNTER: Atomic<u64> = AtomicU64::new(0);
46
47                let mut last = COUNTER.load(Ordering::Relaxed);
48                loop {
49                    let Some(id) = last.checked_add(1) else {
50                        exhausted();
51                    };
52
53                    match COUNTER.compare_exchange_weak(
54                        last,
55                        id,
56                        Ordering::Relaxed,
57                        Ordering::Relaxed,
58                    ) {
59                        Ok(_) => return ThreadId(NonZero::new(id).unwrap()),
60                        Err(id) => last = id,
61                    }
62                }
63            }
64            _ => {
65                use crate::cell::SyncUnsafeCell;
66                use crate::hint::spin_loop;
67                use crate::sync::atomic::AtomicBool;
68                use crate::thread::yield_now;
69
70                // If we don't have a 64-bit atomic we use a small spinlock. We don't use Mutex
71                // here as we might be trying to get the current thread id in the global allocator,
72                // and on some platforms Mutex requires allocation.
73                static COUNTER_LOCKED: Atomic<bool> = AtomicBool::new(false);
74                static COUNTER: SyncUnsafeCell<u64> = SyncUnsafeCell::new(0);
75
76                // Acquire lock.
77                let mut spin = 0;
78                // Miri doesn't like it when we yield here as it interferes with deterministically
79                // scheduling threads, so avoid `compare_exchange_weak` to avoid spurious yields.
80                while COUNTER_LOCKED.swap(true, Ordering::Acquire) {
81                    if spin <= 3 {
82                        for _ in 0..(1 << spin) {
83                            spin_loop();
84                        }
85                    } else {
86                        yield_now();
87                    }
88                    spin += 1;
89                }
90                // This was `false` before the swap, so we got the lock.
91
92                // SAFETY: we have an exclusive lock on the counter.
93                unsafe {
94                    if let Some(id) = (*COUNTER.get()).checked_add(1) {
95                        *COUNTER.get() = id;
96                        COUNTER_LOCKED.store(false, Ordering::Release);
97                        ThreadId(NonZero::new(id).unwrap())
98                    } else {
99                        COUNTER_LOCKED.store(false, Ordering::Release);
100                        exhausted()
101                    }
102                }
103            }
104        }
105    }
106
107    #[cfg(any(not(target_thread_local), target_has_atomic = "64"))]
108    pub(super) fn from_u64(v: u64) -> Option<ThreadId> {
109        NonZero::new(v).map(ThreadId)
110    }
111
112    /// This returns a numeric identifier for the thread identified by this
113    /// `ThreadId`.
114    ///
115    /// As noted in the documentation for the type itself, it is essentially an
116    /// opaque ID, but is guaranteed to be unique for each thread. The returned
117    /// value is entirely opaque -- only equality testing is stable. Note that
118    /// it is not guaranteed which values new threads will return, and this may
119    /// change across Rust versions.
120    #[must_use]
121    #[unstable(feature = "thread_id_value", issue = "67939")]
122    pub fn as_u64(&self) -> NonZero<u64> {
123        self.0
124    }
125}
std/thread/id.rs

std/thread/
id.rs
